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Development of hybrid microsensors for environmental monitoring and biodetection
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Development of hybrid microsensors for environmental monitoring and biodetection
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Development of Hybrid Microsensors for Environmental Monitoring and Biodetection by Simin Mehrabani A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (CHEMICAL ENGINEERING) May 2015 Copyright 2015 Simin Mehrabani ii Acknowledgements “Be grateful for whoever comes, because each has been sent as a guide from beyond.” - Rumi “Life is a journey, not a destination” - Emerson The past five years of my life have been filled with moments of great value: moments of happiness, hope, and excitement and naturally moments of sadness, and despair. I am thankful to many people who helped me experience all the joyous moments and overcome the sad ones. First and foremost, I would like to thank my advisor, Professor Andrea Armani, who accepted me into her research group as a PhD candidate back in January of 2010. I have constantly been amazed and motivated by her hard work and great accomplishments at such a young age. I am thankful for her support, kindness and considerations which have always been beyond what I expected from my advisor. Dr. Armani has provided me with countless opportunities that have made an invaluable impact on my graduate career: attending and presenting my findings at conferences, ensuring funding through numerous fellowships and awards, and engaging in research collaborations both university-wide and beyond. I sincerely thank her for believing in me, inspiring me and guiding me throughout the past years. I immensely appreciate the opportunity to meet with Dr. Deniz Armani and I would like to especially thank him for shining CO 2 laser onto silica microdisks and iii creating microtoroids. His creation enabled various on-chip applications, some of which serve as the basis of my thesis work. My special thanks go to all the professors at USC that I have had the privilege to learn from, especially my qualifying exam and defense committee members, Professor Barry Thompson, Professor Muhammad Sahimi, Professor Pin Wang, and Professor Steven Nutt. I would like to sincerely thank Professor Sahimi for believing in me and my abilities, when I first arrived at USC. Without his support, I could not have started such a great journey. In addition, I had the opportunity to collaborate with Professor Malancha Gupta on the project discussed in Chapter 4 and I really appreciate it. Additionally, I would like to thank Professor Wei Wu for teaching an exceptional course on nano- fabrication techniques. I owe many thanks to my teachers in Iran, especially, Mrs. Kavoosi, Mrs. Janati, Mrs. Razafshar, Mrs. Keramati, Mrs. Samayounkhani, Mrs. Nobakht, Mrs. Izadpanah, Mrs. Zeinolabedin, Mrs. Riazidoost, Mrs. Jalali, Mrs. Shabab, Mrs. Mazdayi (thanks for the awesome lectures on chemistry and life!), Dr. Niknam, Mr. Heravi (thanks for reshaping my understanding of physics), Mr. Ghalamchi, Mr. Khanof, Mr. Taheri, Dr. Shirvani, Dr. Pak, Dr. Parvari, Dr. Roshanzamir, and Dr. Hashemabadi. During my PhD work, I have had great mentors in the Armani Lab: Dr. Hong Seok Choi, thanks for welcoming me to the group on my first day, giving me a USC campus tour and later training me on the resonator testing process. Your patience during testing has been a great source of inspiration. Professor Heather Hunt, while you were a postdoctoral scholar in the Armani Lab, I had the privilege of conducting my first wet iv chemistry experiments under your instructions. I am thankful to you for teaching me sol- gel synthesis and surface chemistry techniques. Also, I would like to thank you for the collaboration on the quantum dot project discussed in Chapter 6. Dr. Xiaomin Zhang, thanks for training me on the cleanroom fabrication process. I have learnt a lot from your fabrication skills, attention to detail and patience. I feel truly lucky to be part of the Armani lab and I would like to thank all the past and current members for being welcoming, friendly and supportive: Dr. Ce Shi (thanks for being a great office mate and also for our collaboration on the work discussed in Appendix B), Dr. Biliang Hu, Alice Hu (the first Armani Lab baby), Dr. Eda Gungor (my dear sister), Victoria Sun (thanks for all the fun experiments, jokes, and awesome books), Sahar Elyahoodyan, Soheil Soltani (thanks for all the helpful discussions), Dr. Ashley Maker (thanks for all the yummy baked goods, the fun English terms!! and the collaboration on the hybrid sensors review paper), Matt Reddick, Maria Chistiakova, Kelvin Kuo, Melody Kuo, Audrey Harker (thanks for the collaboration on the project discussed in Chapter 3), Sam Kushner-Lenhoff (thanks for the collaboration on the project discussed in Appendix C), Emma Meinke (thanks for the collaboration on the project discussed in Appendix B), Dr. Jason Gamba, Dr. Rasheeda Hawk, Professor Cecilia Lopez, Nishita Deka, Vinh Diep, Mark Harrison, Nicole Harrison, Andre Kovach (thanks for being such an awesome activity czar), Michele Lee, Alexa Hudnut, Sam McBirney, Akshay Panchavati, Dr. Tushar Rane, Leah Tsui (the most cheerful person I have ever met), Sekiharu Kure (thanks for teaching me Japanese), Dania Neiroukh, Chaitanya Murthy, Shehzad Ismail, Dr. Xiaoquin Shen, Nic Murillo, Brian Rose, Max v Reynolds, Bradley Biggs, Tobias Wienhold, Eric Moen, Tara Assi, Alexei Naumann, Gumi Sethi, Martin Siron, Jason Pang, Grace Kim, and Lili Lash-Rosenberg. I would like to thank the researchers outside the Armani Lab who I had the opportunity to collaborate with: Philip Kwong (thanks for the collaboration on the project discussed in Chapter 5), Dr. Hari Mahalingam (thanks for the collaboration on the project discussed in Appendix D), Liubing Huang (thanks for the collaboration on the project discussed in Chapter 7), and Dr. Imran Cheema (thanks for the collaboration on the project discussed in Appendix A). I would also like to thank Dr. Saber Naserifar for his awesome job as the teaching assistant for the transport courses. There are various research facilities at USC that I have used at different stages of my research and I would like to express my sincerest gratitude to the coordinators and trainers at the following facilities: Vivian Hall of Enngineering Cleanroom (Dr. Donghai Zhu), Center for Electron Microscopy and Microanalysis (John Curulli, Yuzheng Zhang, Ian Mc Farlane, and Dr. Matthew Mecklenburg), and Ellipsometry Measurements (Professor William Steier, and Dr. Hari Mahalingam). My time at USC could have not been this great without the awesome job of the USC staff. I would like to thank all of them, especially, Andy Chen, Angeline Fugelso, Heather Alexander, Dr. Jenny Vazquez- Akim, Karen Woo, Margine Martinez, Martin Olekszyk, Shokry Bastorous, and Tina Silva. I have been blessed with invaluable friendships throughout my life and my deepest gratitude goes to my truly caring friends: Bita Pournajhdi, Ghazal Salimian, vi Fereshteh Nazimi, Fereshteh Rahnama, Malak Khojasteh, Sahar Dehgaei, Sahra Homayounian, Sara Abdolahzadeh, and Shima Haghighat. I would like to thank all of my family members including my aunts, uncles, grandparents, cousins, and all of my in-laws for their love and support. I have been especially inspired by my dear uncle, Dr. Anooshiravan Ghafari, and his life’s journey of moving to the US, where he received his Master’s degree in mechanical engineering from USC and his PhD from UC Berkeley. I am truly thankful for his support and kindness throughout my whole life. I cannot thank my parents, Parvaneh Abyari and Ardeshir Mehrabani, enough for all the sacrifices that they have made in order to provide me with a better life. They are not only my parents but also my dearest friends who have constantly encouraged me to follow my passion, provided me with exceptional learning opportunities and most important of all, loved me unconditionally. I am thankful to them for always listening to my concerns, cheering me up and holding my hands even though they are thousands of miles away in my motherland Iran. During my pre-college studies, I usually referred to my brother, Kaveh, for random science-related questions. I would like to thank him for encouraging me to search for the answers on my own. At the time, I might have been disappointed with him, but I am thankful now! Lastly, but certainly not least, I would like to thank my dear husband, Hooman Rashidi. Ever since we met, he has had an incredible ability to talk to my heart and make me laugh. I would like to thank him for understanding me, supporting my goals, motivating me and believing in me. vii Table of Contents Acknowledgements ............................................................................................................. ii List of Figures ..................................................................................................................... x Abstract ............................................................................................................................ xix Chapter 1 Introduction ..................................................................................................... 1 1.1 Motivation ........................................................................................................... 1 1.2 Chapter overview ................................................................................................ 2 Chapter 1 References ...................................................................................................... 7 Chapter 2 Background ..................................................................................................... 9 2.1 Light .................................................................................................................... 9 2.2 Refractive index ................................................................................................ 10 2.3 Total internal reflection..................................................................................... 11 2.4 Optical fiber ...................................................................................................... 12 2.5 Optical resonators ............................................................................................. 14 2.6 Whispering gallery mode optical microresonators ........................................... 14 2.7 Quality factor .................................................................................................... 17 2.8 Free spectral range ............................................................................................ 21 2.9 Mode volume .................................................................................................... 21 2.10 Different geometries of whispering gallery mode optical resonators ............... 22 2.11 Silica microtoroid fabrication ........................................................................... 24 2.12 Evanescent field coupling ................................................................................. 34 2.13 Testing setup ..................................................................................................... 38 2.14 Quality factor measurement .............................................................................. 40 2.15 Characterizing the free spectral range ............................................................... 44 2.16 Nonlinear effects: thermal broadening and mechanical vibration .................... 45 2.17 Application of silica microtoroids in sensing ................................................... 47 2.18 Tracking the resonant wavelength .................................................................... 48 Chapter 2 References .................................................................................................... 50 viii Chapter 3 Ultraviolet sensor based on a silica microtoroid ........................................... 54 3.1 Introduction ....................................................................................................... 54 3.2 Experiment ........................................................................................................ 59 3.3 Results and discussion ...................................................................................... 61 3.4 Conclusion ........................................................................................................ 66 Chapter 3 References .................................................................................................... 67 Chapter 4 Relative humidity sensor based on a polymer coated silica microtoroid...... 69 4.1 Introduction ....................................................................................................... 69 4.2 Experiment ........................................................................................................ 72 4.3 Results and discussion ...................................................................................... 77 4.4 Conclusion ........................................................................................................ 90 Chapter 4 References .................................................................................................... 91 Chapter 5 Blue microlaser based on a thulium doped silica microtoroid ...................... 93 5.1 Introduction ....................................................................................................... 93 5.2 Experiment ...................................................................................................... 103 5.3 Results and discussion .................................................................................... 108 5.4 Conclusion ...................................................................................................... 120 Chapter 5 References .................................................................................................. 121 Chapter 6 Integration of semiconductor quantum dots and silica microtoroids .......... 124 6.1 Introduction ..................................................................................................... 124 6.2 Experiment ...................................................................................................... 128 6.3 Results and discussion .................................................................................... 129 6.4 Conclusion ...................................................................................................... 138 Chapter 6 References .................................................................................................. 139 Chapter 7 Studying the integration of zinc oxide and silica microtoroids .................. 141 7.1 Introduction ..................................................................................................... 141 7.2 Experiment ...................................................................................................... 142 7.3 Results and discussion .................................................................................... 148 7.4 Conclusion ...................................................................................................... 164 Chapter 7 References .................................................................................................. 165 ix Bibliography ................................................................................................................... 169 Appendix A Phase shift cavity ring-down measurement based biosensor .................. 190 A.1 Introduction ..................................................................................................... 190 A.2 Experiment ...................................................................................................... 195 A.3 Results and discussion .................................................................................... 197 A.4 Conclusion ...................................................................................................... 203 Appendix A References .............................................................................................. 204 Appendix B Approaches for improving biodetection ................................................. 205 B.1 Improving the detection specificity utilizing bimodal kinetics ....................... 205 B.2 Controlling binding sites on surface of silica microresonators ....................... 210 Appendix B References .............................................................................................. 213 Appendix C Thulium cerium co-doped silica microtoroids ........................................ 214 Appendix C References .............................................................................................. 219 Appendix D Titania-silica hybrid microresonators ..................................................... 221 Appendix D References .............................................................................................. 228 x List of Figures Figure 2-1 Demonstration of Snell’s law (a) Incident angle is smaller than the critical angle (b) Critical angle (c) Total internal reflection. ................................................ 12 Figure 2-2 Schematic of an optical fiber where total internal reflection is responsible for trapping light within the core of the fiber. .......................................................... 13 Figure 2-3 (a) The dome of St. Paul’s Cathedral [9] (b) The whispering gallery at St. Paul’s Cathedral [10] (c) The snapshots of sound waves resonating in a cylinder of air of the same size as the whispering gallery at St. Paul’s Cathedral. Red and blue show the higher and lower air pressures, respectively [6]. ....................................... 15 Figure 2-4 Schematic of on-resonance and off-resonance conditions. ............................. 16 Figure 2-5 Different geometries of silica whispering gallery mode optical resonators (a) Scanning electron microscope (SEM) image of a silica microdisk (b) Scanning electron microscope (SEM) image of a microtoroid (c) Widefield microscope image of a microsphere. ............................................................................................ 22 Figure 2-6 Silica microtoroid fabrication process (a) Two micron thick thermally grown silica on a silicon wafer (b) Photoresist is spin-coated on top of the silica (c) Sample is exposed to UV radiation through a photomask (d) The UV-exposed areas of photoresist is washed off in the developer solution (e) Buffered oxide etchant (BOE) etches the uncovered areas of silica (f) Photoresist is washed off with acetone (g) XeF 2 gas undercuts the silica, forming microdisks (h) Microdisks are reflowed with a CO 2 laser, forming the microtoroids. ........................................ 25 Figure 2-7 Picture of HMDS deposition set up. ............................................................... 26 Figure 2-8 Picture of a photomask with rows of various sizes of disks. .......................... 27 Figure 2-9 Karl Suss MJB 3 photomask aligner. .............................................................. 28 Figure 2-10 XeF 2 etching unit (a) Front view (b) Side view. ........................................... 31 Figure 2-11 XeF 2 etcher LabVIEW program front panel. ................................................ 32 Figure 2-12 CO 2 laser reflow set up. ................................................................................ 33 Figure 2-13 Taper puller setup where tapered optical fiber is formed by simultaneous pulling and melting the optical fiber. ........................................................................ 36 Figure 2-14 (a) Optical image of an optical fiber (b) Optical image of a tapered optical fiber. .......................................................................................................................... 37 xi Figure 2-15 Schematic of the testing setup. ...................................................................... 38 Figure 2-16 (a) Resonator testing setup (b) Zoomed in view of sample holder stage. ..... 39 Figure 2-17 Schematic of ring-down measurement, where Q=ωτ o . ................................. 40 Figure 2-18 Schematic of a resonance dip and linewidth measurement, where Q= λ o /Δλ .................................................................................................................... 41 Figure 2-19 (a) Transmission signal of a silica microtoroid on resonance where the red and green curves represent Lorentzian fits to the resonant peaks (b) Graph of quality factor versus coupling which is used to calculate the intrinsic quality factor based on the y-intercept of the linear fit. .................................................................. 43 Figure 2-20 Broad scan spectra of a silica microtoroid with major diameter of 50μm which has free spectral range of 4.9nm. ................................................................... 44 Figure 2-21 Thermal broadening in a silica microtoroid. ................................................. 45 Figure 2-22 Mechanical vibration in a silica microtoroid. ............................................... 46 Figure 3-1 Electromagnetic spectrum [3]. ........................................................................ 54 Figure 3-2 Schematic of side view of a microtoroid depicting major diameter (D) and minor diameter (d). ................................................................................................... 57 Figure 3-3 Schematic of testing setup for UV measurement experiments [19]. ............... 60 Figure 3-4 Transmission spectrum of silica microtoroid on resonance with taper in contact. ...................................................................................................................... 61 Figure 3-5 resonant wavelength shift versus time as the intensity of UV radiation is increased from 14mW/cm 2 to 23mW/cm 2 and subsequently decreased back to 14mW/cm 2 in the same increments........................................................................... 62 Figure 3-6 resonant wavelength shift as a function of cumulated fluence (product of UV intensity by the exposure time) where forward is increase in UV intensity and backward is decrease in UV intensity. ...................................................................... 63 Figure 3-7 Histogram of the distribution of noise in tracking the resonant wavelength shift. .......................................................................................................................... 64 As explained in the introduction, the shift in the resonant wavelength is due to the absorption of the UV radiation by silica, which subsequently increases the temperature of the device. The relation between the UV intensity and resonant wavelength shift can be predicted using Equation 3-12. In Figure 3-8, the results xii of the theoretical predictive model are plotted along with the experimental measurements. There is agreement between the model and the experiments. The slight difference between them could be due to the fact that, in theory, it is assumed that all the UV radiation reaching the sample will be absorbed based on the absorption coefficient; however, scattering could cause lower absorption [19]. 64 Figure 3-8 Experimental and theoretical shift in the resonant wavelength as a function of UV cumulated fluence. ......................................................................................... 65 Figure 4-1 poly (N-isopropylacrylamide) (pNIPAAm) structure. .................................... 70 Figure 4-2 Schematic of iCVD reactor. ............................................................................ 71 Figure 4-3 Pictures of iCVD reactor used for deposition of pNIPAAm film on silica microtoroids. ............................................................................................................. 74 Figure 4-4 (a) Rendering image of the testing setup for relative humidity measurements [8] (b) Picture of the testing setup............................................................................. 76 Figure 4-5 Fourier transform infrared (FTIR) spectra of pNIPAAm. .............................. 77 Figure 4-6 Refractive indices of fabricated pNIPAAm films measured by ellipsometry in comparison with refractive index of silica based on reference [14]. .................... 79 Figure 4-7 Optical images of (a) thin film coated microtoroid (b) thick film coated microtoroid. ............................................................................................................... 80 Figure 4-8 FEM simulation of (a) thin film coated microtoroid (b) thick film coated microtoroid. ............................................................................................................... 81 Figure 4-9 Results of quality factor measurements (a) Sample transmission spectra of resonant peak for thin polymer film (b) Intrinsic quality factor measurement for thin polymer film (c) Sample transmission spectra of resonant peak for thick polymer film (d) Intrinsic quality factor measurement for thick polymer film. ....... 82 Figure 4-10 Response of reference relative humidity sensor over time (black squares) along with the resonant wavelength shift as a function of time (red circles) for bare silica microtoroid. ............................................................................................. 83 Figure 4-11 Reference relative humidity sensor response over time (black squares) along with the resonant wavelength shift as a function of time (red circles) for (a) thin film coated microtoroid (b) thick film coated microtoroid [8]. ......................... 84 xiii Figure 4-12 Resonant wavelength shift as a function of changes in relative humidity for the bare silica microtoroid and the thin and thick polymer coated devices along with the noise measurements. The error bars in the experiment are smaller than the symbols [8]. ......................................................................................................... 85 Figure 4-13 The response of hybrid sensors with (a) thin polymer film (b) thick polymer film [8]. ....................................................................................................... 86 Figure 4-14 Hysteresis of hybrid silica microtoroids with (a) thin polymer film and (b) thick polymer film [8]. .............................................................................................. 88 Figure 4-15 Characteristic curve of the changes in the resonant wavelength vs. changes in the relative humidity at various temperatures for the (a) thin polymer coated device and (b) thick polymer coated device. ............................................................ 89 Figure 5-1 Four main types of interaction of photons and electrons in a laser gain medium. .................................................................................................................... 95 Figure 5-2 Three level lasing scheme, where the energy of the pumping photon is higher than the laser photon. ..................................................................................... 96 Figure 5-3 Sequential two-photon absorption in a three level upconversion scheme, where the energy of the pumping photon is lower than the energy of the released photon. ...................................................................................................................... 98 Figure 5-4 Schematic of the sol-gel process used to fabricate pure and thulium doped silica films on a silicon substrate. ........................................................................... 104 Figure 5-5 Rendering of testing setup. ............................................................................ 106 Figure 5-6 Modified sample holder for measurements in water. .................................... 107 Figure 5-7 Testing setup with the modified sample holder for experiments in water. ... 107 Figure 5-8 FTIR spectra of fabricated sol-gel silica film along with the FTIR spectra of commercial thermally grown silica. ................................................................... 108 Figure 5-9 Scanning electron microscope (SEM) image of a 0.094at.% Tm doped silica microtoroid [5]............................................................................................... 109 Figure 5-10 (a) Sample transmission spectra of a 0.033at.% Tm doped silica microtoroid with Q=1.6x10 6 in air (b) Quality factor of sol-gel silica microtoroids with various levels of Tm doping measured in the air. ..................... 110 xiv Figure 5-11 Images of 0.033at.% Tm doped silica microtoroids with tapered optical fiber taken from top view camera in air (a) Off resonance (b) On resonance with lights on and (c) On resonance with lights off [5]. ................................................. 111 Figure 5-12 Side view images of 0.043at.% Tm doped silica microtoroids while on resonance and lasing [43]. ....................................................................................... 112 Figure 5-13 (a) Multimode blue lasing at 450nm and 461nm in air, inset: lasing threshold graph of 0.043at.% Tm sample with threshold power of 32μW in air (b) Lorentzian fit to the lasing spectra in (a) which shows a constant distance of 1.1nm between peaks which is in agreement with the calculated free spectral range of the device [5]. .......................................................................................... 113 Figure 5-14 (a) Multimode lasing at 784nm ,802nm ,816nm in air, inset: lasing threshold graph of 0.043at.% Tm with threshold power of 17μW in air (b) Lorentzian fit to the lasing spectra in (a) which shows a constant distance of 2.7nm between each peak in agreement with the calculated free spectral rang of the device [5]........................................................................................................... 114 Figure 5-15 Energy level diagram of Tm 3+ [5]. .............................................................. 115 Figure 5-16 Lasing threshold in air versus thulium concentration for (a) Blue lasing (b) Near-IR lasing. .................................................................................................. 116 Figure 5-17 (a) Representative transmission spectrum of a 0.062at.% Tm doped silica microtoroid in water (b) Intrinsic quality factor measurement for a 0.062at.% Tm doped silica microtoroid in water. .......................................................................... 117 Figure 5-18 Images of 0.062at.% Tm doped silica microtoroids with tapered optical fiber taken from the top view camera in water (a) Off resonance (b) On resonance with lights on and (c) On resonance with lights off. .............................. 118 Figure 5-19 (a) Multimode blue lasing at 450nm and 461nm in water (b) Lasing threshold graph of 0.062at.% Tm with a threshold power of 230μW in water (c) Multimode lasing at 784nm ,802nm ,816nm in water (d) Lasing threshold graph of 0.062at.% Tm with a threshold power of 40μW in water. ................................. 119 Figure 6-1 Bioconjugation technique for attachment of biotin on surface of silica. ...... 126 Figure 6-2 Process of attaching streptavidin conjugated quantum dots onto silica microtoroids. ........................................................................................................... 128 xv Figure 6-3 Optical and fluorescence microscope images of a silica microtoroid under consecutive attachment and removal of two different streptavidin conjugated quantum dots. .......................................................................................................... 131 Figure 6-4 Quality factor of silica microtoroids at different stages of attachment and removal of quantum dots (Qdot655). ...................................................................... 132 Figure 6-5 Quality factor of silica microtoroids coated with CdSe/ZnS quantum dots.. 134 Figure 6-6 Emission from the quantum dot coated silica microtoroid as a function of time. ........................................................................................................................ 135 Figure 6-7 Emission from the quantum dot coated silica microtoroid as a function of time where the center of the emission is found by fitting a Gaussian curve (red line). ........................................................................................................................ 136 Figure 7-1 (a) Tube furnace where the ZnO nanowires were grown (b) Mass flowmeters used to control the flow rate of argon and oxygen gasses entering the tube furnace. ............................................................................................................ 143 Figure 7-2 Scanning electron microscope (SEM) images of ZnO grown on silica/silicon chips. Sample (a) is the farthest from the Zn source and sample (c) is the closest to the Zn source. ................................................................................ 149 Figure 7-3 (a) Results of spectrofluorometery measurements on reference wafers with low and high density ZnO nanowires along with a sample without ZnO nanowires (b) Zoomed-in view of the graph in part (a) showing the signal collected from the lower density ZnO sample. ....................................................... 151 Figure 7-4 Scanning electron microscope (SEM) images of silica microtoroids with ZnO nanowires grown on them at different reaction times. As moving from (a) to (e) the reaction time drops from 20min to 6min. (a) 20min (b) 15min (c) 8min (e) 6min. Scale bars (a2) 15μm (a3) 2.5μm (b1) 15μm (b3) 1μm (c1) 10μm (c2) 2.5μm (c3) 500nm (d1) 2μm. .................................................................................. 153 Figure 7-5 Representative transmission signal and corresponding Q factors, the red and green curves are Lorentzian fits (a) Corresponds to sample d in Figure 7-4 (b) Corresponds to sample e in Figure 7-4............................................................. 154 xvi Figure 7-6 Three representative silica microdisks with ZnO nanowires grown on them (top images) and the structures formed after reflowing them with CO 2 laser (bottom images). ..................................................................................................... 155 Figure 7-7 Quality factor of the microtoroid in Figure 7-6b. ......................................... 156 Figure 7-8 Pure ZnO sol-gel sample fabricated using approach A (a) and (b) scanning electron microscope (SEM) images (c) Optical image (d) Transmission signal showing Q factor of 7.92x10 5 (the red curve is the Lorentzian fit). ....................... 157 Figure 7-9 Pure ZnO sol-gel sample B (a) transmission signal showing Q factor of 7.96x10 7 (the red and green curves are the Lorentzian fits) (b) optical image. ...... 158 Figure 7-10 ZnO:silica sol-gel sample fabricated trough approach C (a), (b) Scanning electron microscope (SEM) images (c) Optical image (d) FTIR spectra. .............. 159 Figure 7-11 ZnO:silica sol-gel sample fabricated through approach D1 (a) Optical image (b) Transmission signal showing Q factor of 8.25x10 5 (red curve is the Lorentzian fit). ........................................................................................................ 160 Figure 7-12 ZnO:silica sol-gel sample fabricated through approach D2 (a), (b) Optical images of two microtoroids. ................................................................................... 160 Figure 7-13 (a) A representative signal showing the laser peak in the middle and the ghost peaks on both sides of the parent laser peak (b) Demonstration of the movement of the ghost peak on the left side of the laser peak as the laser is scanned over a range of wavelengths (c) Demonstration of the movement of the ghost peak on the right side of the laser peak as the laser is scanned over a range of wavelengths. ....................................................................................................... 162 Figure 7-14 A representative signal of spectrograph showing peaks that appear only when light is on resonance with ultra-high quality factors and at certain couplings (this graph is from a silica microtoroid with Q factor of 10 8 pumped with 765-781nm laser). ........................................................................................... 163 Figure A-1 The principle of PS-CRDS technique: the intensity of the continuous wave laser source is modulated which results in phase shift in the output light from the cavity as well as decrease in the modulation depth [10]. ........................................ 192 Figure A-2 PS-CRDS experimental set up where FG: function generator, CW: continuous wave [8]. ............................................................................................... 197 xvii Figure A-3 (a) Transmission signal through the tapered optical fiber coupling light into the silica microtoroid while light is on resonance (FSP: forward scanning peak, BSP: backward scanning peak) (b) Phase shift experienced by the sinusoid coming out of the microcavity when light is on resonance [8]. .............................. 198 Figure A-4 (a) Changes in the phase shift as a function of time (b) Zoom in view of the graph in part (a) [8]. .......................................................................................... 199 Figure A-5 (a) Changes in the quality factor as a function of time during the detection process (b) resonant wavelength shift as a function of time during the detection process [8]. .............................................................................................................. 201 Figure A-6 A representative measurement of the error signal, where the taper is moved away from the microcavity: Mean: ±0.2662 o , Variance: 0.1076 o , mode: ±0.3387 o , Signalnoisemin=16, andSignalnoisemax=38 [8]. .................................. 202 Figure B-1 A sample transmission signal while light is on resonance in the device immersed in HEPES buffer. The red curve is the Lorentzian fit to the black signal. The quality factor is 3.27x10 6 which is calculated based on linewidth measurements [3]. ................................................................................................... 207 Figure B-2 resonant wavelength shift as a function of time while mixture of free streptavidin and streptavidin-labeled polystyrene beads was injected [3]. ............. 208 Figure B-3 Calculation of dissociation constant for K d1 (top graph) and for K d2 (bottom graph) [3]. .................................................................................................. 209 Figure B-4 Fluorescence intensity versus reacting biotin solution concentration where each point represents three wafers. ......................................................................... 212 Figure C-1 Fourier Transform Infrared (FTIR) spectra of thermally grown silica, pure sol-gel silica and different Tm +3 and Ce +3 co-doped sol-gel silica. ........................ 215 Figure C-2 Transmission spectra for various co-doped silica sol-gel microtoroids. The red curve is the Lorentzian fit. ................................................................................ 216 Figure C-3 A representative transmission spectra for one of the co-doped silica microtoroids. ........................................................................................................... 217 Figure C-4 A representative transmission of 0.05at.% Ce doped silica microtoroid. .... 218 Figure D-1 Scanning electron microscope (SEM) Images of hybrid titania-silica fabricated micropads and microdisks [1]. ............................................................... 223 xviii Figure D-2 Top view and side view images of a hybrid microdisk (~80μm) next to a tapered optical fiber [1]. .......................................................................................... 224 Figure D-3 Spectra of transmission through the tapered optical fiber coupling light into the hybrid microdisk (device scan) and the background transmission (taper scan) [1]. ................................................................................................................. 225 Figure D-4 Scanning electron micrographs of hybrid titania-silica microdisks after shining a CO 2 laser on them [1]. ............................................................................. 226 xix Abstract Ever since silica microtoroid optical resonators were developed in 2003, a significant amount of research has focused on utilizing their unique properties in various applications, ranging from fundamental physics research to chemical and biological detection. However, many of these applications require further design improvements for their optimum performance to be fully realized. Additionally, as the technology evolves, new applications are discovered. The focus of this thesis is to outline the development of such optimized microtoroids and their technological role. While biological detection has been the focal point of optical resonant cavity sensing, monitoring the environment can also benefit from these unique microsensors. Slight variations in environmental factors such as temperature, pressure, humidity and electromagnetic radiations such as ultraviolet, can significantly alter our lives. In addition, these factors play major roles in various industrial processes. Considering the rapid increase in the complexity of these processes, sensors that monitor the environment quickly, reliably, and sensitively will become more important. In this thesis, it is first shown that silica microtoroids provide an exceptional platform for monitoring different levels of ultraviolet radiation (UV) and relative humidity (RH) in the environment. The second part of this thesis discusses the possibility of utilizing silica microtoroids in creating various types of microlasers, which will improve the sensing performance metrics of silica microtoroids for environmental monitoring and biodetection. 1 Chapter 1 Introduction 1.1 Motivation Dielectric whispering gallery mode (WGM) optical microresonators are a unique group of optical resonators with interesting applications in various basic and applied research venues including biological and chemical detection [1-4], microlasers [5-8], and fundamental physics studies such as cavity quantum electrodynamics (CQED) [9-12]. Among the different materials and geometries of WGM microresonators, silica microtoroids, first developed in 2003 [13], offer superior properties. These include ultra- high quality factors, planar structure, and a silicon compatible fabrication process. This unique combination of properties has opened up doors towards the realization of compact, on-chip, highly sensitive biosensors that are capable of label-free biodetection. On the other hand, the excellent blend of properties that silica microtoroids offer can benefit other types of sensors, namely environmental monitoring sensors. There are various environmental factors such as temperature, pressure, relative humidity, and electromagnetic radiation (e.g. ultraviolet) that require constant monitoring due to their effect on our well-being. For example, a combination of high temperature and low relative humidity can contribute to catastrophic wildfires. More directly, over- exposure to ultraviolet radiation is linked to skin cancer. In addition, these factors require continuous control in industrial processes to ensure safety and desired yields. As a result, there is a strong demand for fast, highly sensitive, compact and robust sensors for environmental monitoring. 2 This thesis starts by presenting the development of bare silica microtoroids for monitoring different levels of ultraviolet radiation accompanied by theoretical models that can predict the response of the sensor. This sensor offers unique properties such as a linear response and low hysteresis. Monitoring relative humidity is another application that is investigated in this thesis. In this case, an organic/inorganic hybrid system is developed to improve the sensitivity of silica microtoroids in the detection of relative humidity. The second part of this thesis is focused on the application of silica microtoroids in the development of microlasers to improve the sensitivity of detection. A blue emitting microlaser is demonstrated based on doping the silica microtoroids with thulium ions. Here, upconversion provides a straight-forward route for converting the readily available near infrared (IR) lasers to the visible or even ultraviolet wavelength range. 1.2 Chapter overview The organization of this thesis is as follows: Chapter 2 includes an overview of the background information regarding the basics of operation of optical resonators. Whispering gallery mode optical resonators are specifically discussed along with their fundamental properties such as mode volume and quality factor. In addition, various geometries of these resonators such as microspheres and microtoroids are presented accompanied by the detailed fabrication process of silica microtoroids. Evanescent field coupling using tapered optical fibers and their fabrication process is explained. The chapter concludes with an explanation of the testing procedure, device characterization and detection based on silica microtoroids. 3 Chapter 3 is about the detection of ultraviolet (UV) radiation using bare silica microtoroids [14]. First, the importance of monitoring levels of UV exposure in the environment is explained. In addition, various types of UV sensors that have been developed in the literature are reviewed. Next, the mechanism of UV detection based on silica microtoroids has been analyzed and a theoretical model is developed. The experimental measurements demonstrate various advantages including a high signal-to- noise ratio, low hysteresis, and linear response. In addition, there is a good agreement between the theoretical model and the experimental results. Chapter 4 is outlines environmental monitoring using a relative humidity (RH) sensor [15]. This chapter includes the fabrication process required for coating nanometer thick films of a hydrophilic polymer named poly(N-isopropylacrylamide) on the surface of silica microtoroids. The technique applied here is initiated chemical vapor deposition (iCVD), which had not previously been used on silica microtoroids. Experimentally it was confirmed an efficient way to coat microtoroids uniformly with thin polymer films. During the validation of the process, the effect of polymer thickness on the sensor performance was investigated and is discussed in detail. The methods and measurements applied to the developed hybrid sensor reveal nearly two orders of magnitude improvement in RH detection using silica microtoroids. In Chapter 5, the development of a blue emitting microlaser based on upconversion of thulium atoms within silica microtoroid is described [16]. The fundamentals of upconversion and its advantages over other non-linear techniques have been discussed. The sol-gel technique used for integration of thulium dopants into silica network is explained in detail. Using this approach, silica microtoroids with various 4 dopant concentrations were developed. Finally, the experimental set up, quality factor, lasing spectra and threshold are measured. Here, the ultra-high quality factors in silica microtoroids are the main source of low threshold lasing in these microlasers. Chapter 6 includes information about nanocrystal quantum dots and their interesting properties, which have motivated their integration with optical resonators. However, these attempts do not allow for a reconfigurable microlaser, which imposes a limitation to these systems. Therefore, a novel approach based on bioconjugation strategies was developed to create a reconfigurable system. This technique allows for the surface of a silica microtoroid to be reversibly configured with various types of commercially available quantum dots based on the high affinity of streptavidin and biotin biomolecules [17]. In Chapter 7, the integration of ZnO and silica microtoroids is investigated. ZnO is a wide bandgap semiconductor with the possibility of ultraviolet lasing at room temperature. Two different approaches were pursued and analyzed for the integration of ZnO and silica microtoroids, (1) growing ZnO nanowires and (2) spin-coating ZnO thin films on silica microtoroids. Appendix A presents the results of a research project focused on biodetection using a phase shift cavity ring-down measurement [18]. It is known that microcavity detection based on tracking the resonant wavelength is very sensitive to the intensity fluctuations of the laser source. These sources of noise can be removed by application of cavity ring-down spectroscopy (CRDS) technique. Therefore, in this project, the testing setup is modified to accommodate the requirement of CRDS experiments. In addition, the 5 surface of the silica microtoroids is functionalized with biotin using a reversible and non- destructive bioconjugation technique. Biodetection of streptavidin is then performed. The application of this technique is the ability to detect a wider variety of biological molecules. In Appendix B, a method of improving the specificity of label free biodetection using silica microtoroids is discussed [19]. The surface of the silica microtoroids is functionalized with biotin using a non-destructive bioconjugation method. Analytes of interest are free streptavidin and polystyrene beads functionalized with streptavidin. A combination of resonant wavelength shift tracking and analysis of the kinetics of dissociation allows for the identification of the individual species in the mixed solution. In the second part of Appendix B presents the preliminary work done to modifying the density of binding sites on the surface of silica microtoroids. Following the successful demonstration of functionalizing surface of silica microtoroids with biotin [17], a secondary blocker agent is added to tune the number of these binding sites. Appendix C includes the results of building an ultraviolet upconversion laser based on upconversion of 1064nm laser radiation in a co-doped silica microtoroid. Cerium is a rare earth dopant that has shown UV upconversion in YAP (YAlO 3 ) crystal once it is pumped at ~800nm. On the other hand, upconversion of 1064nm laser radiation in thulium doped silica microtoroids generates ~800nm emission [16]. Therefore, the focus of this section is to investigate the possibility of energy transfer between thulium and cerium dopants in silica matrix. 6 In Appendix D, preliminary studies on fabrication and characterization of hybrid titanium dioxide (titania)-silica microdisks and microtoroids are discussed. Similar to silica, titania is transparent over a large wavelength range of visible and near infrared. Interestingly, titania has higher refractive index compared to silica and based on previous studies, adding titania into silica sol-gel and coating a thin film on silica microtoroids can tune the behavior of light within the device [20]. Therefore, it is interesting to study microresonators that are completely made out of hybrid titania and silica sol-gel, especially when the major component is titania. 7 Chapter 1 References 1. Armani, A.M., et al., Label-free, single-molecule detection with optical microcavities. science, 2007. 317(5839): p. 783-787. 2. Armani, A.M. and K.J. Vahala, Heavy water detection using ultra-high-Q microcavities. Optics Letters, 2006. 31(12): p. 1896-1898. 3. Hunt, H.K. and A.M. Armani, Label-free biological and chemical sensors. Nanoscale, 2010. 2(9): p. 1544-1559. 4. He, L., et al., Detecting single viruses and nanoparticles using whispering gallery microlasers. Nature nanotechnology, 2011. 6(7): p. 428-432. 5. Hsu, H.-S., C. Cai, and A.M. Armani, Ultra-low-threshold Er: Yb sol-gel microlaser on silicon. Optics Express, 2009. 17(25): p. 23265-23271. 6. Yang, L. and K.J. Vahala, Gain functionalization of silica microresonators. Opt Lett, 2003. 28(8): p. 592-4. 7. Maker, A.J. and A.M. Armani, Nanowatt threshold, alumina sensitized neodymium laser integrated on silicon. Optics express, 2013. 21(22): p. 27238-27245. 8. Chistiakova, M.V. and A.M. Armani, Cascaded Raman microlaser in air and buffer. Optics Letters, 2012. 37(19): p. 4068-4070. 9. Mabuchi, H. and H. Kimble, Atom galleries for whispering atoms: binding atoms in stable orbits around an optical resonator. Optics letters, 1994. 19(10): p. 749-751. 10. Lin, H., et al., Cavity-modified spontaneous-emission rates in liquid microdroplets. Physical Review A, 1992. 45(9): p. 6756. 11. Vernooy, D. and H. Kimble, Quantum structure and dynamics for atom galleries. Physical Review A, 1997. 55(2): p. 1239. 12. Ilchenko, V.S. and A.B. Matsko, Optical resonators with whispering-gallery modes-part II: applications. IEEE Journal of Selected Topics in Quantum Electronics, 2006. 12(1): p. 15- 32. 13. Armani, D., et al., Ultra-high-Q toroid microcavity on a chip. Nature, 2003. 421(6926): p. 925-928. 14. Harker, A., S. Mehrabani, and A.M. Armani, Ultraviolet light detection using an optical microcavity. Optics letters, 2013. 38(17): p. 3422-3425. 15. Mehrabani, S., et al., Hybrid microcavity humidity sensor. Applied Physics Letters, 2013. 102: p. 241101. 16. Mehrabani, S. and A.M. Armani, Blue upconversion laser based on thulium-doped silica m icrocavity. Optics letters, 2013. 38(21): p. 4346-4349. 8 17. Hunt, H.K., C. Soteropulos, and A.M. Armani, Bioconjugation strategies for microtoroidal optical resonators. Sensors, 2010. 10(10): p. 9317-9336. 18. Cheema, M.I., et al., Simultaneous measurement of quality factor and wavelength shift by phase shift microcavity ring down spectroscopy. Optics express, 2012. 20(8): p. 9090-9098. 19. Shi, C., S. Mehrabani, and A. Armani, Leveraging bimodal kinetics to improve detection specificity. Optics Letters, 2012. 37(10): p. 1643-1645. 20. Maker, A.J., B.A. Rose, and A.M. Armani, Tailoring the behavior of optical microcavities with high refractive index sol-gel coatings. Opt Lett, 2012. 37(14): p. 2844-6. 9 Chapter 2 Background 2.1 Light Light is an electromagnetic (EM) wave with time varying magnetic and electric fields that are propagating perpendicular to each other and to the propagation direction. Based on Faraday’s law, a time varying electric field generates a time varying magnetic field with the same frequency, orthogonal to the electric field. Therefore, the two fields are related where the electric field component is usually used to describe the EM wave when light is traveling within a non-conducting material [1]. The speed of light in vacuum is expressed through the following equation: s m 1.1 8.7 299,792,45 ε μ 1 C o o o ± = = 2-1 where m henry 10 4π μ 7 o − × = is the magnetic permittivity of free space and m farad 10 8.854 ε 12 o − × = is the electric permittivity of free space [2]. Based on Maxwell’s electromagnetic wave equation, the electric field in a dielectric medium should satisfy the following equation: 2 2 o r o 2 2 2 2 2 2 t E μ ε ε z E y E x E ∂ ∂ = ∂ ∂ + ∂ ∂ + ∂ ∂ 2-2 E is the electric field, x, y, and z are the Cartesian coordinates, t is the time, o ε is absolute permittivity, r ε is relative permittivity, and o μ is absolute permeability. This 10 equation is valid for an isotropic and linear material in which the relative permittivity is the same in all directions and independent of the electric field [1]. Because the present thesis is focused on dielectrics, the background will be limited to this type of material. 2.2 Refractive index When an electromagnetic wave is propagating in a dielectric medium, the electric field induces molecular dipoles and polarizes the medium with the frequency of the EM wave. The coupling of the EM electric field and the dipoles decreases the speed of the propagation of light compared to its speed in the vacuum [1]. Refractive index or index of refraction (n) of a medium is defined as the ratio of the speed of light in vacuum to the speed of light in that medium. Based on the dependence of the refractive index on direction materials can be classified into the following two groups [1]: 1. Isotropic: refractive index does not depend on the direction of propagation. e.g., non-crystalline materials such as glasses 2. Anisotropic: refractive index depends on the direction of propagation. e.g., non-cubic crystalline materials It is important to note that the frequency of light is independent of the medium in which it is propagating. Wavelength of light, on the other hand, depends on the speed of light and therefore on the refractive index based on the following equation: n λ C C λ λ o o m o m = = 2-3 11 where λ m is the wavelength in the medium, λ o is the wavelength in vacuum, C m is the speed of light in the medium, C o is the speed of light in the vacuum and n is the refractive index [1]. 2.3 Total internal reflection Snell’s law describes the behavior of light at the interface of two dielectric media with different refractive indices (Figure 2-1). Assume light is traveling from the medium with higher refractive index (n 1 ) to the medium with lower refractive index (n 2 ) with an incident angle of (θ I ). When light reaches the boundary of the two media, it will undergo reflection and refraction, where the reflection angle (θ R ) is equal to the incident angle, and the refraction angle (θ T ) is calculated using Snell’s law as follows [3]: n 1 Sin θ I = n 2 Sin θ T 2-4 As the incident angle (θ I ) increases, the refraction angle (θ T ) increases until it reaches 90 degrees where the refracted beam is parallel to the boundary of the two media. The incident angle, in this case, is called the critical angle (θ c ) [3]: θ c =arcsin (n 2 /n 1 ) 2-5 If the incident angle is larger than critical angle (θ I > θ c ), the boundary of the two media will act as a mirror and the light will be reflected back to the medium with higher refractive index. This is called total internal reflection (TIR) [3]. 12 Figure 2-1 Demonstration of Snell’s law (a) Incident angle is smaller than the critical angle (b) Critical angle (c) Total internal reflection. At the boundary of these two media, there are two boundary conditions that must be satisfied [1]: 1. Continuity of the electric field that is tangential to the boundary 2. Continuity of the magnetic field that is tangential to the boundary In the case of TIR, based on these boundary conditions, there must be an electric field in the lower refractive index medium. This wave is called an evanescent wave, and it propagates along the boundary. Its amplitude decays exponentially as it moves into the lower refractive index medium [1]. 2.4 Optical fiber Optical fibers confine light based on the concept of total internal reflection. Optical fibers are made of two main sections where the middle part is the core and the outer part is the cladding (Figure 2-2). The refractive index of the core is slightly larger than the cladding; therefore, as long as the incident angle of light is larger than the critical angle, light will be reflected back to the core. Optical fibers are usually made of glass (SiO 2 ) 13 where the central part is doped to increase the refractive index slightly. For fibers operating in the near-IR, Germanium (Ge) is the typical core dopant, and the refractive index difference between the core and the cladding is as small as 0.01 which corresponds to the critical angle of 6.6 degrees [4]. Figure 2-2 Schematic of an optical fiber where total internal reflection is responsible for trapping light within the core of the fiber. Optical fibers can be classified into single mode and multimode optical fibers. This classification is based on the number of possible paths of light in the fiber. In a multimode fiber, one ray of light can travel along the central line of the core and another ray could travel along the periphery. This is due to total internal reflection from the boundary of the core and the cladding. In a single mode fiber, the size of the core is decreased such that only one path is allowed. Multimode fibers are used in short distance applications where high power is required. On the other hand, single mode fibers provide lower loss and are used in long distance applications [4]. 14 2.5 Optical resonators Optical resonators or optical cavities are devices in which photons can be confined for a specific amount of time [5]. These devices are divided into two major categories: standing wave resonators and traveling wave resonators. In a standing wave resonator, light waves will propagate back and forth between the walls of the cavity. Fabry-Perot cavities are an example of this type of resonator and are two flat mirrors facing each other [1]. On the other hand, photons in traveling wave resonators will propagate in a uni- directional manner inside the cavity where they retrace their path without reversing their direction. Whispering gallery mode optical microresonators are a typical example of these resonators [5]. 2.6 Whispering gallery mode optical microresonators In order to better understand different properties of whispering gallery mode microresonators, it is helpful to look into the basic definition of an optical mode. When light is confined in an optical resonator, it will reflect multiple times and as a result of the interference phenomena, certain frequencies of radiation are being suppressed by destructive interference and others are sustained due to constructive interference. These interactions will generate stable radiation patterns called Eigenmodes, or modes of the resonator. In other words, a mode is a wave that reproduces itself after a single round trip [5]. Mathematically, the possible solutions of the Helmholtz equation for waves are the modes of the system. The Helmholtz equation is obtained by combining Maxwell’s equation and the boundary conditions imposed on the system. 15 The name “whispering gallery” is inspired by the sound effect noticed in some buildings such as in St. Paul’s Cathedral in London (Figure 2-3), where sound waves travel close to the circumference of the building in such a way that whispers can be heard clearly in different parts of the building [6-8]. Figure 2-3 (a) The dome of St. Paul’s Cathedral [9] (b) The whispering gallery at St. Paul’s Cathedral [10] (c) The snapshots of sound waves resonating in a cylinder of air of the same size as the whispering gallery at St. Paul’s Cathedral. Red and blue show the higher and lower air pressures, respectively [6]. 16 In a whispering gallery mode (WGM) optical resonator, light propagates inside a circular path confined within the periphery of the resonator via total internal reflection. Only certain wavelengths of light called resonant wavelengths can stay on-resonance depending on the material and geometry of the cavity as well as the material of the surrounding environment [11, 12]. Figure 2-4 represents on-resonance and off-resonance conditions. In the case of on-resonance, the total round trip of light (optical path length) equals an integral number of the wavelength of the circulating light, so it constructively interferes with the light entering the device. In the off-resonance case, the total round trip is not an integral number of wavelengths; therefore, the light is not in phase with the entering light, resulting in non-continuous circulation. Figure 2-4 Schematic of on-resonance and off-resonance conditions. 17 The optical path length is defined as the refractive index of the medium multiplied by the geometric length that light travels. When the radius of the WGM resonator is much larger than the wavelength of light and incident angle is close to 90 degrees, we can approximate the geometrical length with the circumference of the resonator. This is mathematically expressed as follows [5, 13]: Mλ n R 2π = 2-6 where R is the radius of the resonator, λ is the wavelength of light in vacuum, n is the refractive index of the resonator and M is an integer number (interference order). 2.7 Quality factor Quality factor (Q) is used to characterize the photon lifetime (τ) in resonators, and it is a measurement of temporal confinement of light. Higher values of quality factor correspond to longer circulation of photons inside a cavity [14]. For example, a quality factor of 10 8 at 1550nm corresponds to a photon lifetime of ~80ns [15]. The origin of this expression goes back to the radio engineering early days where it described the performance of an oscillating circuit. A narrow band pass and sharply tuned radio required a high Q and low loss circuit [16]. The relation between quality factor and photon lifetime is expressed as follows [14]: Q = ωτ = 2πcτ/λ 2-7 where ω is the optical frequency of circulating photons, c is the speed of light in vacuum and λ is the wavelength of the circulating photons in vacuum. 18 In dielectric cavities, different loss mechanisms that affect the circulation time of photons in a whispering gallery mode optical resonator are described by the following equation: 1 coup 1 cont 1 rad 1 ss 1 mat 1 ext 1 int 1 measured 1 loaded 1 tot Q Q Q Q Q Q Q Q Q Q − − − − − − − − − − + + + + = + = = = 2-8 where 1 mat Q − is the material loss (absorption loss or material attenuation), 1 ss Q − is the surface scattering loss, 1 rad Q − is the radiation (curvature, bending or whispering gallery) loss, 1 cont Q − is the contamination loss, and 1 coup Q − is the coupling loss. Combination of material loss, surface scattering loss, radiation loss, and contamination loss are intrinsic to the cavity and are collectively called intrinsic quality factor ( 1 int Q − ) or cold cavity quality factor. Coupling loss is an extrinsic loss ( 1 ext Q − ) and it is related to the light coupling back into the coupler after a round trip [17-20]. Q mat depends on the material from which the cavity is made as well as the environment surrounding the cavity. Q mat can be described by the following equation: eff eff mat λα 2π Q n = 2-9 where n eff is the effective refractive index, λ is the wavelength of light, and α eff is the effective absorption coefficient which is also called effective material attenuation or effective material loss. The n eff of the system is calculated by adding up the product of refractive index and the portion of optical field present in each part of the system, where the system is the 19 cavity and its surrounding environment. For instance, if we have a silica cavity in the air, the effective refractive index is calculated by the following equation: n eff = βn silica + δn air 2-10 β+δ=1 2-11 where β is the portion of optical field in the silica cavity, δ is the portion of optical field in air, n silica is the refractive index of silica and n air is the refractive index of air. The α eff of the system is calculated by adding up the product of material attenuation and the portion of optical field present in each part of the system, where the system is the cavity and its surrounding environment. For example, if we have a silica cavity in the air, the effective material loss is calculated by the following equations: α eff = βα silica + δα air 2-12 β+δ=1 2-13 where β is the portion of optical field in the silica cavity, δ is the portion of optical field in the air, α silica is the absorption coefficient of the silica and α air is the absorption coefficient of the air. The Q ss of the system depends on the scattering of light from the cavity. It is a combination of two scattering losses, surface scattering (Q ss ) and internal scattering (Q is ) which are expressed with the following equations: 20 2 2 2 eff 3 ss σ B π 8n R 3λ 1 K K Q + = 2-14 7 eff T 2 2 3 is n Tβ p 4π K 3λ Q κ = 2-15 where K defines the internal reflection condition, λ is the wavelength of light, R is the radius of the cavity, n eff is the effective refractive index, p is the Pocklet coefficient, κ is the Boltzman constant, T is the melting temperature, β T is the iso-thermic compressibility, σ and B are the rms (root mean square) size and the correlation length of surface inhomogeneities, respectively. Q rad is the radiation loss due to the curvature of the cavity. It has been shown that 1 rad Q − vanishes exponentially with increasing the microcavity size. The minimum diameter to eliminate radiation loss is defined by the material of the cavity and the surrounding environment. If the device diameter is smaller than this minimum value, the incident angle is smaller than the critical angle, and total internal reflection will no longer occur which will cause light to escape the cavity due to refraction. Therefore, appropriate choice of the cavity diameter can eliminate radiation loss [20-22]. 1 cont Q − is due to the contamination on the surface of the cavity and any contamination introduced into the cavity while fabricating the device. Moreover, the presence of water molecules will contribute to the contamination losses. Therefore, by keeping the fabrication process clean and by storing the device in a clean and dry container, this source of loss can be minimized. 21 2.8 Free spectral range Free spectral range or FSR of a resonator is the distance between sequential longitudinal modes of the resonator, and it is approximated using the following equation [23]: nR 2π λ FSR 2 ≈ 2-16 where λ is the resonant wavelength, n is the refractive index and R is the radius of the resonator. Within one FSR there is a resonant wavelength that corresponds to each accessible mode inside the resonator. 2.9 Mode volume Mode volume is defined as the volume that an optical mode occupies in an optical resonator and it is a measurement of the spatial confinement of light. There are different definitions of mode volume. In relation to the energy density of the optical mode, it is defined as the equivalent volume that the mode occupies assuming the homogenous distribution of optical energy at its maximum value: 2 max V 3 2 m E r d E ) r ε( V Q ∫ = 2-17 22 where V Q is the integration volume, ) r ε( is the refractive index at r squared , 2 E is the strength of the electric field, and max E is the maximum strength of the electric field [8, 22]. A combination of high temporal confinement (high Q factor) and high spatial confinement (low mode volume) provides a tightly confined optical energy which can provide an exceptional platform for various applications [7, 18, 22, 24-29]. 2.10 Different geometries of whispering gallery mode optical resonators Whispering gallery mode microresonators can be fabricated from different materials in various shapes such as microdisks, microspheres and microtoroids (Figure 2-5). Figure 2-5 Different geometries of silica whispering gallery mode optical resonators (a) Scanning electron microscope (SEM) image of a silica microdisk (b) Scanning electron microscope (SEM) image of a microtoroid (c) Widefield microscope image of a microsphere. 23 Silica microdisks suffer from scattering losses due to the roughness on the edge of the device. In the case of silica microspheres and microtoroids, the loss due to surface scattering is negligible since the laser reflow during the device fabrication process results in near atomically smooth surfaces [5]. Therefore, once the size of the device is large enough to avoid the radiation losses, and the contamination loss is minimized by keeping the device clean and dry, the intrinsic quality factor of silica microtoroids and microspheres are only material loss limited. Silica microspheres have shown Q factors as high as 1x10 9 , whereas silica microtoroids have Q factors on the order of 10 8 . Studies have shown that slightly the lower Q factor in silica microtoroids is related to the fact that microtoroids are fabricated from thermally grow silica on silicon substrates. The silicon substrate is doped with boron which diffuses into the oxide layer during the thermal growth process. As a result, thermally grown silica has higher material absorption as compared to fused silica from which microspheres are made [19]. Microspheres are fabricated by melting a piece of optical fiber, and as a result, they cannot be integrated on silicon substrate. Microtoroids, on the other hand, are fabricated on a silicon chip using complementary metal-oxide-semiconductor (CMOS) technology. Therefore, silica microtoroids can be potentially integrated with other electronic and optical components on the same silicon chip for applications that require a compact platform such as in sensors. Comparing the mode volume of silica microspheres and microtoroids shows that microtoroids provide significantly smaller mode volumes which results in higher energy densities [22]. Based on numerical calculations, 24 microtoroids confine whispering gallery modes in three times smaller effective area than microspheres [22, 30]. 2.11 Silica microtoroid fabrication Silica microtoroids are fabricated using 2µm thick thermally grown silica on silicon substrate (WRS Materials) using a well-established process [23, 31] which is summarized in Figure 2-6. To minimize the contamination losses ( 1 cont Q − ), all the fabrication steps are performed in a cleanroom facility (class 10/100), except the final CO 2 reflow step. First, the surface of the silica is cleaned with acetone, methanol, isopropyl alcohol (IPA) and blow-dried with dry nitrogen. The sample is placed on a 120°C hotplate for two minutes to dehydrate the surface. 25 Figure 2-6 Silica microtoroid fabrication process (a) Two micron thick thermally grown silica on a silicon wafer (b) Photoresist is spin-coated on top of the silica (c) Sample is exposed to UV radiation through a photomask (d) The UV-exposed areas of photoresist is washed off in the developer solution (e) Buffered oxide etchant (BOE) etches the uncovered areas of silica (f) Photoresist is washed off with acetone (g) XeF 2 gas undercuts the silica, forming microdisks (h) Microdisks are reflowed with a CO 2 laser, forming the microtoroids. During the priming step a few drops of hexamethyldisilazane (HMDS) (Aldrich 99%) are added to a small beaker. The beaker and the sample are then placed under a larger inverted beaker for two minutes (Figure 2-7). This process deposits a layer of HMDS molecules on the silica surface. HMDS is an adhesion promoter between silica and the photoresist. Photoresist is hydrophobic and silica is hydrophilic which results in discontinuous coverage of photoresist on the surface of silica (dewet). HMDS is a silane 26 which reacts with silanol molecules on the dehydrated silica surface to substitute hydrogen with a silicon atom through the following reaction [32]: 2SiOH + [(CH 3 ) 3 Si] 2 NH → 2SiOSi(CH 3 ) 3 +NH 3 2-18 Figure 2-7 Picture of HMDS deposition set up. S1813 photoresist (Shipley) is then spin-coated on the surface of the sample for five seconds at 500rpm and 45s at 3000rpm. The sample is then soft-baked (pre-baked) at 95°C for two minutes as the remaining solvent evaporates and photoresist hardens. [32]. 27 Next, the sample is exposed to ultraviolet (UV) radiation through a photomask (Figure 2-8) using a Karl Suss MJB3 photomask aligner (Figure 2-9). The mask contains arrays of circular pads of desired sizes. The exposure dose of the UV radiation is 80mJ/cm 2 , selected based on the photoresist datasheet. Figure 2-8 Picture of a photomask with rows of various sizes of disks. 28 Figure 2-9 Karl Suss MJB 3 photomask aligner. After UV exposure, the sample is immersed in MF-321 developer (Shipley) solution. Since S1813 photoresist is a positive resist, UV radiation degrades the exposed areas and they are subsequently washed with the developer. The sample is then rinsed with deionized (DI) water and blow dried with nitrogen. Once circular pads of photoresist are confirmed to be uniform by optical microscopy, the samples are hard-baked (post-baked) at 110°C for two minutes. This process hardens the photoresist and restores any weakened adhesion between the photoresist and the wafer that may have occurred due to the penetration of developer [32]. 29 The photoresist pads are used as mask for patterning silica pads on silicon substrate. The sample is immersed in solution of buffered oxide etchant (BOE) (Transene). BOE is hydrofluoric acid (HF) (a selective isotropic etchant for silica) mixed with a buffering agent (typically ammonium fluoride (NH 4 F)). The overall reaction is outlined in the following reaction [33]: SiO 2 + 6HF→H 2 + SiF 6 + 2H 2 O 2-19 As the reaction goes on, HF is consumed and therefore the rate of reaction decreases with time. To overcome this problem, the buffering agent is added to maintain the concentration of etching ions at a constant level to control the etching process. The dissociation of ammonium fluoride is as follows [33]: NH 4 F ↔ HF + NH 3 2-20 The sample is then rinsed with DI water and blow dried with nitrogen. Next, the uniformity of the etched silica micropads on silicon substrate is checked using optical microscopy. It is important to note that there is an inclination on the sidewalls of the micropads. This is caused during the isotropic etch of BOE where silica is etched both in vertical direction and in horizontal direction underneath the photoresist. The inclination angle depends on the adhesion between the photoresist and the substrate. The sample is then cut into smaller pieces each containing eight to ten microdisk pads. They are rinsed with acetone, methanol, and isopropyl alcohol to remove the photoresist and blow dried with nitrogen. 30 Next, the cut and cleaned samples are placed in the xenon difluoride (XeF 2 ) etcher unit (Figure 2-10). XeF 2 is an isotropic etchant that will selectively etch silicon through the following reaction [34-36]: 2XeF 2 + Si → 2Xe (g) + SiF 4 (g) 2-21 31 Figure 2-10 XeF 2 etching unit (a) Front view (b) Side view. XeF 2 is a white crystalline solid at room temperature; however, it will sublimate once the pressure is lower than its vapor pressure at room temperature (~4.5mTorr) [37]. Once samples are in the etching chamber, the unit is purged with nitrogen gas to remove 32 any water molecules present in the system. This controls for the fact that XeF 2 can react with water to form HF, which will etch silica pads and cause defects in the fabricated devices. After purging the system with nitrogen, the samples are exposed to the XeF 2 vapor at 2800mTorr through a cyclic (pulsed) process. Each pulse is ~80s and the chamber is evacuated between each cycle to remove the byproducts formed during the etching process. The etching unit is equipped with both computer controlled valves and manual valves. A LabVIEW program is used to facilitate the process (Figure 2-11). Figure 2-11 XeF 2 etcher LabVIEW program front panel. 33 The required number of pulses for a desired undercut depends on the size and the number of samples. Once the desired number of etching pulses is finished, the chamber is purged with nitrogen. The samples are then removed and checked under optical microscope to ensure the desired undercut is achieved. The last step in the microtoroid fabrication process is to reflow the silica microdisks into microtoroids using the CO 2 laser set up (Figure 2-12), which is equipped with Synrad CO 2 laser (10.6µm) with the beam intensity profile of approximate Gaussian distribution. Figure 2-12 CO 2 laser reflow set up. First, the center of the laser spot is found using a thin glass slide. Next, a microdisk is centered at the laser spot and surface normal-irradiated. Since silicon has far 34 less absorption of 10.6µm light, CO 2 reflow does not affect the silicon parts of the sample. During the reflow process, silica absorbs the CO 2 laser light, melts down and due to surface tension collapses to a toroidal shape. The final size of the microtoroid is defined by the thickness of silica disk and the pillar size. The silica disk melts along its periphery toward the center because the optical extinction coefficient of silica is strongly temperature dependent around 10.6µm [38, 39], and silicon is 100 times more thermally conductive than silica (thermal conductivity of silicon= 0.309cal/cm·s·C and silica = 0.003cal/cm·s·C [40]). Therefore, the silicon pillar acts as a heat sink for the silica located closer to the pillar. 2.12 Evanescent field coupling In order to experimentally characterize the unique properties of whispering gallery mode microresonators and to implement them in various applications, we couple light into the cavity. There are various methods for coupling light into whispering gallery mode cavities [41], such as using a prism coupler [18], an angle polished fiber tip [42], and a polished half block coupler [43]. However, these methods are either bulky or suffer from low coupling efficiency and/or high coupling loss ( 1 coup Q − ). Tapered optical fibers [21, 44-47] provide an alternative route to couple light into whispering gallery mode microresonators. In an optical fiber, the refractive index difference between the cladding and the core allow light to be guided through total internal reflection. By adiabatically pulling the optical fiber, it is possible to form a thin region (waist region) where the fiber core cannot fully confine the light inside of the waveguide. In this region, an evanescent field is created because the light is guided due 35 to the refractive index contrast between silica and the environment. This field can be coupled into the microcavity as long as there is a phase-match between the mode of the microcavity and the guided light in the tapered fiber. Advantages of coupling using tapered optical fiber waveguides are [14, 30]: 1- Same material as the microtoroid (silica) 2- Adjustable thickness (radius) to phase match the mode of the microtoroid 3- Low-loss i.e. transmitting light with minimal degradation of power 4- Highly efficient delivery of the optical power to the device with negligible induced losses such as scattering and non-resonant insertion loss 5- Relatively easy-to-align coupler due to the small transverse dimensions The optical tapered fiber is fabricated by first removing about one inch length of the polymer protective coating of the appropriate single mode optical fiber (Newport) and cleaning it with methanol or isopropyl alcohol. The appropriate fiber is selected based on the wavelength of light that is going to be studied. For example, F-SC fiber is used for 980nm. Next, the fiber is placed in the fiber holder, which is positioned on a motorized stage (SIGMA KOKI, Stepping Motor Drive SHOT-102) to pull the fiber from both sides (Figure 2-13). 36 Figure 2-13 Taper puller setup where tapered optical fiber is formed by simultaneous pulling and melting the optical fiber. A hydrogen torch is used to provide the flame for melting the fiber. The flow rate of hydrogen is adjusted using a flowmeter such that the flame has a nearly circular shape with the fiber nearly at the tip of it. The pulling process is started when the flame is lit and is continuously monitored on the camera. The process is stopped once the desired width of the tapered region is achieved. Figure 2-14 shows an optical fiber, before and after the optical fiber is tapered by the process described. 37 Figure 2-14 (a) Optical image of an optical fiber (b) Optical image of a tapered optical fiber. There are three coupling regimes based on the distance between the taper and the resonator. These three regions are: under-coupled, critically coupled and over-coupled. In the under-coupled regime, the coupler is far from the resonator; therefore, there is weak coupling between the resonator and the coupler modes. Coupling losses are smaller in the under-coupled regime. When the coupler and the resonator get closer, the overlap between their modes increases to such a point that all the light is coupling into the device. This point is the critical coupling point and the intrinsic loss of the resonator is equal to the coupling loss. After this point, decreasing the distance between the coupler and the device will result in an increase in the overlap of the modes. This region is called the over-coupled region and coupling loss is the maximum [48]. 38 2.13 Testing setup Figure 2-15 represents a schematic of the testing setup used for measuring different properties of the whispering gallery mode microresonators such as the quality factor. It is also used for investigating new applications of these devices. The microtoroid chip is placed on a stage, which is equipped with manual positioning knobs as well as a three axis fine stage piezo positioning system (SIGMA KOKI, FINE-503) (Figure 2-16b). Two cameras (top and side views) are used to help with positioning the sample and the taper. Figure 2-15 Schematic of the testing setup. Light is coupled from a fiber coupled narrow linewidth (<300kHz) continuous wave tunable laser (Newport, Velocity TLB6700 or TLB6300-LN) into the microresonator using a tapered optical fiber. One end of the fiber is connected to the laser and its other end is connected to a photodetector (Thorlabs) of the correct wavelength 39 range (Figure 2-16a). The photodetector is then connected to an oscilloscope card (National Instruments, NI PCI-5114). The laser is connected through a PCIe GPIB card (National Instruments) to the computer. It is also connected to a function generator card (National Instruments, NI PCI-5402) through a BNC cable. The function generator is used to tune the wavelength of laser with a triangular wave (100Hz and 1V peak-to-peak). LabVIEW programs are used to maintain the desired scan speed of the different wavelengths of light through the cavity to find the resonant wavelength. Figure 2-16 (a) Resonator testing setup (b) Zoomed in view of sample holder stage. 40 2.14 Quality factor measurement The quality factor is the figure of merit for resonators, and it shows the capability of a resonator to store light. Therefore, it is one of the fundamental properties of a resonator which must be to be determined when developing novel applications. Two different methods can be applied to measure the Q: cavity ring-down measurement and linewidth measurement. In the cavity ring-down measurement (Figure 2-17), a single laser pulse is injected into the device and the decay of light out of the cavity is recorded. In this case, quality factor is the product of the resonance frequency by the photon decay time i.e.: Q=ωτ o 2-22 Figure 2-17 Schematic of ring-down measurement, where Q=ωτ o . In the linewidth measurement (Figure 2-18), a narrow range of wavelengths is coupled into the resonator from the tunable laser where the transmission through the coupler is monitored continuously. A dip in the transmission signal represents resonance 41 of light of a certain wavelength in the cavity. The quality factor is then defined as the central wavelength divided by the linewidth of the resonance dip i.e.: Δλ λ Q 0 = 2-23 The linewidth ( Δλ) is evaluated by fitting a Lorentzian curve to the dip and measuring its full width at half maximum (FWHM). This technique is applicable as long as the laser linewidth is narrower than the resonance linewidth. The reason a Lorentzian fit is used is that in order to convert time domain into frequency (wavelength) domain, a Fourier transform is used. The resonance in time domain is an exponential decay and the Fourier transform of an exponential decay is a Lorentzian function. Figure 2-18 Schematic of a resonance dip and linewidth measurement, where Q= λ o / Δ λ In order to find the intrinsic quality factor ( 1 int Q − ) using the linewidth measurement, a series of resonances are recorded with various couplings. The depth of the resonant 42 peak defines the coupling. Quality factors are plotted versus their corresponding coupling values and a linear fit is used to extrapolate the quality factor at zero coupling which is the intrinsic quality factor. Figure 2-19 demonstrates a representative transmission of a microtoroid on resonance along with the intrinsic quality factor measurement. The double peak occurs due to the clockwise and counterclockwise resonance modes [49-51]. All of these measurements are performed with low power coupled into the device to minimize the influence of non-linear effects such as thermal broadening and mechanical vibration on the measurement. The peaks are symmetric which confirms that the device is operating in the linear regime. 43 Figure 2-19 (a) Transmission signal of a silica microtoroid on resonance where the red and green curves represent Lorentzian fits to the resonant peaks (b) Graph of quality factor versus coupling which is used to calculate the intrinsic quality factor based on the y-intercept of the linear fit. 44 2.15 Characterizing the free spectral range A broad scan is used to measure the free spectral range (FSR) of a silica microtoroid. To perform the broad scan, the function generator is turned off and a range of wavelengths is scanned through the device. In Figure 2-20, the broad scan of a silica microtoroid 50μm in major diameter is shown where the fundamental mode resonant peaks are 4.9nm apart. This value is in agreement with the calculated value from the free spectral range equation. Figure 2-20 Broad scan spectra of a silica microtoroid with major diameter of 50μm which has free spectral range of 4.9nm. 45 2.16 Nonlinear effects: thermal broadening and mechanical vibration Due to small mode volume and high photon lifetime in silica microtoroids, nonlinear effects such as thermal broadening and mechanical vibration can occur at moderate input powers. Thermal broadening is a result of the absorption losses where the optical power absorbed by the silica increases the temperature of the microresonator. Due to the thermal expansion and thermo-optic effect, the optical path of the microresonator can change which results in the change in the resonant wavelength [52-55]. Figure 2-21 demonstrates an example of thermal broadening in a silica microtoroid. Figure 2-21 Thermal broadening in a silica microtoroid. 46 Mechanical vibration occurs as a result of radiation pressure induced opto- mechanical oscillation, which is the pressure applied on a surface due to the electromagnetic radiation. Based on Maxwell’s theory, an electromagnetic wave holds momentum which can be transferred to a surface if they come in contact. In the electromagnetic particle model, electromagnetic radiation is quantized in photons. Even though photons do not have mass, they possess energy and momentum. Therefore, when they hit a surface their momentum can change. The radiation pressure of the high circulating optical power inside a microtoroid applies force radially on the microresonator. As a result, the microtoroid undergoes mechanical deformation causing the resonant wavelength to shift [8, 56, 57]. Figure 2-22 demonstrates an example of mechanical vibration in a silica microtoroid. Figure 2-22 Mechanical vibration in a silica microtoroid. 47 2.17 Application of silica microtoroids in sensing The whispering gallery mode resonant wavelength of whispering gallery mode optical resonators depends on their refractive index (n) and radius (R). Any change to the radius or the refractive index, it will alter the resonance condition and as a result it will induce a shift in the resonant wavelength. Therefore, by tracking the shift in the resonant wavelength, detection is possible. These shifts can be calculated mathematically, following Equation 2-6 [30]: n Δn R ΔR λ Δλ + = 2-24 where λ is the resonant wavelength, Δλ is the change in the resonant wavelength, R is the radius of the device, ΔR is the induced change in the radius of the device, n is the refractive index and Δn is the induced change in the refractive index. Due to the ultra-high quality factor of whispering gallery modes in silica microtoroids, photons circulate several thousand times before they exit the device due to loss mechanisms. This creates a long interaction optical path which provides high sensitivity in detection [8, 13]. These devices have been used in biosensing where the presence of biomolecules in the evanescent field around the resonator will induce a shift in the resonant wavelength [58]. In addition to biosensing, whispering gallery mode microresonators can be used as temperature sensors [59-61]. In this case, following Equation 2-24, we calculate [8]: 48 T )d dT dn n 1 dT dR R 1 λ( Δλ + = 2-25 Thermal expansion (ε) is defined as: dT dR R 1 ε = 2-26 Therefore, combining Equation 2-25 and 2-26 will result in the following equation: T ) dT dn n 1 λ(ε Δλ ∆ + = 2-27 where dT dn is the thermo-optic coefficient. 2.18 Tracking the resonant wavelength In order to perform detection using resonant wavelength tracking, the experimental setup described in Section 2.13 can be used. Light is coupled into the microresonator and the resonant wavelength is found. The power coupled into the device is adjusted to make sure the linewidth is not distorted due to non-linear effects such as thermal broadening or mechanical vibration. It should be noted that there are various resonance modes that can be excited. 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Optics letters, 2011. 36(11): p. 2152-2154. 54 Chapter 3 Ultraviolet sensor based on a silica microtoroid 3.1 Introduction Ultraviolet or UV radiation corresponds to the part of the electromagnetic spectrum right below the shortest wavelength that our eyes can detect (violet) (Figure 3-1). Sunlight is the natural source of ultraviolet radiation. There are various diseases that are related to UV over-exposure, the most widespread of them being skin cancer. Oxidative DNA (deoxyribonucleic acid) damages can occur due to UV exposure which can cause DNA mutations and consequently uncontrolled cell proliferation [1, 2]. Figure 3-1 Electromagnetic spectrum [3]. 55 In addition to solar UV radiation, artificial sources of UV such as tanning beds can also be harmful, especially for the working personnel in tanning bed salons [1]. Therefore, it is of great importance to monitor UV exposure in the environment with reliable sensors. In addition, such a sensor can be utilized in a wide variety of applications such as flame detection, water purification, combustion engineering, and pollution engineering [4]. There are various organic and inorganic UV-responsive materials that have been integrated in optical or electrical transducers for UV sensing. For example, different dye- sensitized polymer films have been applied in optical UV sensors where the dye undergoes reversible changes as it is exposed to UV radiation [5-7]. Zinc oxide (ZnO) is an inorganic UV responsive material that has been studied for UV detection. ZnO is a wide band gap semiconductor with photoconductive properties, meaning that when it absorbs UV radiation, its electrical conductivity changes. Therefore, ZnO have been used in electronic UV sensors [8-13]. There are various issues that can make the application of these UV-responsive materials challenging. For example, they degrade over time as a result of exposure to environmental changes. There are also parasitic effects, which can mask the actual response of the sensor. ZnO is an example of such a material because, in addition to having photoconductivity properties, it is a piezoelectric material. For such materials the response of these sensors is hysteretic – they exhibit different behaviors when UV intensity is increasing or decreasing. 56 It is clear that building a reliable UV sensor requires specific material properties that do not limit their application in UV detection. In this work, silica is studied for UV detection since it is robust and does not have parasitic behaviors. Here, silica is used in the form of a microtoroidal optical resonator which provides several other advantages such as high sensitivity, immunity to radio frequency sources of noise (e.g. cell phones), compatibility with semiconductor processing, miniature size, and possibility of integration with other electronic and optical components on the same chip. At first glance, silica does not seem to be responsive toward UV radiation. However, a closer look at the material absorption (α) of silica shows a weak absorption of about 3.262m -1 at 385nm [14]. The absorbed UV radiation by silica can be calculated using Lambert’s law [15, 16] which expresses the effect of thickness of a material on the absorption through the following equation: I=I 0 e -αX 3-1 where I is the intensity of radiation after passing through the absorbing material, I 0 is the initial intensity of radiation, and X is the thickness of the absorbing material. The above equation can be written in the following form based on exponential series expansion: ( ) ( ) + − + − = ... 3! αX 2! αX 1! αX 1 I I 3 2 0 3-2 In the present system, αX is very small; therefore, we can re-write the above equation in the following form: 57 ( ) αX 1 I I 0 − = 3-3 Therefore, the intensity of radiation absorbed is expressed as: αX I I - I 0 0 = 3-4 The absorbed UV generates some heat (Q) within the silica microtoroid, which can be calculated through the following equation: X Aα I Q 0 = 3-5 where A is the surface area of the toroid which is calculated using the following equation: Dd π A 2 = 3-6 where D is the major diameter of toroid and d is the minor diameter of toroid (Figure 3-2). Figure 3-2 Schematic of side view of a microtoroid depicting major diameter (D) and minor diameter (d). 58 The generated heat will increase the temperature of the microtoroid based on the following equation: T mc Q ∆ = 3-7 where c is the specific heat of silica, ΔT is the increase in the temperature, and m is mass of the device, which can be calculated as following equation : ρV m = 3-8 where ρ is density of silica and V is volume of microtoroid, which can be calculated through the following equation: 2 2 Dd π V = 3-9 Therefore, by combining the above equations and assuming a uniform thickness of X=d, an increase in the temperature of the silica microtoroid can be calculated: ρ α c I ΔT 0 = 3-10 On the other hand, as explained in Chapter 2, silica microtoroids can be used for detection of temperature through tracking the changes in the resonant wavelength shift described by the following equation: T ) dT dn n 1 λ(ε Δλ ∆ + = 3-11 where Δλ is the change in the resonant wavelength, λ is the resonant wavelength, ε is the thermal expansion of silica, n is the refractive index of silica, dT dn is the thermo-optic coefficient of silica, and ΔT is the increase in the temperature of the device. 59 Combining the last two equations gives Equation 3-12, which relates the intensity of UV radiation to the resonant wavelength shift of the silica microresonator: ρ α c I ) dT dn n 1 λ(ε Δλ 0 + = 3-12 where the material properties of silica required in the above equation are n=1.45 at 765nm [17], ρ=2.2 kg/m 3 [18], ε=0.55E−6C −1 [18], dn/dT=1.19E−5C −1 [18], α=3.262m −1 at 385nm [14], and c=703J∕kgC [18]. 3.2 Experiment Silica microtoroids of about 50 microns in diameter were fabricated following the process explained in Chapter 2. To perform the experimental measurements, the setup described in Chapter 2 was used with a 765-781nm tunable laser (Newport, TLB6312). In order to expose the device to controlled levels of UV radiation, a UV lamp (DYMAX, 385nm, BlueWave LED DX-1000) connected to a 5mm-diameter lightguide (DYMAX, 5720 Lightguide) was positioned above the silica microtoroid chip (schematic is shown in Figure 3-3). The intensity of UV radiation at the sample level was measured by a radiometric UV intensity meter (DYMAX, ACCU-CAL 50-LED) [19]. 60 Figure 3-3 Schematic of testing setup for UV measurement experiments [19]. First, the resonant wavelength of the silica microtoroid was determined. The taper was in contact with the sample to ensure constant coupling during the measurements. The coupled power was adjusted to eliminate any potential non-linear broadening of the resonant peak which could interfere with the sensing measurement [19]. The UV lamp was turned on for 20min for a certain value of UV intensity to make sure the resonant wavelength reached equilibrium. Next, the UV lamp was turned off for 20min to allow the resonant wavelength to return to the baseline. This process was repeated for different values of UV intensity while the exact location of the resonant wavelength was tracked as explained in Chapter 2 [19]. 61 The noise of these measurements was evaluated by tracking the location of the same resonant wavelength for 15min while the UV lamp was off. A Gaussian distribution was fit to the distribution of the resonance shifts where the noise level was set to three times the standard deviation (3σ). Each set of measurements was performed in one sitting under ambient conditions using the same resonant wavelength [19]. 3.3 Results and discussion Figure 3-4 demonstrates a sample transmission spectrum of the silica microtoroid on resonance in contact with the taper. Quality factor is determined by fitting a Lorentzian curve (red dashed line) to the spectrum, giving a loaded Q factor of ~ 9.6x10 6 . The resonant peak is symmetric, indicative of the absence of any non-linear effects [19]. Figure 3-4 Transmission spectrum of silica microtoroid on resonance with taper in contact. 62 In Figure 3-5, the result of the resonant wavelength tracking at various levels of UV radiation over time is plotted. These results show the dynamic behavior of the sensor. It is important to note that the UV intensity increases from 14mW/cm 2 to 23mW/cm 2 and subsequently returns to 14mW/cm 2 in the same increments. For similar values of UV intensity, the resonant wavelength shift is consistent, demonstrating the fact that this silica microsensor is robust, reusable, and not damaged by UV radiation [19]. Figure 3-5 resonant wavelength shift versus time as the intensity of UV radiation is increased from 14mW/cm 2 to 23mW/cm 2 and subsequently decreased back to 14mW/cm 2 in the same increments. In order to highlight various characteristics of this sensor, the experimental data shown in Figure 3-5 is re-plotted as the resonant wavelength shift as a function of 63 cumulated fluence in Figure 3-6. This graph is the sensor characteristic curve. Cumulated fluence is known to be the product of the UV intensity and the exposure time. Based on Figure 3-6, the sensor shows linear behavior when UV intensity is both increasing and decreasing. This is an important characteristic of a sensor and defines its working range. Secondly, the response of the sensor is independent of the forward or backward operation highlighting the reproducibility of the detection [19]. Figure 3-6 resonant wavelength shift as a function of cumulated fluence (product of UV intensity by the exposure time) where forward is increase in UV intensity and backward is decrease in UV intensity. The distribution of noise in the experiments is plotted in Figure 3-7. The noise level is calculated to be 0.0983pm. Therefore, the signal-to-noise ratio (SNR) is larger than 100. It is important to note that this extremely high signal-to-noise ratio is achieved 64 under ambient conditions where there could be various fluctuations that might interfere with the measurements [19]. Figure 3-7 Histogram of the distribution of noise in tracking the resonant wavelength shift. As explained in the introduction, the shift in the resonant wavelength is due to the absorption of the UV radiation by silica, which subsequently increases the temperature of the device. The relation between the UV intensity and resonant wavelength shift can be predicted using Equation 3-12. In Figure 3-8, the results of the theoretical predictive model are plotted along with the experimental measurements. There is agreement 65 between the model and the experiments. The slight difference between them could be due to the fact that, in theory, it is assumed that all the UV radiation reaching the sample will be absorbed based on the absorption coefficient; however, scattering could cause lower absorption [19]. Figure 3-8 Experimental and theoretical shift in the resonant wavelength as a function of UV cumulated fluence. 66 Comparing Figure 3-6 and Figure 3-8 (experimental data), it can be seen that the same value of cumulated fluence does not show the same resonant wavelength shift. This occurred because the data on these two graphs are taken on two different days with different silica microtoroids and different resonant wavelengths [19]. 3.4 Conclusion In conclusion, the application of silica microtoroids for detection of UV radiation in the ambient environment was discussed. Since silica is a robust material, it provided a suitable platform for UV sensing. Silica absorbs the UV radiation very slightly and raises the temperature. Temperature changes are detected as a change in the resonant wavelength through thermo-optic effect. The experimental measurements verified the reusability of the sensor and reproducibility of its response. This sensor exhibits a linear response with similar behavior when UV intensity both increases and decreases. Also, in spite of possible ambient fluctuations, the sensor has very high signal-to-noise ratio. In parallel with the experimental measurements, a theoretical mode is developed which is able to predict the response of the sensor with good agreement [19]. 67 Chapter 3 References 1. Saladi, R.N. and A.N. Persaud, The causes of skin cancer: a comprehensive review. Drugs of Today, 2005. 41(1): p. 37-54. 2. Diffey, B.L., Sources and measurement of ultraviolet radiation. Methods, 2002. 28(1): p. 4- 13. 3. Wikipedia. Electromagnetic spectrum. 2014 [cited 2014 June 10th]; Available from: http://en.wikipedia.org/wiki/Electromagnetic_spectrum. 4. Tsai, C. and M.-S. Young, Measurement system using ultraviolet and multiband infrared technology for identifying fire behavior. Review of scientific instruments, 2006. 77(1): p. 014901. 5. Kim, H.-K., W. Shin, and T.-J. Ahn, UV sensor based on photomechanically functional polymer-coated FBG. Photonics Technology Letters, IEEE, 2010. 22(19): p. 1404-1406. 6. Kim, K.T., N.I. Moon, and H.-K. Kim, A fiber-optic UV sensor based on a side-polished single mode fiber covered with azobenzene dye-doped polycarbonate. Sensors and Actuators A: Physical, 2010. 160(1): p. 19-21. 7. Yoon, J.-K., et al., Controllable in-line UV sensor using a side-polished fiber coupler with photofunctional polymer. Photonics Technology Letters, IEEE, 2003. 15(6): p. 837-839. 8. Liu, J., et al., Synthesis of high crystallinity ZnO nanowire array on polymer substrate and flexible fiber-based sensor. ACS applied materials & interfaces, 2011. 3(11): p. 4197-4200. 9. Wang, Z., et al., A flexible UV nanosensor based on reduced graphene oxide decorated ZnO nanostructures. Nanoscale, 2012. 4(8): p. 2678-2684. 10. Zhai, T., et al., Recent Developments in One ‐Dimensional Inorganic Nanostructures for Photodetectors. Advanced Functional Materials, 2010. 20(24): p. 4233-4248. 11. Basak, D., et al., Photoconductive UV detectors on sol–gel-synthesized ZnO films. Journal of Crystal Growth, 2003. 256(1): p. 73-77. 12. Gimenez, A.J., J. Yanez-Limon, and J.M. Seminario, ZnO− Paper Based Photoconductive UV Sensor. The Journal of Physical Chemistry C, 2010. 115(1): p. 282-287. 13. Fang, X., et al., ZnO and ZnS nanostructures: ultraviolet-light emitters, lasers, and sensors. Critical Reviews in Solid State and Materials Sciences, 2009. 34(3-4): p. 190-223. 14. Kitamura, R., L. Pilon, and M. Jonasz, Optical constants of silica glass from extreme ultraviolet to far infrared at near room temperature. Applied optics, 2007. 46(33): p. 8118- 8133. 15. Parker, S.P., Optics sources book. 1988. 16. Hecht, E., Optics. 3rd ed. 1998, Reading, Mass.: Addison-Wesley. vi, 694 p. 68 17. Malitson, I., Interspecimen comparison of the refractive index of fused silica. JOSA, 1965. 55(10): p. 1205-1208. 18. Wakaki, M., T. Shibuya, and K. Kudo, Physical properties and data of optical materials. 2010: CRC Press. 19. Harker, A., S. Mehrabani, and A.M. Armani, Ultraviolet light detection using an optical microcavity. Optics letters, 2013. 38(17): p. 3422-3425. 69 Chapter 4 Relative humidity sensor based on a polymer coated silica microtoroid 4.1 Introduction Humidity is defined as the amount of water in the air. There are various ways of measuring humidity. Absolute humidity is the total amount of water vapor in a certain volume of air. Relative humidity (RH) reports the ratio of the water vapor pressure to the saturated vapor pressure at a given temperature. RH is expressed as a percentage and compares the existing absolute humidity to the maximum absolute humidity at a specific temperature [1]. Relative humidity is an important environmental factor that directly affects our daily life. Our body is cooled down through evaporation of perspiration which slows down in the case of high levels of relative humidity. As a result, our body perceives a higher temperature. Relative humidity plays a major role in many industrial processes such as food processing, semiconductor fabrication, petrochemical processing, and chemical synthesis. For example, in a food processing facility, high levels of relative humidity increase the probability of mold formation. In semiconductor production lines, relative humidity is required to be held at specific values since the yield of the process is closely related to this factor. It is therefore vital to build reliable sensors to monitor relative humidity [1]. Various relative humidity sensors have been developed based on electrical or optical principles. One advantage of optical sensors is that unlike electrical sensors, optical sensors are not susceptible to electromagnetic interference [2-4]. One optical 70 approach that has recently been applied for relative humidity detection is based on whispering gallery mode optical resonators. Ma et al. coated the surface of a silica microsphere with a silica nano-coating. As the coating absorbed the humidity, its refractive index changed which induced a shift in the resonant wavelength [5]. Bhola et al. developed a relative humidity sensor based on a polymer microring coated with silica sol- gel. Due to porosity of sol-gel film, it absorbed water molecules which changed the refractive index of the resonator causing the resonant wavelength to shift [6]. In this work, a hybrid polymer coated silica microtoroid is built for relative humidity monitoring. The polymer is poly(N-isopropylacrylamide) (pNIPAAm) (Figure 4-1) [7] which is hydrophilic and swells as it is exposed to water vapor. Therefore it changes both the refractive index and the size of the hybrid microresonator, simultaneously. The surface of the microtoroids is uniformly coated with pNIPAAm using initiated chemical vapor deposition (iCVD) [8]. Figure 4-1 poly (N-isopropylacrylamide) (pNIPAAm) structure. 71 The iCVD process involves free radical polymerization in the vapor-phase. In free radical polymerization, the initiator will decompose into highly active species (free radicals) which attack the monomer molecules, bond with them and form radicals. This process initiates the polymer chain propagation. There are alternative routes to generate initiators; such as thermal decomposition, photolysis, and ionizing radiation [9]. The process of polymer deposition using iCVD is generally accomplished in a vacuum chamber in a pancake shape as shown in Figure 4-2. The chamber pressure is kept constant in range of 50 to 1000mTorr. The substrate is placed at the bottom of the chamber which is cooled down using recirculated water. Above the substrate, there is a hot filament array that is resistively heated. The monomer and the initiator enter the reactor in the gas phase and their flow rates are either controlled using mass flow controllers or through adjusting their temperature. The initiator molecules decompose through pyrolysis upon contacting the hot filament array. On the other hand, monomer molecules adsorb to the substrate and the chain propagation starts [10]. Figure 4-2 Schematic of iCVD reactor. 72 The iCVD process has the following advantages [10]: 1. Temperature of the substrate is independent of the initiator decomposition temperature allowing polymerization to occur on temperature sensitive substrates 2. No use of highly flammable or toxic solvents 3. The absence of solvent surface tension, enables the coating of small, high aspect ratio, and three dimensional substrates with uniform thin films 4. Demands low energy which keeps the functional groups of the monomer intact 4.2 Experiment Silica microtoroids of ~50μm in diameter were prepared according to the fabrication process described in Chapter 2. iCVD of poly(N-isopropylacrylamide) (pNIPAAm) was performed in the reaction chamber shown in Figure 4-3. The pressure of the chamber was held at 100mTorr. The silicon chip containing silica microtoroids was positioned at the bottom of the chamber which was held at 40°C. The filament array above the substrate is set at 270°C. The monomer, N-isopropylacrylamide (NIPAAm) (Aldrich, 97%) entered the reactor at 60°C. The initiator, t-butyl peroxide (TBPO) (Aldrich, 98%) entered at room temperature with controlled flow rate using a mass flow controller (MKS, 1479A). Polymer films with different thicknesses were fabricated by changing the reaction time [8]. In order to characterize the thickness and refractive index of the pNIPAAm films, small pieces of silicon wafer were placed adjacent to the silica microtoroid chips in the 73 iCVD chamber. A variable wavelength and angle of incidence spectroscopic ellipsometer (V-VASE J.A. Wollam Co.) was used to evaluate the refractive index and the thickness of the fabricated films [8]. To confirm the composition of the fabricated film, Fourier transform infra-red (FTIR) spectra were obtained on polymer coated silicon wafer. The measurements were performed using a Thermo Scientific Nicolet iS10 FTIR (128 scans). The distribution of the optical field in the hybrid device and the surrounding medium was modelled using COMSOL Multiphysics finite element method (FEM) [11, 12]. The mesh size in the region of the optical field was 0.001μm 2 . The resonant wavelength was set to 980nm by controlling the azimuthal mode number (M). The optical field distribution was defined as the magnitude of the electric field squared in the silica microtoroid, pNIPAAm film, and air. 74 Figure 4-3 Pictures of iCVD reactor used for deposition of pNIPAAm film on silica microtoroids. To evaluate the performance of hybrid polymer coated silica microtoroids, the testing setup described in Chapter 2 was used. The laser was a 975-985nm tunable laser 75 (Newport, TLB6320). Three modifications were implemented on the setup which are highlighted in Figure 4-4 and explained below: 1. A reference relative humidity sensor (OMEGAETTE, Model: HH314A) was placed close to the sample to measure the relative humidity and temperature of the experiments. It is important to monitor the temperature due to the dependence of relative humidity on temperature. The recordings from this sensor were automatically recorded on a computer using an RS232 interface. 2. Since changes in the temperature can shift the resonant wavelength, the temperature of the sample was controlled with a heater (Omega, CSH- 102100=120V) and a thermocouple (Omega, SA1XL) attached to a bench- top controller (Omega, CSC32 series). All the experiments were performed at 23°C. 3. In order to vary the level of relative humidity, an ultrasonic humidifier (Stadler Form Jerry) was integrated into the setup. The advantage of using this type of humidifier is that it does not change the temperature of the environment (Initially, a bubbler system with nitrogen gas and boiling water was implemented to change relative humidity; however, it changed the temperature as well. Therefore, in order to allow for studying the pure effect of relative humidity on the sensor, the ultrasonic humidifier was preferred). The intrinsic quality factor of polymer coated silica microtoroids was determined using the linewidth measurement method explained in Chapter 2. Once the resonant 76 wavelength was found, the ultrasonic humidifier was turned on and the shift in the resonant wavelength was tracked as described in Chapter 2. Figure 4-4 (a) Rendering image of the testing setup for relative humidity measurements [8] (b) Picture of the testing setup. 77 4.3 Results and discussion Figure 4-5 shows the FTIR spectra of a fabricated polymeric film. The following characteristic peaks confirm that the film is composed of pNIPAAm [13]: 1. Peaks near 3330cm -1 correspond to NIPAAm secondary amine (N-H) stretching 2. Peaks near 1530cm -1 correspond to NIPAAm amide (C-N-H) bending (amide II band) 3. Peaks near 1654-1645cm -1 correspond to NIPAAm C=O stretching (amide I band) Figure 4-5 Fourier transform infrared (FTIR) spectra of pNIPAAm. 78 Based on ellipsometry measurements, pNIPAAm films with two thicknesses of 71.5nm (thick film) and 38.5nm (thin film) were achieved using the iCVD process. Figure 4-6 demonstrates the refractive index of the two films as a function of wavelength measured by ellipsometry [8]. For comparison, refractive index of silica is also plotted which is based on reference [14]. It can be seen that the polymer film has higher refractive index compared to the silica. Moreover, the thick film has slightly higher refractive index as compared to the thin film. 79 Figure 4-6 Refractive indices of fabricated pNIPAAm films measured by ellipsometry in comparison with refractive index of silica based on reference [14]. Figure 4-7 shows optical images of silica microtoroids coated with the thin and thick pNIPAAm films. From such optical images it is clear that the polymer deposition using iCVD does not create any micron-scale damage to the surface of the devices. 80 Figure 4-7 Optical images of (a) thin film coated microtoroid (b) thick film coated microtoroid. 81 Figure 4-8a shows FEM results for the thin polymer coated device with 52(8)μm major (minor) diameter and Figure 4-8b shows the FEM results for the thick coating with 57(8)μm major (minor) diameter. It can be seen that a portion of the optical field leaks into the polymer coating as well as the surrounding environment which is air. Figure 4-8 FEM simulation of (a) thin film coated microtoroid (b) thick film coated microtoroid. 82 Sample spectra of the linewidth measurements for microtoroids with both polymer thicknesses are shown in Figure 4-9. The intrinsic quality factors of the hybrid devices are above 2.70x10 5 for both thick polymer coated and thin polymer coated devices. The lower quality factor of the polymer coated devices as compared to bare silica microtoroids (above 10 8 ) is mainly due to the increase in the absorption losses caused by the polymer films. Figure 4-9 Results of quality factor measurements (a) Sample transmission spectra of resonant peak for thin polymer film (b) Intrinsic quality factor measurement for thin polymer film (c) Sample transmission spectra of resonant peak for thick polymer film (d) Intrinsic quality factor measurement for thick polymer film. 83 Figure 4-10 demonstrates the response of the bare silica microtoroid toward changes in the relative humidity. The black squares correspond to the response of the reference relative humidity sensor over time. The red circles show the shift in the resonant wavelength as the relative humidity changes. Figure 4-10 Response of reference relative humidity sensor over time (black squares) along with the resonant wavelength shift as a function of time (red circles) for bare silica microtoroid. 84 The characteristic sensing curve for both thin and thick polymer coated devices are plotted in Figure 4-11. The graph includes the response of the reference humidity sensor (control sensor) shown in black squares. The red circles show the shift in the resonant wavelength (Δλ) of hybrid sensors as a function of time as the relative humidity changes. For both film thicknesses, there is an excellent agreement between the control sensor and the hybrid microtoroid sensors. As the relative humidity increases, the polymer swells increasing both the refractive index and the size of the resonator, which results in the resonant wavelength redshift. These measurements also demonstrate the viability of the sensor in iterative measurements [8]. Figure 4-11 Reference relative humidity sensor response over time (black squares) along with the resonant wavelength shift as a function of time (red circles) for (a) thin film coated microtoroid (b) thick film coated microtoroid [8]. 85 The comparison between the responses of bare silica microtoroid and hybrid polymer coated microtoroids is shown in Figure 4-12. The linear portion of resonant wavelength shift versus time is plotted against the changes in relative humidity recorded by the reference relative humidity sensor. In addition, the noise level is plotted. The slope of the linear fit to each data set defines the sensitivity of the sensor response which are listed below [8]: • Bare silica microtoroid: 0.29pm/%RH • Thin polymer coated silica microtoroid: 12.98pm/%RH • Thick polymer coated silica microtoroid: 10.35pm/%RH The nanometers thick pNIPAAm coating improves the response of the bare silica microtoroid by almost two orders of magnitude [8]. Figure 4-12 Resonant wavelength shift as a function of changes in relative humidity for the bare silica microtoroid and the thin and thick polymer coated devices along with the noise measurements. The error bars in the experiment are smaller than the symbols [8]. 86 In Figure 4-13, the single sensing responses of the hybrid devices are shown. It can be seen that the forward responses (increase in relative humidity) of both hybrid sensors are very linear; however, the backward response (decrease in relative humidity) is extremely non-linear. Figure 4-13 The response of hybrid sensors with (a) thin polymer film (b) thick polymer film [8]. 87 To evaluate the temporal behavior of the sensors, the slope of linear fit to the forward responses (response rates) are compared and listed below [8]: • Bare silica microtoroid: 4pm/s • Thin polymer coated silica microtoroid: 74pm/s • Thick polymer coated silica microtoroid: 40pm/s The recovery rates ( τ ) of the sensors are also evaluated by fitting an exponential function of the following form to the backward response of the sensors [8]: o y ) xτ Aexp( y + − = 4-1 Based on the exponential fit, the recovery times for both polymer thicknesses are [8]: • Thin polymer coated silica microtoroid: 0.2s -1 • Thick polymer coated silica microtoroid: 0.6s -1 It is important to note that even though both film thicknesses generate almost similar responses toward relative humidity changes (Figure 4-12), they have different temporal behaviors. This could be due to the difference in the rearrangement kinetics of the polymer films [8]. One of the important parameters of a sensor is the hysteresis, the difference between its forward and backward response. The data in Figure 4-11 is re-plotted in Figure 4-14 to show the hysteretic behavior of the hybrid sensors. This behavior could be due to the difference in mechanism of water desorption from the pNIPAAm hydrophilic polymer. It can be seen that the thick film provides more linear and therefore more desirable response; however, the thin film has more reproducible behavior. 88 Figure 4-14 Hysteresis of hybrid silica microtoroids with (a) thin polymer film and (b) thick polymer film [8]. One of the unique features of pNIPAAm is that it exhibits a lower critical solution temperature (LCST) in water. Below the LCST, the polymer chains are fully hydrated, and therefore it is hydrophilic. Above the LCST, the polymer chains collapse into a globular state, and it becomes hydrophobic. The LCST of the bulk polymer is known to be ~32°C [13]. The behavior of two different film thicknesses over a range of temperatures has been characterized using the hybrid microcavity device. The scatter plots in Figure 4-15 show the sensor response at different temperatures. In the case of the thin polymer coating, a sharp change in the behavior of the device occurs around 40°C. The thick polymer coating shows a similar trend, but at the slightly higher temperature of 54°C. 89 Figure 4-15 Characteristic curve of the changes in the resonant wavelength vs. changes in the relative humidity at various temperatures for the (a) thin polymer coated device and (b) thick polymer coated device. 90 4.4 Conclusion A relative humidity sensor was successfully developed, by coating based on a hybrid polymer coated silica microtoroid. The nanometers thick pNIPAAm polymer was deposited using the iCVD technique which was capable of coating the microstructure uniformly. The polymer coating improved the response of the bare silica device toward relative humidity by almost two orders of magnitude by utilizing both refractive index and geometry change detection mechanisms. Comparing two different film thicknesses showed that the thin film had a faster response, but slower recovery [8]. This work demonstrated an example of leveraging the properties of polymers in hybrid sensors to improve the performance of their single-material counterparts [15, 16]. In addition, this work demonstrated the application of iCVD technique for deposition of various functional polymers to enable future innovations. Examples of these functional polymers include: pH-sensitive polymers [17], UV-responsive polymers [18] and polymers which allow biomolecule attachment [19]. 91 Chapter 4 References 1. Fraden, J., Handbook of modern sensors: physics, designs, and applications. 2004: Springer. 2. Xiao, G. and W.J. Bock, Photonic Sensing: Principles and Applications for Safety and Security Monitoring. Vol. 227. 2012: John Wiley & Sons. 3. Yu Francis, T. and Y. Shizhuo, Fiber Optic Sensors. 2002, Marcel Dekker Inc., New York. 4. Grattan, K.T.V. and B.T. Meggitt, Optical Fiber Sensor Technology. 1995: Chapman & Hall. 5. Ma, Q., et al., Spectral shift response of optical whispering-gallery modes due to water vapor adsorption and desorption. Measurement Science and Technology, 2010. 21(11): p. 115206. 6. Bhola, B., et al., Sol-gel-based integrated optical microring resonator humidity sensor. Sensors Journal, IEEE, 2009. 9(7): p. 740-747. 7. Schild, H., Poly (N-isopropylacrylamide): experiment, theory and application. Progress in polymer science, 1992. 17(2): p. 163-249. 8. Mehrabani, S., et al., Hybrid microcavity humidity sensor. Applied Physics Letters, 2013. 102: p. 241101. 9. Chanda, M., Introduction to Polymer Science and Chemistry: A Problem-Solving Approach. 2013: CRC Press. 10. Seidel, S., C. Riche, and M. Gupta, Chemical vapor deposition of polymer films. Encyclopedia of Polymer Science and Technology, 2011. 11. Choi, H.S., X. Zhang, and A.M. Armani, Hybrid silica-polymer ultra-high-Q microresonators. Optics letters, 2010. 35(4): p. 459-461. 12. Oxborrow, M., Traceable 2-D finite-element simulation of the whispering-gallery modes of axisymmetric electromagnetic resonators. Microwave Theory and Techniques, IEEE Transactions on, 2007. 55(6): p. 1209-1218. 13. Alf, M.E., T.A. Hatton, and K.K. Gleason, Novel N-isopropylacrylamide based polymer architecture for faster LCST transition kinetics. Polymer, 2011. 52(20): p. 4429-4434. 14. Bass, M., et al., Handbook of Optics, Volume IV: Optical Properties of Materials, Nonlinear Optics, Quantum Optics (set). Vol. 4. 2009: McGraw Hill Professional. 15. Mehrabani, S., A.J. Maker, and A.M. Armani, Hybrid Integrated Label-Free Chemical and Biological Sensors. Sensors, 2014. 14(4): p. 5890-5928. 16. Clevenson, H., et al., High sensitivity gas sensor based on high-Q suspended polymer photonic crystal nanocavity. Applied Physics Letters, 2014. 104(24): p. 241108. 92 17. Kwong, P. and M. Gupta, Vapor phase deposition of functional polymers onto paper-based microfluidic devices for advanced unit operations. Analytical chemistry, 2012. 84(22): p. 10129-10135. 18. Haller, P.D., C.A. Flowers, and M. Gupta, Three-dimensional patterning of porous materials using vapor phase polymerization. Soft Matter, 2011. 7(6): p. 2428-2432. 19. O'Shaughnessy, W.S., et al., Initiated Chemical Vapor Deposition of a Surface ‐Modifiable Copolymer for Covalent Attachment and Patterning of Nucleophilic Ligands. Macromolecular Rapid Communications, 2007. 28(18 ‐19): p. 1877-1882. 93 Chapter 5 Blue microlaser based on a thulium doped silica microtoroid 5.1 Introduction The word laser is derived from the acronym, Light Amplification by Stimulated Emission of Radiation. In May 1960, Theodore Maiman demonstrated the first solid state laser based on a ruby crystal (aluminum oxide doped with transition metal ion Cr 3+ ). Shortly after the first gas laser based on helium-neon was made by Ali Javan in December 1960 at Bell Lab. Following these great inventions, various types of lasers emerged [1]. A laser has three main components: 1. Gain medium: where electronic transitions occur and laser photons are generated 2. Optical resonator: directs the generated photons to feedback into the system. 3. Pump source: provides the required energy for the electronic transitions within the gain medium To better understand the laser process, it is helpful to discuss several different possible interactions of light and matter. Consider two energy levels in a material (e.g. atoms, molecules, ions) where level 1 is lower in energy than level 2 (Figure 5-1). There is an infinite number of energy levels in a given material. For convenience level 1 is taken as the ground state and the energy of level 1 is denoted by E 1 . E 2 denotes the energy of level 2 and the energy difference of the two levels (E 1 -E 2 ) is equal to hν where 94 h is the Planck’s constant and ν is frequency [2]. As depicted in Figure 5-1, there are four main cases of light-matter interaction: 1. Absorption: the energy of the incident photon is absorbed by the electron in the ground state and causes the electron to be excited to a higher energy level. The energy of the incident photon matches the energy difference between the ground state and the excited state (hν). 2. Spontaneous emission: the electron in the excited energy level will decay to a lower energy level spontaneously and a photon is emitted as a result of this process. The energy of the emitted photon is equal to the energy difference between the ground state and the excited state (hν). 3. Non-radiative decay: the electron in the excited energy level decays to the ground state. However, the energy released as a result of this decay is not in the form of light. It may transfer into kinetic or internal energy of its surroundings. In solid-state lasers, it is usually in the form of phonons, which are related to the crystal lattice vibrations or glass vibrational modes. 4. Stimulated emission: when the electron in the excited state is hit by a photon of energy equal to the difference of the energy of the two energy levels (hν). Since the incident photon has the same frequency (therefore energy) as the atomic frequency, there is a finite probability that it will force (stimulate) the electron to decay to the ground state. The stimulating photon is not absorbed. As a result, two photons of same energy (hν) are emitted. It is important to note that in stimulated emission, unlike 95 spontaneous emission, the emitted photons are in phase and in the same direction as the incident (stimulating) photons. Figure 5-1 Four main types of interaction of photons and electrons in a laser gain medium. In a laser gain medium, a combination of all the above interactions occurs. Consider a three level lasing scheme (Figure 5-2), where photons of appropriate energy (emitted from the pump source) are absorbed by the ground state electrons, moving them into the excited states. The excited electrons decay non-radiatively to intermediate energy levels, from where they decay. The stimulating photons are the ones generated through spontaneous emission that feedback into the laser cavity. The released photons hit the walls of the laser’s cavity and bounce back to the gain medium where they cause more 96 stimulated emissions. In this case, the energy of the emitted photons is always lower than the pumping photons. As a result the wavelength of the emitted light is longer than the pump source wavelength. It is important to note that for lasing to occur, a population inversion must form. This describes a state of the system when the number of excited electrons is greater than the number of electrons in the lower energy levels [2]. Figure 5-2 Three level lasing scheme, where the energy of the pumping photon is higher than the laser photon. 97 Development of a blue light emitting solid-state microlaser integrated on a silicon chip enables various new applications. Most notably it will advance compact biodetection platforms and telecommunication systems such as data storage. Different approaches have been taken to build such lasers. Since the majority of modern lasers generate infrared (IR) and near-IR emissions, the main approach to achieve shorter wavelength lasers is to convert these emissions into the desired wavelength. This requires applying non-linear optical methods such as harmonic generation and optical parametric oscillation. However, these techniques suffer from several rigorous requirements: crystal temperature, crystal axis alignment, beam divergence, beam quality and polarization orientation [3]. An alternative approach for visible lasing is based on an upconversion process. In a simple upconversion lasing scheme (Figure 5-3), pump photons are absorbed sequentially to excite the electrons from the ground state to a meta-stable energy level and then to a higher energy level. In this case the spontaneous emission of the excited electron will release a photon whose energy is higher than that of the pump source and consequently has a wavelength is shorter than the pump source [3]. The first upconversion laser was developed by Johnson and Guggenheim in 1971 in Yb, Er : BaY 2 F 8 and Yb, Ho : BaY 2 F 8 crystals [4]. 98 Figure 5-3 Sequential two-photon absorption in a three level upconversion scheme, where the energy of the pumping photon is lower than the energy of the released photon. Development of a blue upconversion laser requires careful attention to the three main components of the laser system [5]: 1. The laser gain medium must provide energy levels that enable the right path for blue emission. In addition, the gain material needs to possess metastable energy levels to act as energy reservoirs for sequential multi- photon absorption. 99 2. The laser resonator needs to have high photon life time (high Q factor) to provide a low laser threshold. The laser threshold is the minimum pump power at which a population inversion occurs. A higher quality factor results in a much larger power build-up. Therefore, losses in the system can be overcome with much lower input powers. Once the laser threshold is reached, incremental changes in the pump power cause the laser power to increase by orders of magnitude. 3. The pump laser has to have the correct wavelength (energy) matching the absorption of the gain medium. Also in order to achieve continuous lasing and due to the multi-photon nature of the process, the pump light should be delivered to the gain medium efficiently. In order to address all of the above-mentioned requirements, thulium doped silica microtoroids that are optically pumped using tapered optical fibers are presented. The gain medium of this microlaser is thulium trivalent ions (Tm 3+ ). Rare earth elements have unique electronic structures consisting of several intermediate metastable levels, which make them suitable for upconversion [3, 5-9]. The first thulium blue upconversion laser was developed by Nguyen et al. in 1989. They pumped 1% Tm doped in YLF (yttrium lithium fluoride) crystal at 75K with lasing threshold powers above 100mW [10]. Since then several other crystals such as YLiF 2 [11] and YAG (Yttrium aluminum garnet, Y 3 Al 5 O 12 )) [12] were also used as the host lattice for thulium blue emission; however, they all required both operation at cryogenic temperatures and very high pump powers. In 1992, Grubb et al. developed the first blue thulium doped ZBLAN (ZrF 4 -BaF 2 -LaF 3 -AlF 3 -NaF) fiber laser operating at room 100 temperature with a lasing threshold of 46mW [13]. After that, several other Tm doped ZBLAN blue fiber lasers were developed [14-22]. However, to obtain the necessary density of photons in the fiber for upconversion, very high threshold powers are required. In addition, the incompatibility of ZBLAN with silicon processing techniques further limits its application in fiber lasers. [5]. Interestingly, integration of thulium in silica microtoroids can solve both of these issues. First, as explained in Chapter 2, silica microtoroids are fabricated on silicon chips using CMOS compatible techniques. Second, they provide ultra-high quality factors which results in a high intensity of the optical field circulating within the microresonator. The following equation relates the quality factor (Q) and the input power (P in ) to the intensity of the circulating light (I circ ) in a whispering gallery mode microresonator: I c irc = P in ( λ 2πn )( Q V ) 5-1 where λ is the resonant wavelength, n is the group index, and V is the mode volume [23, 24]. Due to the high intensity of the optical field present in WGM optical resonators, these resonators have been successfully used in the development of ultra-low threshold microlasers [25-30]. The high intensity of the optical field inside the microtoroid overcomes problems associated with the high phonon energy of silica. This high phonon energy generally results in a rapid non-radiative decay of the excited states, which makes silica an undesirable upconversion host. The high photon lifetime and high intensity of optical field inside silica microtoroids compensates for this potential drawback [2, 5]. Another 101 advantage of using silica microtoroids as opposed to other resonators is the ability to couple light into the resonator. As explained in Chapter 2, light can be delivered very efficiently to the silica microtoroids using tapered optical fibers [5]. In order to build thulium doped silica microtoroids, thulium first needs to be integrated within the silica film. To accomplish this task, the sol-gel technique is used because this technique offers advantages such as ion implantation [31]. Some of the these advantages are summarized below [32, 33]: 1. Relatively low cost 2. Low temperature requirements which minimizes thermal degradation of the material and entrapped species 3. Controllable and homogenous doping, due to the wet chemical nature of the synthesis process 4. Straightforward fabrication of various shapes such as thin films without the machining or melting (achieved by adding liquid precursors in the process) As the name of this technique implies, it involves a transition from a colloidal liquid phase (sol) to a semi-solid phase (gel). Colloids are solid particles of 1-100nm in diameter and gels are networks of rigid polymer chains of more than 1micron long with submicron pores. The sol-gel process involves the following reaction steps: hydrolysis, water condensation, alcohol condensation, gelation and drying [33]. To start the sol-gel process, a metal alkoxide precursor is added to water. Since water and alkoxides are not miscible, an alcohol is added as a co-solvent. During the 102 hydrolysis reaction, hydroxyl groups (OH) replace the alkoxide groups (OR). This is shown in the following reaction [33]: 5-2 The speed and the extent of the hydrolysis reaction can increase in acidic environments since the alkoxide group is protonated rapidly Therefore electron density is withdrawn from the silicon atom. This will make the silicon atom electrophilic with partial positive charges, and it will become more susceptible to nucleophilic attack by water molecules [33]. Next, the silanol groups (Si-OH) produce siloxane bonds (Si-O-Si) resulting in a by-product of water (water condensation) or alcohol (alcohol condensation). As the number of siloxane group increases they bridge and the silica network is formed. These reactions are represented below where Equation 5-3 is the water condensation reaction and Equation 5-4 is the alcohol condensation reaction [33]: 5-3 5-4 The condensation reaction is also efficient in an acidic environment, which helps the formation of the weakly cross-linked polymer network. During the gelation process, 103 the cross-linking continues and the gel shrinks as the non-bonded contacts are replaced by covalent links. Finally, during the drying step, the trapped solvents and organic residues in the silica network will be eliminated, and the densified silica film is formed [33]. 5.2 Experiment To form the thulium doped silica films, the process described in Figure 5-4 was applied. Tetraethyl orthosilicate (TEOS) (Alfa Aesar 99.999%), ethanol (BDH1156), DI (deionized) water, hydrochloric acid (EMD 36%), and thulium (III) nitrate pentahydrate (N 3 O 9 Tm · 5H 2 O) (Sigma-Aldrich 99.99%) were added step wise with five minutes of stirring at 500rpm in between each addition. The final mixture was stirred for two hours at room temperature and then set aside to age for 24hr at room temperature. Next, it was passed through a syringe filter (VWR 0.45μm PTFE) and moved to the 4°C fridge to stop further gelation. The relative molar ratio of components was set at TEOS: Ethanol: H 2 O: HCl = 1:4:2:0.1. The amount of thulium was adjusted to achieve 0.033, 0.043, 0.062, 0.08, and 0.094 atomic% (at.%) Tm along with a control (no thulium). Pieces of bare silicon were then cleaned with acetone, methanol, isopropyl alcohol and blow dried with an air gun. The sol-gel mixture was spin-coated (Laurell, WS-400 Lite spin processor) on the cleaned silicon chips at 7000rpm for 30s and pre- baked at 75°C on a hotplate for five minutes. The samples were then transferred in porcelain Boats (VWR) to a tube furnace (Thermo scientific, Lindberg blue M) where they were annealed at 1000°C for one hour in normal atmospheric conditions. The process of spin-coating and annealing was repeated three times to achieve films of approximately 1μm thick [5]. The main challenge during this process was the formation 104 of cracks after the annealing step. Cracks were minimized using fresh sol-gel mixtures (not older than three weeks). Figure 5-4 Schematic of the sol-gel process used to fabricate pure and thulium doped silica films on a silicon substrate. In order to verify the formation of the silica network, Fourier Transform Infra- Red (FTIR) spectroscopy (Bruker, ALPHA-P) techniques were applied to the fabricated sol-gel films. The spectra were then compared with the FTIR spectra of thermally grown silica films (WRS) measured with the same FTIR unit. After the sol-gel films were prepared, microtoroids were fabricated using the process outlined in Chapter 2. It is important to note that the CO 2 laser reflow step required slightly lower power than thermally grown silica. The fabricated devices were checked with a wide-field optical microscope (Nikon) and scanning electron microscope (SEM) (HITACHI, TM3000 Tabletop Microscope). 105 The quality factor and lasing emission from the fabricated devices were characterized using the testing setup described in Chapter 2. A tunable laser of 1055- 1070nm (Newport, TLB6321) was used for quality factor measurement and pumping the thulium laser. All measurements were done for the fundamental mode. The fundamental mode was identified through a broad scan of the wavelengths. The intrinsic quality factor was determined using linewidth measurements as discussed in Chapter 2. To collect the lasing emission, a fiber coupled spectrograph (Andor, Shamrock spectrograph: SR-163, Newton charge-coupled device (CCD) detector: DU92ON-BR-DD, fiber: SR-OPT-8002, Grating: SR1-GRT-1200-500) was mounted on the side view camera positioned right in front of the sample (Figure 5-5). The spectrograph was connected to the computer with a USB cable and the signal was recorded on the computer using Andor SOLIS software. The laser threshold was evaluated by recording the emission intensity (area under emission curve) for a range of pump powers measured using an optical power meter (Thorlabs). These measurements were performed on various concentrations of thulium doped silica microtoroids of similar size and on their fundamental modes. 106 Figure 5-5 Rendering of testing setup. In addition to testing in the air, the possibility of lasing in water was also investigated. In this case, the size of the microtoroids was increased as compared to the previous experiments in air. This increase is necessary to compensate for the decrease in refractive index contrast due to additional radiation losses in the water. The sample holder was also modified slightly with coverslips to create a water chamber (Figure 5-6) [34]. 107 Figure 5-6 Modified sample holder for measurements in water. The picture of the setup with the modified sample holder is also shown in Figure 5-7. First, the taper and the sample were aligned, and then deionized (DI) water was injected into the chamber to cover the chip and fill the chamber. The rest of the testing procedure was similar to testing in air as explained previously. Figure 5-7 Testing setup with the modified sample holder for experiments in water. 108 5.3 Results and discussion FTIR spectra of the fabricated sol-gel silica film and commercial thermally grown silica were compared (Figure 5-8). Based on the following peaks that are identical between the two samples, it is clear that the sol-gel technique is successful in the formation of a silica network [35-42]: 1. 450cm -1 corresponds to Si-O-Si bending 2. 800cm -1 corresponds to Si-O-Si symmetric stretching 3. 1000-1100cm -1 corresponds to Si-O-Si asymmetric stretching Figure 5-8 FTIR spectra of fabricated sol-gel silica film along with the FTIR spectra of commercial thermally grown silica. 109 Figure 5-9 shows a scanning electron microscope (SEM) image of a 0.094at.% Tm doped silica microtoroid. The smooth surface of the microtoroid is clear in this figure. Figure 5-9 Scanning electron microscope (SEM) image of a 0.094at.% Tm doped silica microtoroid [5]. In Figure 5-10a, a sample transmission spectra of a 0.033at.% thulium doped silica microtoroid is shown. Figure 5-10b shows the dependence of the intrinsic quality factor on thulium concentration. As explained in Chapter 2, the main loss mechanism in silica microtoroids is the material absorption loss. In these thulium doped silica microtoroids, the absorption losses increase due to the presence of the dopant. Since Q mat = 2πn∕λα there is inverse relationship between the Q factor and absorption coefficient or the concentration of the dopant. In Figure 5-10b an equation of the form y=a∙x b is fitted to the data. A value of b= -1.00 is in good agreement with the material-limited quality factor model [5]. 110 Figure 5-10 (a) Sample transmission spectra of a 0.033at.% Tm doped silica microtoroid with Q=1.6x10 6 in air (b) Quality factor of sol-gel silica microtoroids with various levels of Tm doping measured in the air. 111 In Figure 5-11, a set of pictures of a 0.033at.% silica microtoroid with tapered optical fiber are shown when the device is on and off resonance. It can be seen that when the device is off resonance, there is no blue lasing emission. Once it is on resonance, blue lasing is observed along the periphery of the device, which coincides with the optical field of the circulating whispering gallery modes. In addition, the uniform intensity of the blue emission is an indication of the uniform distribution of thulium within the silica network. Figure 5-12 also shows the side view image of a 0.043at.% Tm doped microtoroid [5, 43]. Figure 5-11 Images of 0.033at.% Tm doped silica microtoroids with tapered optical fiber taken from top view camera in air (a) Off resonance (b) On resonance with lights on and (c) On resonance with lights off [5]. 112 Figure 5-12 Side view images of 0.043at.% Tm doped silica microtoroids while on resonance and lasing [43]. A representative blue lasing signal from a 0.043at.% Tm sample, collected on the spectrograph is shown in Figure 5-13. It is clear that the emission is multi-mode centered at 450nm and 461nm with whispering gallery mode oscillations. The distance between the WGM’s are 1.1nm which is in agreement with the free spectral range (FSR) of 450nm and 461nm emissions calculated based on FSR equation discussed in Chapter 2 (FSR ∼ λ 2 ∕2πnR) [5]. A representative threshold measurement is shown in the inset of Figure 5-13a. The lasing threshold of 32μW is calculated based on the x-intercept of the fitted line. This lasing threshold is three orders of magnitude improvement compared to previous upconversion blue fiber lasers which have had lasing thresholds on the order of milliwatts [5]. 113 Figure 5-13 (a) Multimode blue lasing at 450nm and 461nm in air, inset: lasing threshold graph of 0.043at.% Tm sample with threshold power of 32μW in air (b) Lorentzian fit to the lasing spectra in (a) which shows a constant distance of 1.1nm between peaks which is in agreement with the calculated free spectral range of the device [5]. In addition to blue emission, near-IR multi-mode upconversion lasing with WGM oscillations was also observed at 784nm, 802nm, and 816nm. Figure 5-14 shows a representative lasing spectra for a 0.043at.% Tm doped microtoroid. The distance between whispering gallery modes is ~2.7nm in accordance with the free spectral range (FSR) calculated based on the FSR equation [5]. The inset in Figure 5-14a shows an example of a lasing threshold measurement for a 0.043at.% Tm doped sample. The lasing threshold of 17μW is an improvement of three orders of magnitude over the previously discussed fiber lasers which have thresholds on the order of milliwatts [5]. 114 Figure 5-14 (a) Multimode lasing at 784nm ,802nm ,816nm in air, inset: lasing threshold graph of 0.043at.% Tm with threshold power of 17μW in air (b) Lorentzian fit to the lasing spectra in (a) which shows a constant distance of 2.7nm between each peak in agreement with the calculated free spectral rang of the device [5]. As explained before, thulium is a rare earth dopant containing the desired energy levels for blue upconversion using 1064nm light. The energy levels of thulium are shown in Figure 5-15 along with the corresponding emission paths. The spectrograph signal showed weak emissions at 660nm and 360nm; however, WGM laser oscillation was not observed. The specific transitions responsible for the observed emissions occur as three 1064nm photons excite a ground state electron to the 1 G 4 level. The excitation of the 1 D 2 level happens due to absorption of two 1064nm photons in addition to absorption of a 660nm photon at the 3 H 4 level [5]. 115 Figure 5-15 Energy level diagram of Tm 3+ [5]. The lasing threshold is inversely related to the intensity of the optical field present within the microresonator. The intensity of the optical field is directly related to the Q factor and the Q factor is inversely proportional to the concentration of the dopant. Therefore, lower concentrations can increase the intensity of the optical field and consequently decrease the threshold. At the same time, dopants are an essential part of solid-state lasers since they form the gain medium. As a result, to achieve the lowest threshold possible, a balance between increasing the concentration of dopants and decreasing the Q factor must be maintained while minimizing the lasing threshold. 116 In order to define this optimum concentration for thulium, in Figure 5-16 the dependence of the lasing threshold on thulium concentration is plotted. In case of 0.033at.% concentration, even though the Q factor is the highest, because the gain medium concentration is too low, the lasing threshold is relatively high. The optimum concentration is reached at around 0.043at.%, because after this point increasing the thulium concentration decreases the Q which increases the lasing threshold. Figure 5-16 Lasing threshold in air versus thulium concentration for (a) Blue lasing (b) Near-IR lasing. A representative transmission spectra of a 0.062at.% Tm doped silica microtoroid on resonance in water is shown in Figure 5-17a. The intrinsic Q factor measurement results are also shown in Figure 5-17b. It can be seen that the Q factor in the water is lower than in the air. This is largely due to the increase in the effective absorption losses as a result of the high absorbance of water at 1064nm compared to air. 117 Figure 5-17 (a) Representative transmission spectrum of a 0.062at.% Tm doped silica microtoroid in water (b) Intrinsic quality factor measurement for a 0.062at.% Tm doped silica microtoroid in water. 118 The top view images of 0.062at.% Tm doped silica microtoroids being tested in water are shown in Figure 5-18. When the device is off resonance, there is no lasing; however, once it is on resonance, blue emission can be observed from the circumference of the device. Figure 5-18 Images of 0.062at.% Tm doped silica microtoroids with tapered optical fiber taken from the top view camera in water (a) Off resonance (b) On resonance with lights on and (c) On resonance with lights off. 119 The lasing spectra and the corresponding lasing thresholds of a 0.062at.% Tm sample in the blue and near-IR spectral ranges are plotted in Figure 5-19. It can be seen that the lasing thresholds in both cases are higher than in air, which is mainly due to the lower quality factor of the device in water. Figure 5-19 (a) Multimode blue lasing at 450nm and 461nm in water (b) Lasing threshold graph of 0.062at.% Tm with a threshold power of 230μW in water (c) Multimode lasing at 784nm ,802nm ,816nm in water (d) Lasing threshold graph of 0.062at.% Tm with a threshold power of 40μW in water. 120 5.4 Conclusion In this chapter the application of silica microtoroids for upconversion to blue wavelengths is outlined in detail. Although, silica is not an ideal host for upconversion, due to high phonon energy the high intensity optical field present inside this microresonator overcomes this challenge. In addition, the fact that the fabrication process for these device is compatible with conventional CMOS processing, allows for their integration on a silicon substrate. These results enable the potential for further integration of these devices with other components on the same chip and their utilization in applications that require micron scale devices. The integration of a thulium dopant within the silica device was accomplished using sol-gel technique. This gives the synthesis process for these microlasers several advantages over other potential techniques such as ion implantation. These microlasers have been characterized with measurements of two of the most important properties of WGM resonator devices, namely the quality factor and lasing threshold in both an air and water environment. 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Young, T., et al., Study on the Si–Si Vibrational States of the Near Surface Region of Porous Silicon. Journal of Porous Materials, 2000. 7(1-3): p. 339-343. 43. Mehrabani, S. and A.M. Armani. Low-threshold integrated microlaser emitting in the blue formed from thulium-doped silica. in SPIE OPTO. 2014. International Society for Optics and Photonics. 124 Chapter 6 Integration of semiconductor quantum dots and silica microtoroids 6.1 Introduction Electrons in individual atoms have discrete energy levels; however in the bulk state these discrete energy levels are merged into energy bands. In a semiconductor, the highest band containing electrons is called the valance band; the next highest is the conduction band. The difference between them is called the band gap. In a bulk semiconductor, the band gap is independent of the size. However, once the semiconductor nanocrystal size is less than the de Broglie wavelength of a charge carrier, the band gap becomes size dependent. Such a size limit in one, two or three dimensions of a semiconductor crystal leads to the formation of a quantum well, quantum wire or a quantum dot (QD), respectively. Here, the term “quantum” originates from the requirement of treating these systems with the physics of quantum mechanics [1-4] . The focus of this chapter is semiconductor quantum dots, which were originally discovered by Luis Brus at Bell laboratories in 1980s [5, 6]. In a quantum dot of a semiconductor, as the size decreases, the energy gap increases which translates into a blue shift in its emission wavelength. This phenomenon is mainly due to the quantum size effect i.e. the electronic excitations (responsible for the emissions) “feel” the boundaries of the system and respond to it through changing the energy gap. The energy of the quantum dot band gap can be approximately calculated using the following expression: 125 r ε ε 4π 1.8e m 1 m 1 8r h E E o 2 * h * e 2 2 bulk g QD g − + + ≅ 6-1 where, QD g E is the band gap of the quantum dot, bulk g E is the band gap of the bulk material, h is the Planck’s constant, r is the particle radius, * e m is the effective mass of conduction band electron, * h m is the effective mass of valence band hole, e is the elementary charge, ε is the relative permittivity of the quantum dot and o ε is the permittivity of the vacuum [5]. When a quantum dot absorbs a photon, an electron is excited. This electron is confined within the core because the size of the quantum dot is smaller than its exciton Bohr radius. This phenomenon leads to quantum confinement [7]. Interestingly, without altering the chemical composition of semiconductors, the emission wavelength of quantum dots can be tuned by adjusting their size. For example, consider bulk CdSe which has a band gap in the infrared. In contrast, changing the size of CdSe quantum dots from 3nm to 6.5nm can tune its emission from 470nm to 630nm. In addition to the alteration of the band gap, the quantum confinement effect results in the breakdown of the energy bands into discrete energy levels, turning the quantum dots into “synthetic atoms” [2, 8]. Due to these unique properties of quantum dots, there has been a great interest in combining these properties with WGM optical microresonators in applications such as lasing [9-15]. In these studies, quantum dots are either grown by molecular beam epitaxy or spin-coated on microresonators. Consequently, the lasing wavelength of the final device is fixed and cannot be reconfigured. Therefore, the goal was to develop a 126 reconfigurable technique for the attachment of quantum dots on the surface of silica microtoroids. The basis of this approach is a bioconjugation technique that has been developed for the reversible attachment of biotin molecules on the surface of silica microtoroids [16]. The overall technique is summarized in Figure 6-1. First, the surface of the silica is hydroxylated using an oxygen plasma treatment. Then a monolayer of amine-terminated silane is grafted using chemical vapor deposition. Finally, biotin is attached through N- hydroxysuccinimide (NHS) ester chemistry. It is important to note that this process is reversible through oxygen plasma treatment. Figure 6-1 Bioconjugation technique for attachment of biotin on surface of silica. 127 The above-mentioned technique allows covalent attachment of biotin on the surface of silica microtoroids. Biotin is a biomolecule which has very high binding affinity with a protein called streptavidin. Even though this bond is non-covalent, it is very strong and resistant to extreme conditions such as pH and heat. This unique property has been widely used for applications ranging from highly specific protein detection to protein separation and purification [7]. Here, this high affinity is utilized to attach streptavidin conjugated quantum dots to the surface of biotinylated microtoroids. To accomplish this task, quantum dots comprised of a CdSe core with a ZnS shell, conjugated with streptavidin were purchased from Invitrogen. The ZnS shell protects the core from the surrounding environment, photo-oxidation or tunneling of the excited electrons out of the core. In addition, it increases the quantum efficiency to values more than 50% [17, 18]. This core/shell assembly is then coated with an amphiphilic polymer that facilitates the conjugation of streptavidin [19-21]. The overall process of attaching these streptavidin conjugated quantum dots onto the silica microtoroids is shown in Figure 6-2. 128 Figure 6-2 Process of attaching streptavidin conjugated quantum dots onto silica microtoroids. 6.2 Experiment Silica microtoroids were fabricated according to the process explained in Chapter 2. Next, the silica surface of the device was terminated with hydroxyl groups using oxygen plasma treatment for five minutes at 120W and ~200mTorr. Then, the hydroxylated samples were aminated through chemical vapor deposition where they are exposed to 3-aminopropyltrimethoxysilane (APTMS, Aldrich) vapor under vacuum for 15min. The samples were then incubated in a 10mM solution of NHS-Biotin (Pierce) in dimethylsulfoxide (DMSO, anhydrous, Aldrich) at room temperature for 30min. The samples were soaked in deionized (DI) water to remove any physically absorbed biotin from the surface for five minutes. 129 The streptavidin conjugated quantum dot (Invitrogen, Qdot 655, 705) stock solution was centrifuged for three minutes and diluted to a 0.02µM solution in 1x PBS (Phosphate Buffer Saline) prior to use. The samples were incubated in the diluted quantum dot solutions at room temperature for one hour. To dehydrate the samples, they were submerged sequentially for five seconds in 30%, 50%, 70%, 90% ethanol / H 2 O solutions, twice in pure ethanol for five seconds and air-dried at room temperature. In order to check the uniformity of the attached quantum dots they were verified under fluorescence microscope (Nikon, ECLIPSE LV100D-U). To investigate the possibility of reconfiguring the devices, different types of quantum dots were attached and removed from silica microtoroids. Wide field (Nikon) and fluorescence optical microscopy (Nikon, ECLIPSE LV100D-U) were performed after each step of attachment and removal. The optical performance of the fabricated devices was evaluated using the testing setup described in Chapter 2. This process was done at two different wavelengths using two lasers of 765-781nm tunable laser (Newport, TLB6312) and 635-637nm tunable laser (Newport, TLB6304). To capture any emission from the quantum dot conjugated silica microtoroids, the testing setup was equipped with the fiber coupled spectrograph explained in Chapter 5. 6.3 Results and discussion Figure 6-3 shows the optical images along with the fluorescence images of silica microtoroids while different quantum dots (Qdot 655 and 705) were attached and removed. This set of images shows the uniform surface coverage of different quantum dots on a single device with no micron scale damage to the surface of the cavity. The 130 black fluorescence images prove the complete removal of quantum dots after oxygen plasma treatment. One may wonder why this technique does not functionalize the silicon substrate considering the thin layer of native oxide. Research on this topic has shown that this is mainly due to the fluorosilyl species that form on the silicon surface during the XeF 2 etching step in the fabrication of silica microtoroids. In fact, these species interfere with the O 2 plasma and hinder hydroxylation of the surface which translates into no surface functionalization [22]. 131 Figure 6-3 Optical and fluorescence microscope images of a silica microtoroid under consecutive attachment and removal of two different streptavidin conjugated quantum dots. . 132 Figure 6-4 summarizes the quality factor of three silica microtoroids after attachment and removal of quantum dots (Invitrogen, Qdot655). As can be noted, despite functionalizing the surface of the microtoroids and the presence of quantum dots, the quality factor is still above 10 5 . The decrease in the quality factor of the quantum dot bioconjugated microtoroids at 635nm compared to 765nm is caused by the increase in the surface absorption loss due to the presence of the quantum dots. After the removal of the quantum dots the high quality factor of the device is retrieved. This set of data shows that the optical performance of the device does not degrade after attachment and removal of quantum dots. Figure 6-4 Quality factor of silica microtoroids at different stages of attachment and removal of quantum dots (Qdot655). 133 Even though the quality factor of the quantum dot coated microtoroids were promising, no emission was observed on the spectrograph. This could be due to the low absorption of quantum dots at 635nm which translates into no excitation and consequently no emission. At this point, 635nm laser was the lowest wavelength laser available in the lab. However, a few years later, the lab was equipped with a tunable blue laser of 408-413nm (Newport, TLB-6702). Therefore, at this point, the possibility of exciting the quantum dots with this laser was investigated. To simplify the process, CdSe/ZnS core-shell nanocrystals in toluene were purchased from NN-Labs (CZ560-10) [23]. To prepare the samples, toluene (Mallinckrodt Chemicals 99.7%) was added to 10μL of the quantum dot stock solution to reach 1mL volume for the total solution. Then it was spin-coated on the surface of as-fabricated silica microtoroids at 4000rpm for one minute. Finally, the samples were kept on a hotplate set at 120°C for one hour to remove the solvent. Figure 6-5 shows a representative quality factor of a silica microtoroid coated with the CdSe/ZnS quantum dots measured with the tunable blue laser. 134 Figure 6-5 Quality factor of silica microtoroids coated with CdSe/ZnS quantum dots. The emission from the quantum dot coated silica microtoroid collected with the spectrograph is shown in Figure 6-6 as a function of time as the device is pumped with the blue laser with maximum power of 100μW. It can be seen that the signal decays over time and vanishes quickly. 135 Figure 6-6 Emission from the quantum dot coated silica microtoroid as a function of time. In addition to the quick decay of the emission, it was noticed that the center of the emission blue shifts. This effect is shown in Figure 6-7. 136 Figure 6-7 Emission from the quantum dot coated silica microtoroid as a function of time where the center of the emission is found by fitting a Gaussian curve (red line). 137 Previous studies have shown that CdSe/ZnS quantum dots exhibit fast fading emissions along with blue shifting of the emission wavelength in the air as compared to the nitrogen ambient. The exact origin of the fast fading (bleaching) phenomena is not yet well understood. Most likely the formation of lattice defects creates more non-radiative pathways i.e. quenching states. The main reason for blue shift of the emission wavelength is photoinduced oxidation of the quantum dot core (CdSe). This forms non-radiative recombination centers at the interface of CdSe and CdSeO x . As a result of the oxidation, the size of the quantum dot shrinks which results in the blue shifting of the emission wavelength. It is important to note that the oxidization of the core demonstrates that the ZnS shell is not completely closed, which could be due to either grain boundaries or pores due to the oxidation of the ZnS. In fact, when an electron in the CdSe core is excited, it is most likely expelled from the core. At the interface of the core and the shell, a diffusing oxygen molecule scavenges the expelled electron. There is also the possibility that the expelled electron reacts with the oxygen that has been adsorbed onto the outer surface of the ZnS shell [24-26]. 138 6.4 Conclusion In conclusion, a new approach for reversible attachment of quantum dots on silica microtoroids has been developed. This technique utilizes the strong affinity between biotin and streptavidin. Since CdSe/ZnS quantum dots conjugated with streptavidin are commercially available and biotin can be covalently grafted onto silica microtoroids this is an easily reproducible model. Interestingly, this process is reversible through oxygen plasma treatment. Fluorescence imaging after attachment and removal of different types of quantum dots shows minimal damage to the surface of the device. Quality factor measurement also shows the same results. However, lasing emission from the quantum dots attached to the silica microtoroids was not achieved. Experiments on CdSe/Zn quantum dots without any streptavidin, shows a fast fading emission along with blue shifting of the emission wavelength. These effects are related to the photo-oxidation of the quantum dots in the air under continuous pumping. Since, pulsed excitation of CdSe/ZnS quantum dots spin-coated on silica microtoroids have already been studied [26], future work may include testing the devices in a nitrogen environment. 139 Chapter 6 References 1. Bhushan, B., Springer handbook of nanotechnology. 2010: Springer. 2. Klimov, V.I., Nanocrystal quantum dots. 2010: CRC Press Boca Raton, FL. 3. Bimberg, D., Semiconductor Nanostructures. 2008: Springer. 4. Michler, P., Single semiconductor quantum dots. 2009: Springer. 5. Brus, L., Electronic wave functions in semiconductor clusters: experiment and theory. The Journal of Physical Chemistry, 1986. 90(12): p. 2555-2560. 6. Brus, L.E., Electron–electron and electron ‐hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state. The Journal of chemical physics, 1984. 80(9): p. 4403-4409. 7. Hermanson, G.T., Bioconjugate techniques. 2013: Academic press. 8. Flory, F., L. Escoubas, and G. Berginc, Optical properties of nanostructured materials: a review. Journal of Nanophotonics, 2011. 5(1): p. 052502-052502-20. 9. Srinivasan, K., et al., Optical loss and lasing characteristics of high-quality-factor AlGaAs microdisk resonators with embedded quantum dots. Applied Physics Letters, 2005. 86(15): p. 151106-151106-3. 10. Min, B., et al., Ultralow threshold on-chip microcavity nanocrystal quantum dot lasers. Applied physics letters, 2006. 89(19): p. 191124. 11. Reitzenstein, S., et al., Single quantum dot controlled lasing effects in high-Q micropillar cavities. Optics express, 2008. 16(7): p. 4848-4857. 12. Michler, P., et al., Quantum Dot Lasers Using High ‐Q Microdisk Cavities. physica status solidi (b), 2001. 224(3): p. 797-801. 13. Schafer, J., et al., Quantum dot microdrop laser. Nano letters, 2008. 8(6): p. 1709-1712. 14. Cao, H., et al., Optically pumped InAs quantum dot microdisk lasers. Applied Physics Letters, 2000. 76(24): p. 3519-3521. 15. Michler, P., et al., Laser emission from quantum dots in microdisk structures. Applied Physics Letters, 2000. 77(2): p. 184-186. 16. Hunt, H.K., C. Soteropulos, and A.M. Armani, Bioconjugation strategies for microtoroidal optical resonators. Sensors, 2010. 10(10): p. 9317-9336. 17. Reiss, P., M. Protiere, and L. Li, Core/shell semiconductor nanocrystals. small, 2009. 5(2): p. 154-168. 140 18. Hines, M.A. and P. Guyot-Sionnest, Synthesis and characterization of strongly luminescing ZnS-capped CdSe nanocrystals. The Journal of Physical Chemistry, 1996. 100(2): p. 468- 471. 19. Qdot® 625 Streptavidin Conjugate. [cited 2014 July 23]; Available from: http://www.lifetechnologies.com/order/catalog/product/Q22063?ICID=search-product. 20. Qdot Nanocrystals—Section 6.6. [cited 2014 July 23]; Available from: http://www.lifetechnologies.com/us/en/home/references/molecular-probes-the- handbook/ultrasensitive-detection-technology/qdot-nanocrystal-technology.html. 21. Wu, Y., et al., Spectroscopic characterization of streptavidin functionalized quantum dots. Analytical biochemistry, 2007. 364(2): p. 193-203. 22. Biggs, B.W., H.K. Hunt, and A.M. Armani, Selective patterning of Si-based biosensor surfaces using isotropic silicon etchants. Journal of colloid and interface science, 2012. 369(1): p. 477-481. 23. Cadmium Selenide Zinc Sulfide Quantum Dots. [cited 2014 July 23]; Available from: http://www.nn-labs.com/product-catalog/cdsezns/. 24. van Sark, W.G., et al., Blueing, bleaching, and blinking of single CdSe/ZnS quantum dots. ChemPhysChem, 2002. 3(10): p. 871-879. 25. van Sark, W.G., et al., Photooxidation and photobleaching of single CdSe/ZnS quantum dots probed by room-temperature time-resolved spectroscopy. The Journal of Physical Chemistry B, 2001. 105(35): p. 8281-8284. 26. Min, B., Ultrahigh-Q microtoroid on-chip resonators for low threshold microlasers. 2006, California Institute of Technology. 141 Chapter 7 Studying the integration of zinc oxide and silica microtoroids 7.1 Introduction Zinc oxide (ZnO) is a biocompatible direct band-gap semiconductor with large band-gap energy of 3.37eV at room temperature. In addition, it has a high exciton binding energy of 60meV. This value is much larger than its thermal energy of 26meV at room temperature, calculated by finding kT where k is Boltzmann constant= 8.62x10 -5 eV and T is temperature. The combination of these factors makes ZnO a great candidate for various applications such as room temperature ultraviolet (UV) lasing. Specifically, the high exciton binding energy of ZnO enables room temperature operation and the wide band gap allows for UV emission [1-5]. Optically pumped UV emission from ZnO nano-structures and thin films has been demonstrated with pump wavelengths in the UV spectrum [4-10]. M. H. Huang et al. developed the first room temperature ZnO nanowire UV laser (385nm) in 2001 [11]. They grew the nanowires epitaxially using patterned gold thin films. The pump source was the fourth harmonic of Nd: yttrium-aluminum-garnet laser (266nm, 3ns pulse width) with a threshold power of 40kW/cm 2 . In 2004, Liu et al. developed an ultraviolet laser based on silica microdisks coated with thin films of ZnO. They used the third harmonic (λ=355nm) of a mode-locked Nd:YAG laser (10Hz repetition rate, 20ps pulse width) to pump ZnO. Interestingly, whispering gallery mode lasing was observed [12]. Due to the difficulties associated with UV lasers required to pump ZnO, researchers also explored multiple photon absorption schemes to pump ZnO with visible 142 or near-IR lasers [13-30]. Various pump sources that were used include pulsed lasers in the wavelength ranges of 500nm-1400nm with pulse durations of 80fs-8ns and repetition rates of 10Hz-82MHz. Threshold powers were on the order of TW/cm 2 . As explained previously, silica microtoroids provide exceptionally high intensities of circulating optical fields within a small volume. Therefore, the goal of this project was to explore the possibility of integrating ZnO nanowires and thin films with silica microtoroids. The second goal was to explore multi-photon excitation of ZnO to achieve UV emission. 7.2 Experiment Silica microtoroids were fabricated following the process described in Chapter 2. To grow ZnO nanowires, chemical vapor deposition (CVD) was used. This process was carried out inside a tube furnace (Lindberg, Blue M, Model# TF55035A-1) with a quartz tube (Figure 7-1). The chips containing silica microtoroids were placed on a bare silicon substrate. A quartz container was filled with 0.5g metal zinc power (Aldrich 99%). The Zn container and the silicon wafer were placed inside the tube furnace. A vacuum pump was used to drop the pressure to ~0.06Torr inside the tube. Argon gas was flowed in to reach a pressure of 1atm. The pressure was kept constant at 1atm with argon flow rate of 200sccm (standard cubic centimeter per minute) using a mass flow controller (MKS, Model# M100B00423CS1BV). The temperature was raised to 750°C in 20min and held at 750°C for one hour. Once the temperature was stable at this value, oxygen gas (0.5% diluted in argon, 50sccm) was flowed in using a mass flow controller (MKS, Model# M100B00452CS1BV). The reaction duration and the relative location of the Zn container and microtoroid chips determined the morphology and density of the nanowires [31]. 143 Initial experiments were performed on bare silica/silicon chips (no microtoroids) to check for the possibility of growing ZnO nanowires on silica. After this was successfully achieved the process was repeated on silica microtoroids. In addition, in an attempt to incorporate the ZnO nanowires within silica microtoroid, ZnO nanowires were first grown on silica microdisks and then reflowed using the CO 2 laser. The size and distribution of the ZnO nanowires were checked using a scanning electron microscope (SEM) (Hitachi, S4800 and JEOL, JSM 7001F). A spectrofluorometer (Horiba, FluoroMax-4) was used to check the emission from samples under excitation with 325nm light with excitation slit=5nm, and emission slit=2nm. Figure 7-1 (a) Tube furnace where the ZnO nanowires were grown (b) Mass flowmeters used to control the flow rate of argon and oxygen gasses entering the tube furnace. 144 Four different approaches were followed for the preparation of pure ZnO and ZnO: silica sol-gel which are described as follows: A. Sample A: Pure ZnO [32-34]: Zinc acetate dihydrate (Zn(CH 3 COO) 2 .2H 2 O, Alfa Aesar 98%), ethanol (BDH 94%) and monoethanolamine (C 2 H 7 NO, 2-aminoethanol or ethanolamine or MEA, Alfa Aesar 99%) were mixed in a glass vial with molar ratios of Zn(CH 3 COO) 2 .2H 2 O: C 2 H 5 OH: C 2 H 7 NO = 1:35:1. Here, MEA acts as a stabilizer. After each addition, the mixture was stirred for five minutes at 60°C and after all components were added it was stirred at 60°C for two hours. The solution was then aged at room temperature for 24hr. Next, it was passed through a syringe filter (VWR 0.45μm PTFE) and moved to the 4°C fridge to stop further gelation. To spin-coat the sol- gel, it was first left out of the fridge to reach the room temperature. It was then spin-coated on silica microtoroids (previously O 2 plasma treated for five minutes at 120W and oxygen flow rate of 30sccm). The spin-coating was performed at 7000rpm for 30s. The samples were placed on a hotplate set at 300°C for ten minutes. Finally, the samples were thermally annealed at 600°C for two hours (the temperature was increased from room temperature to 600°C in 11.44hr). It is important to note that the sol-gel did not spin-coat well on the surface of silicon or silica/silicon wafers 145 (cleaned with acetone, methanol, isopropyl alcohol and dried with an air gun) which is most probably due to the hydrophobic nature of ZnO [35]. B. Samples B: Pure ZnO [36]: Zinc acetate dihydrate (Zn(CH 3 COO) 2 .2H 2 O, Alfa Aesar 98%), ethanol (BDH 94%), acetic acid (CH 3 COOH, Alfa Aesar 99.7%) and deionized (DI) water were mixed in a glass vial with molar ratios of Zn(CH 3 COO) 2 .2H 2 O: C 2 H 5 OH: CH 3 COOH: H 2 O = 1:85:0.01:11. After each addition, the mixture was stirred for five minutes at 65°C after all components were added it was stirred at 65°C for 4hr. It was then aged at room temperature for two hours. Next, it was filtered using a syringe filter (VWR 0.45μm PTFE) and moved to the 4°C fridge. To spin-coat the sol- gel, it was first left out of the fridge to reach the room temperature. Next, it was spin-coated on silica microtoroids at 4500rpm for 30s. They were then placed on a hotplate set at 75°C for five minutes. The samples were thermally annealed at 400°C for one hour (the temperature was increased from room temperature to 400°C in 16hr). C. Sample C: ZnO:Silica [37]: Pure ZnO solution was first prepared by mixing Zinc acetate dihydrate (Zn(CH 3 COO) 2 .2H 2 O, Alfa Aesar 98%), ethanol (BDH 94%) and monoethanolamine (Alfa Aesar 99%) in a glass vial with molar ratios of Zn(CH 3 COO) 2 .2H 2 O: C 2 H 5 OH: C 2 H 7 NO = 1:55:0.01. After each addition, 146 the mixture was stirred for five minutes at 60°C and finally it was stirred at 60°C for one hour. The pure silica solution was prepared by mixing tetraethoxysilane (TEOS, Si(OC 2 H 5 ) 4 , Alfa Aesar 99.999%), ethanol (BDH 94%), DI water, and hydrochloric acid (EMD 36%) with molar ratios of Si(OC 2 H 5 ) 4 : C 2 H 5 OH: H 2 O: HCl = 1:8.3:0.088:1.4. After each addition, the mixture was stirred for five minutes at room temperature. The two solutions were then mixed (ZnO: SiO 2 = 0.1:1) and stirred for one hour at room temperature. After 24hr of aging at room temperature, the solution was moved to the 4°C fridge. To spin-coat the sol-gel, it was first left out of the fridge until it reached room temperature. Next, it was spin- coated on silica microtoroids (previously O 2 plasma treated for five minutes at 120W and oxygen flow rate of 30sccm) at 7000rpm for 30s and prebaked on hotplate set at 300°C for ten minutes. The samples were thermally annealed at 600°C for two hours (the temperature was increased from room temperature to 600°C in 11.44hr). The solution was also spin- coated on silicon wafers (cleaned with acetone, methanol, isopropyl alcohol and dried with an air gun) for reference. D. Sample D1 and D2: ZnO:silica [36]: Pure ZnO solution was prepared by mixing Zinc acetate dihydrate (Zn(CH 3 COO) 2 .2H 2 O, Alfa Aesar 98%), ethanol (BDH 94%), acetic acid (CH 3 COOH, Alfa Aesar 99.7%) and deionized (DI) water were mixed in a 147 glass vial with molar ratios of Zn(CH 3 COO) 2 .2H 2 O: C 2 H 5 OH: CH 3 COOH: H 2 O = 1:85:0.01:11. The mixture was stirred at 65°C for two hours. The pure silica solution was prepared by mixing tetraethoxysilane (TEOS, Si(OC 2 H 5 ) 4 , Alfa Aesar 99.999%), ethanol (BDH 94%), DI water, and acetic acid (Alfa Aesar 99.7%) with molar ratios of Si(OC 2 H 5 ) 4 : C 2 H 5 OH: H 2 O: CH 3 COOH = 1:14:0.05:4. After each addition, the mixture was stirred for five minutes at 65°C. The two solutions were then mixed (ZnO: SiO 2 = 0.08:1) and stirred for three hours at 65°C. Next, it was filtered using a syringe filter (VWR 0.45μm PTFE) and moved to the 4°C fridge. To spin-coat the sol-gel, it was first left out of the fridge until it reached room temperature. Next, it was spin-coated on silica microtoroids at 7000rpm for 30s (sample D1) and at 4500rpm for 30s (sample D2). They were then placed on a hotplate set at 75°C for five minutes. The samples were thermally annealed at 400°C for one hour (the temperature was increased from room temperature to 400°C in 16hr). The solution was also spin-coated on silicon wafers (cleaned with acetone, methanol, isopropyl alcohol and dried with an air gun) for reference. To verify the chemical structure, Fourier Transform Infra-Red (FTIR) spectroscopy (Bruker, ALPHA-P) was used with 50 scans and a resolution of 2cm -1 . The scanning electron microscope (SEM) images of the ZnO sol-gel samples were taken using a HITACHI tabletop microscope (TM3000). 148 In order to check for the optical performance of the fabricated devices, the testing setup described in Chapter 2 was used. The laser that was used for multi-photon pumping was a continuous wave tunable laser 765-781nm (Newport, TLB6712). To collect the emission from the samples, a fiber-coupled spectrograph was positioned next to the sample similar to the process explained in detail in Chapter 5. The only significant difference between the two processes was that the spectrograph fiber was switched to a model more suitable for the ultraviolet/visible wavelength range (SR-OPT-8014). 7.3 Results and discussion In Figure 7-2, the scanning electron microscope (SEM) images of ZnO grown on bare silica/silicon chips are shown where the duration of oxygen flow was 20min and the distance from the Zn source to the first chip was ~2cm. Therefore, it is possible to grow ZnO nanowires on silica without requiring any catalysts such as gold. In addition, from these images, it can be seen that the distance between the Zn source and the sample affects the morphology of the nanowires. They get shorter and thicker by moving away from the Zn source mainly due to different Zn/O ratio. 149 Figure 7-2 Scanning electron microscope (SEM) images of ZnO grown on silica/silicon chips. Sample (a) is the farthest from the Zn source and sample (c) is the closest to the Zn source. 150 The results of spectrofluorometery on reference silica/silicon wafers without ZnO and with low and high density ZnO nanowires are shown in Figure 7-3. The peaks in the ultraviolet and blue range are due to the near band-edge transitions. The green emission is attributed to ionized oxygen vacancies [1, 3, 15, 38-41]. 151 Figure 7-3 (a) Results of spectrofluorometery measurements on reference wafers with low and high density ZnO nanowires along with a sample without ZnO nanowires (b) Zoomed-in view of the graph in part (a) showing the signal collected from the lower density ZnO sample. 152 After the successful growth of ZnO nanowires on silica/silicon chips, the possibility of growing them on silica microtoroids was investigated. The scanning electron microscope (SEM) images of samples with various reaction times are shown in Figure 7-4. As the reaction time decreases from 20min to 6min, the size and density of the nanowires decreases as well. Since the quality factor is directly related to the intensity of the light circulating inside the device, it was measured for the fabricated samples in Figure 7-4. Samples (a) and (b) did not show any resonance. The quality factors of samples (d) and (e) are shown in Figure 7-5. However, despite high quality factor resonances, no emission was observed from these samples. 153 Figure 7-4 Scanning electron microscope (SEM) images of silica microtoroids with ZnO nanowires grown on them at different reaction times. As moving from (a) to (e) the reaction time drops from 20min to 6min. (a) 20min (b) 15min (c) 8min (e) 6min. Scale bars (a2) 15μm (a3) 2.5μm (b1) 15μm (b3) 1μm (c1) 10μm (c2) 2.5μm (c3) 500nm (d1) 2μm. 154 Figure 7-5 Representative transmission signal and corresponding Q factors, the red and green curves are Lorentzian fits (a) Corresponds to sample d in Figure 7-4 (b) Corresponds to sample e in Figure 7-4. One possible reason for the absence of UV emission from the ZnO nanowires could be insufficient pump power. Even though the intensity of the optical field 155 circulating inside silica microtoroids is very high, only a small portion leaks out of the resonator. Since ZnO nanowires are on the surface rather than inside the resonator, they do not experience such high intensities of the circulating optical field. Therefore, other approaches were examined to incorporate ZnO into the silica microtoroid. In addition to growing ZnO nanowires on silica microtoroids, the possibility of first growing them on microdisks and then reflowing them into microtoroid using CO 2 laser was investigated. The results are shown in Figure 7-6. A sample quality factor measurement is also shown in Figure 7-7. No UV emission was observed from these devices. In addition, one major concern with this approach is that it is unclear how the ZnO nanowires are affected by the CO 2 reflow process. Figure 7-6 Three representative silica microdisks with ZnO nanowires grown on them (top images) and the structures formed after reflowing them with CO 2 laser (bottom images). 156 Figure 7-7 Quality factor of the microtoroid in Figure 7-6b. In Figure 7-8 and Figure 7-9, the results of spin-coating pure ZnO sol-gel on silica microtoroids using approaches A and B are demonstrated, respectively. It is clear that approach B does not degrade the quality factor. However, UV emission was not observed in any samples. As explained before, these solutions did not spin-coat on the surface of reference silica/silicon chips (no microtoroids) and formed small droplets on the surface. This could be due to the hydrophobic nature of ZnO [35]. Based on this fact, it is not clear if ZnO covered the silica microtoroid uniformly or if got trapped in the middle section of the device (Figure 7-8a and b). 157 Figure 7-8 Pure ZnO sol-gel sample fabricated using approach A (a) and (b) scanning electron microscope (SEM) images (c) Optical image (d) Transmission signal showing Q factor of 7.92x10 5 (the red curve is the Lorentzian fit). 158 Figure 7-9 Pure ZnO sol-gel sample B (a) transmission signal showing Q factor of 7.96x10 7 (the red and green curves are the Lorentzian fits) (b) optical image. In Figure 7-10, Figure 7-11, and Figure 7-12, the results of spin-coating ZnO:silica sol-gels on silica microtoroids using approaches C and D are shown. In the case of approach C, devices suffer from major cracking and non-uniformities, which contributed to major scattering losses and consequently no resonant peak. As a result no UV emission was observed from these samples. FTIR measurements on reference silica/silicon chips spin-coated with this sol-gel shows the characteristic peaks of silica (450, 800, 1000-1100cm -1 ) [42-49] and ZnO bonds (480cm -1 ) [50-52]. 159 Figure 7-10 ZnO:silica sol-gel sample fabricated trough approach C (a), (b) Scanning electron microscope (SEM) images (c) Optical image (d) FTIR spectra. In the case of approach D, sample D2, which was spin-coated with a lower speed (4500rpm), shows defects and clumping on the surface. Therefore, the quality factor of sample D1 was only studied. However, no UV emission was observed from these samples. 160 Figure 7-11 ZnO:silica sol-gel sample fabricated through approach D1 (a) Optical image (b) Transmission signal showing Q factor of 8.25x10 5 (red curve is the Lorentzian fit). Figure 7-12 ZnO:silica sol-gel sample fabricated through approach D2 (a), (b) Optical images of two microtoroids. It is important to note that during the experiments involving the spectrograph, various peaks appeared which did not originate from ZnO emission. Different possibilities for this phenomenon are: 161 I. Some peaks appeared on the spectrograph as long as laser light existed near the spectrograph fiber. It did not matter if the laser patch cable was directly pointed toward the spectrograph tip, or if light was coupled from the taper into the microtoroid (similar results were achieved with microdisks and microtoroids). Usually, when the spectrograph fiber was pointed toward the taper while the sample was far (no coupling), these peaks were very weak; however, their intensity increased as the sample was moved closer to the taper. This is due to the increase in scattering of light. Careful attention must be paid to make sure these peaks are not confused with emission from the sample. The main characteristics of these peaks are that, first of all, they shift as the laser wavelength is scanned. Secondly, they exist on both sides of the main laser line (also called parent line). An example of this type of peak is shown in Figure 7-13. These peaks are most probably ghost peaks that originate from defects in ruled gratings [53-57]. 162 Figure 7-13 (a) A representative signal showing the laser peak in the middle and the ghost peaks on both sides of the parent laser peak (b) Demonstration of the movement of the ghost peak on the left side of the laser peak as the laser is scanned over a range of wavelengths (c) Demonstration of the movement of the ghost peak on the right side of the laser peak as the laser is scanned over a range of wavelengths. 163 II. Other peaks that needed more careful attention appeared only when light was on resonance inside the silica microtoroid (similar results were observed with silica microspheres). These peaks only appeared at high input powers, specific couplings and high quality factors. Usually mechanical vibration was observed, too. Since these peaks appeared only under these specific circumstances, it was very easy to confuse them with the emission from the sample. In order to confirm that these peaks were not due to ZnO, bare silica microtoroids were tested. Interestingly, similar peaks appeared. A closer look at these peaks revealed that they appeared only when Raman lasing of the coupled laser light was observed. An example of these peaks is shown in Figure 7-14. Figure 7-14 A representative signal of spectrograph showing peaks that appear only when light is on resonance with ultra-high quality factors and at certain couplings (this graph is from a silica microtoroid with Q factor of 10 8 pumped with 765-781nm laser). 164 7.4 Conclusion In conclusion, ZnO nanowires were grown successfully on the surface of silica microtoroids using a catalyst-free synthesis process. 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Solid State Communications, 1997. 103(8): p. 459-463. 190 Appendix A Phase shift cavity ring-down measurement based biosensor A.1 Introduction Millions of lives and billions of dollars would be saved if a blood test could show the presence of single cancer cells and other biomarkers at early stages of a disease. Currently, there are many researchers all around the world working on developing biodetection techniques that can detect disease biomarkers at very low concentrations. To be useful, these techniques need to be fast, reliable, easy-to-use and have a relatively low fabrication and maintenance cost. Conventional optical detection schemes are either fluorescence-based or label-free. In fluorescence-based sensors, the target biomolecules are initially captured with a targeting molecule and subsequently labeled with fluorescent tags, and the fluorescence intensity indicates the biomarker presence [1, 2]. These techniques are exceptionally sensitive, but they require a pair of molecules: the initial targeting molecule and the fluorescent molecule. This requirement increases the cost of detection. Moreover, pairs of molecules specific to given biomarkers are frequently unavailable. On the other hand, label-free biodetection techniques are able to detect molecules with only the targeting molecule, eliminating half of the reagents required for detection [2]. One of the most promising label-free biodetection methods is based on whispering gallery mode (WGM) optical microresonators. As explained in Chapter 2, specific wavelengths of light that circulate within and close to the surface of these devices are determined by the cavity’s geometry and material. Therefore, when a molecule binds to the surface of the cavity, the confined wavelength will change. Due to 191 the high Q factors in these devices, light will interact with the surface of the microresonator many times, resulting in very high detection sensitivities. Specificity is equally important. Specificity is achieved by functionalizing the surface of the sensor with recognition elements using techniques similar to immunoassays [3]. For example, epoxy-methods [4] and silane chemistries [5] have been developed to add specificity to the detection process of silica microtoroids and microspheres. To date, the majority of the microcavity detection schemes are based on tracking the resonant wavelength as a function of the binding of the biomolecules. While this detection technique has led to significant improvements in this field, it is known to be very sensitive to the intensity fluctuations of the laser source. These sources of noise can be removed by application of cavity ring-down spectroscopy (CRDS) technique [6]. Cavity ring-down spectroscopy has been applied for absorption measurements of gases. Since the rate of the decay of the ring down time is measured instead of the absolute ring down time or photon lifetime in the cavity, the fluctuations in the laser intensity will not add noise to the measurements. In order to find the decay rate of the laser pulse, fitting algorithms are used which add fitting noise. This problem is solved by modulating the laser light intensity using a technique called phase shift cavity ring-down spectroscopy (PS-CRDS) [7]. In this technique, the intensity of the continuous wave laser source is modulated and injected into the WGM microcavity (Figure A-1). As a result of the intensity modulation, the output light from the cavity will experience a phase shift as well as a decrease in modulation depth with 192 respect to the input. These parameters are then related to the rate of decay of the ring down time. Therefore, by applying this method to WGM microresonators, better noise immunity and thus higher sensitivity can be achieved [8, 9]. Figure A-1 The principle of PS-CRDS technique: the intensity of the continuous wave laser source is modulated which results in phase shift in the output light from the cavity as well as decrease in the modulation depth [10]. In PS-CRDS technique, the intensity of light is modulated and injected into the cavity. Here, the intensity of light is modulated in a sinusoidal manner with α as the modulation depth and ω as the modulation frequency. Therefore, the intensity of the modulated light that enters the optical fiber (I input ) can be described by the following equation [8, 9]: I input =I 0 (1+α input sinωt) A-1 193 As explained in Chapter 2, depending on the coupling regime, the amount of light coupled into the cavity changes. Assuming I 1 and I 2 represent the uncoupled light and the coupled light into the cavity, respectively, it can be easily deduced that the modulated light that enters the microcavity is described as [8, 9]: I cavity =I 2 (1+α input sinωt) A-2 Also, the uncoupled light is defined as [8, 9]: I uncoupled =I 1 (1+α input sinωt) A-3 Due to the circulation of the light in the cavity for ring down time of τ, the impulse response is described as below [8, 9]: h(t)=C exp(- t τ )=C � exp � − t − t′ τ � dt′ t − ∞ A-4 Therefore, the power coupled back from the cavity is [8, 9]: I c o up l ed b ac k fro m cav ity = I c avit y h(t) = C � I 2 (1+α input sinωt)exp � − t − t′ τ � dt′ t −∞ A-5 The following equation can be written for the intensity of the light at the output of the fiber as a function of time [8, 9]: 194 I o ut p ut (t)= I un c o up l ed + I c o up l ed back fro m cavity = I 1 (1+α input sinωt)+I c avit y h(t) = I 1 (1+α input sinωt) + C � I 2 (1+α input sinωt)exp � − t − t ′ τ �dt ′ t − ∞ A-6 Assuming the steady state condition, and applying the principle of conservation of energy (i.e. in a certain amount of time (t) total power coupled to the cavity is equal to the power coupled out of the cavity) [8, 9]: I c o up l ed b ac k fro m c avity =I c avity A-7 C � I 2 (1+α input sinωt)exp � − t − t′ τ � dt ′ = t −∞ I 2 (1+α input sinωt) A-8 C � I 2 exp � − t − t′ τ � dt ′ = t −∞ I 2 A-9 C = 1 τ A-10 Substituting the above equations back into Equation A-6 yields [8, 9]: I out put (t)= I 1 (1+α input sinωt)+ I 2 (1+α out put sin(ωt+ φ)) A-11 where 195 tan(φ)= -ωτ/2 A-12 α output = α input √1 + 𝜔 2 𝜏 2 A-13 Consequently, the ring down time τ can be evaluated from the above equation by knowing the phase of the input and output signal. Once the ring down time (τ) is evaluated, the quality factor (Q) of the cavity can be calculated from the below equation [8, 9]: Q= 2 πc τ λ r es o n an c e A-14 Here, in order to investigate and verify the application of the PS-CRDS method for biodetection, streptavidin is selected as the molecule to be detected. This protein is commonly used for characterizing specific biodetection techniques due to its high affinity for biotin [8, 9]. A.2 Experiment Silica microtoroids with major (minor) diameter of ~105µm (5µm) were fabricated following the process described in Chapter 2. This size was chosen to minimize radiation losses in an aqueous environment. In order to add specificity to the detection process of streptavidin, the surface of the silica microtoroids were functionalized with biotin using the bioconjugation process described in Chapter 6 [8]. 196 The schematic of the testing setup is shown in Figure A-2. Light is coupled from a 635-637nm tunable laser (Newport, TLB6304) through a tapered optical fiber into the biotin functionalized microtoroid. This wavelength was chosen due to the low absorption of water at this wavelength, which ensures a high quality factor and therefore higher sensitivity of detection. The taper and the device were immersed in a PBS (Phosphate buffered saline) microaquarium (shown in Chapter 5). The microaquarium was then flushed with 6mL of fresh PBS. Using a function generator (FG 1 ) to modulate the laser wavelength with a triangular wave output (100MHz, 1V peak-to-peak), the resonant peak was located. A second function generator (FG 2 ) was then switched on to provide sinusoidal modulation (13MHz, 4V peak-to-peak) of the intensity of the laser. Here, the phase shift between a reference sinusoid (sinusoidal modulated intensity of the laser before coupling) and the sinusoid at the fiber output indicated that the device was on resonance [8]. Using a syringe pump, 1nM streptavidin solution was injected at the rate of 50μL/min. The syringe pump was turned off after seven minutes as the changes in the quality factor and resonant wavelength were monitored. As explained in Chapter 2, the loaded quality factor depends on the coupled input power. Therefore, all measurements were performed with the taper in contact with the microtoroid. In addition, the depth of the resonant peak was monitored to make sure the coupled power did not play any role in the changes of the quality factor [8]. 197 Figure A-2 PS-CRDS experimental set up where FG: function generator, CW: continuous wave [8]. A.3 Results and discussion Figure A-3a shows a representative transmission signal of the device on resonance. Here the forward scanning peak (FSP) and backward scanning peak (BSP) are symmetric which shows the absence of any non-linear effects during experimental measurements. In Figure A-3b the phase shift in the signal demonstrates that light is on resonance within the microtoroid. Based on the value of the phase shift; the ring down time and the quality factor are evaluated [8]. 198 Figure A-3 (a) Transmission signal through the tapered optical fiber coupling light into the silica microtoroid while light is on resonance (FSP: forward scanning peak, BSP: backward scanning peak) (b) Phase shift experienced by the sinusoid coming out of the microcavity when light is on resonance [8]. 199 Figure A-4 shows the changes in the phase shift over time. As a result of the computer speed limitation, the forward scan peaks and backward scan peaks are separated by about five seconds. It is important to note that the similar value of the phase shifts for these two peaks indicates there are no non-linear effects involved in the experiments. This is similar to the case of the symmetrical resonant wavelength peaks in Figure A-3a [8]. Figure A-4 (a) Changes in the phase shift as a function of time (b) Zoom in view of the graph in part (a) [8]. 200 The results of the changes in the quality factor and resonant wavelength (after the syringe pump was turned off) are shown in Figure A-5. Based on previous studies, the binding kinetics of biotin and streptavidin follow an exponential curve which is in good agreement with the results [8]. 201 Figure A-5 (a) Changes in the quality factor as a function of time during the detection process (b) resonant wavelength shift as a function of time during the detection process [8]. 202 In Figure A-6, a representative measurement of the error signal is shown which is taken with the taper far from the device. The minimum and maximum values of the signal to noise ratios are 12dB and 15dB. Figure A-6 A representative measurement of the error signal, where the taper is moved away from the microcavity: Mean: ±0.2662 o , Variance: 0.1076 o , mode: ±0.3387 o , � Signal noise � min =16, and � Signal noise � max =38 [8]. 203 A.4 Conclusion In conclusion, this work demonstrates the application of phase shift cavity ring- down spectroscopy (PS-CRDS) for detection in aqueous environments. Here, the combination of the high noise immunity of CRDS technique and ultra-high quality factor of silica microtoroids allows for improved biodetection performance. In addition, specific detection of streptavidin protein detection was achieved through reversible bioconjugation of biotin on the surface of silica microtoroids. 204 Appendix A References 1. Hunt, H.K. and A.M. Armani, Label-free biological and chemical sensors. Nanoscale, 2010. 2(9): p. 1544-1559. 2. Fan, X., et al., Sensitive optical biosensors for unlabeled targets: A review. analytica chimica acta, 2008. 620(1): p. 8-26. 3. Hunt, H.K., C. Soteropulos, and A.M. Armani, Bioconjugation strategies for microtoroidal optical resonators. Sensors (Basel). 10(10): p. 9317-36. 4. Hawk, R.M., M.V. Chistiakova, and A.M. Armani, Monitoring DNA hybridization using optical microcavities. Optics letters, 2013. 38(22): p. 4690-4693. 5. Hunt, H.K., C. Soteropulos, and A.M. Armani, Bioconjugation strategies for microtoroidal optical resonators. Sensors, 2010. 10(10): p. 9317-9336. 6. Berden, G. and R. Engeln, Cavity ring-down spectroscopy : techniques and applications. 2009, Chichester, U.K.: Wiley. xix, 322 p., [2] p. of plates. 7. Herbelin, J.M., et al., Sensitive measurement of photon lifetime and true reflectances in an optical cavity by a phase-shift method. Appl Opt, 1980. 19(1): p. 144-7. 8. Cheema, M.I., et al., Simultaneous measurement of quality factor and wavelength shift by phase shift microcavity ring down spectroscopy. Optics express, 2012. 20(8): p. 9090-9098. 9. Cheema, M.I., et al., Erratum: Simultaneous measurement of quality factor and wavelength shift by phase shift microcavity ring down spectroscopy. Optics express, 2013. 21(13): p. 15430-15431. 10. Cheema, M.I., et al. Experimental demonstration of application of ring down measurement approach to microcavities for biosensing. in SPIE BiOS. 2012. International Society for Optics and Photonics. 205 Appendix B Approaches for improving biodetection B.1 Improving the detection specificity utilizing bimodal kinetics As previously discussed in Chapter 2, silica microtoroids can perform label-free biodetection with ultra-high sensitivity [1]. In addition to sensitivity, specificity is essential to a reliable sensor to ensure its operation in complex environments. While sensitivity is inherent to the sensor, specificity is achieved by the attachment of a recognition element specific to the analyte of interest on the surface of the microtoroid [2]. Consequently, it is assumed that the shift in the resonant wavelength is due to the presence of the target analyte. However, this assumption is only valid if the interaction between a given recognition element and its analyte is truly unique, which is rarely achieved [3]. In order to create an ideal recognition element, it is important to understand how the binding site interacts with the analyte of interest. The two factors that enable this process are morphology and chemical interactions. However, in practice, different molecules might bind for a shorter period of time. For instance, smaller molecules can fit into the binding pockets of a large binding site. Interestingly, studying the dissociation constants in these reactions can determine if the correct molecule is attached. Here, the dissociation constant (K d ) defines the affinity between the target analyte and the recognition element. The dissociation constant is related to the resonant wavelength shift through the following expression [3]: ln (λ(t)/λ o ) = -K d t B-1 206 where λ o is the initial resonant wavelength, t is time, and λ(t) is the resonant wavelength at time t. Here, the interaction of streptavidin and biotin are studied because their high affinity makes them a good proof of concept model. The surfaces of the silica microtoroids are biotinylated. Then the shift in the resonant wavelength is tracked while two analytes (streptavidin and streptavidin-labeled polystyrene beads) are injected. These species are identified by measuring the dissociation constants and comparing the results with the dissociation constants of free streptavidin and biotin. Silica microtoroids were fabricated following the process explained in Chapter 2. The surfaces of the silica microtoroids were functionalized with biotin using the reversible non-destructive bioconjugation process described in Chapter 6. The detection experiments were carried out in the testing setup described in Chapter 2. The sample holder was modified with a microaquarium that was explained in detail in Chapter 5. A 765-781nm tunable laser (Newport) was used in these experiments because at this wavelength range water absorption is low, allowing for higher quality factor and, consequently, higher sensitivity in the detection [3]. In order to ensure the conformational stability of streptavidin and biotin, the measurements were done in HEPES buffer (4-(2-hydroxyethyl)-1- piperazineethanesulfonic acid). Once the resonant wavelength was identified, 100fM solution of free streptavidin and streptavidin-labeled polystyrene beads was injected using a syringe pump (0.1μm diameter, Bangs Laboratories, Inc). The continuous flow rate was 100μL/min for two minutes. The resonant wavelength position was tracked 207 using a LabVIEW program which records every 0.5s during the two minutes of injection (association phase) and three minutes after injection (equilibrium and dissociation phase) [3]. In Figure B-1, a representative resonant peak of a biotin functionalized silica microtoroid immersed in HEPES buffer is shown. The quality factor based on the Lorentzian fit to the peak is 3.27x10 6 [3]. Figure B-1 A sample transmission signal while light is on resonance in the device immersed in HEPES buffer. The red curve is the Lorentzian fit to the black signal. The quality factor is 3.27x10 6 which is calculated based on linewidth measurements [3]. 208 The result of resonant wavelength shift over time is shown in Figure B-2. It is important to note that this signal with dual shift is significantly different from detection experiments of free streptavidin, which exhibit only one shift. This is mainly due to the difference in the mass transport and association/dissociation constants of the free streptavidin and the streptavidin-labeled polystyrene beads [3]. Figure B-2 resonant wavelength shift as a function of time while mixture of free streptavidin and streptavidin-labeled polystyrene beads was injected [3]. The dissociation constants of the two phases can be calculated based on the slope of the lines in Figure B-3 and Equation B-1. The slope of the first dissociation phase corresponds to the first dissociation constant (K d1 ) of 2.72x10 -7 . Similarly, the second dissociation constant, (K d2 ) is 8.94x10 -8 . Considering the fact that stronger interaction 209 results in lower dissociation constants, and the fact that streptavidin-labeled polystyrene beads have multiple streptavidin molecules per bead, the second dissociation is related to the streptavidin-labeled polystyrene beads [3]. Figure B-3 Calculation of dissociation constant for K d1 (top graph) and for K d2 (bottom graph) [3]. In conclusion, a novel approach for studying the interaction of recognition elements and target analytes based on measuring dissociation constants has been developed. Here, the surface of the silica microtoroids was functionalized with biotin, and they were used for the detection of a mixture of free streptavidin and streptavidin- 210 labeled polystyrene beads. The detection curve of the resonant wavelength shift shows dual shifts unlike in the case of only free streptavidin. Based on the slope of each dissociation phase, the dissociation constants are calculated, and the nature of the binding analyte in each phase is determined. Using this technique, a single recognition element can be used to distinguish two different analytes. B.2 Controlling binding sites on surface of silica microresonators The surface of silica microtoroids and microspheres can be functionalized with biotin molecules. This bioconjugation process is reversible and non-destructive, which allows for specific detection of streptavidin [2-5]. However, additional research is required to tune the number of binding sites. This is important mainly due to the fact that a dense layer can cause interference between the binding sites. In addition, the detection threshold and response time is affected by the number of binding sites. Therefore, the goal of this project is to develop a method to tune the density of binding sites on silica. The main goal followed in this work was to change the concentration of the biotin solution in the previously developed biotinylation process, described in Chapter 6 [2]. All the experiments were performed on thermally grown silica on silicon wafers that were cut into smaller chips. These samples were rinsed with acetone, methanol, isopropyl alcohol, water, and finishing with one more rinse of isopropyl alcohol. They were then dried using an air gun. The samples were treated using oxygen plasma to covalently graft hydroxyl groups onto the silica surface. Next, a bottle of 3-aminopropyltrimethoxysilane (APTMS, Sigma Aldrich) and the samples were placed in a vacuum desiccator for 25min to allow 211 for chemical vapor deposition (CVD) and attachment of primary amines onto the silica surface. To control the density of the biotin on the surface of the sample, various solutions of biotin were prepared by mixing NHS-LC-Biotin (succinimidyl-6-(biotinamido) hexanoate, Pierce) in DMSO (dimethylsulfoxide, Sigma Aldrich 99.9%) at concentrations ranging from 1µM to 1mM. The samples were then incubated in 250μl aliquots of the biotin solutions in Eppendorf tubes (1ml) for 40min inside a tilt rocker (VWR) set at 5rpm, two degrees and room temperature. To evaluate the amount of biotin attached onto the samples, they were fluorescently labeled with Streptavidin-DyLight 633 (Pierce) molecules. Specifically, samples were incubated in Eppendorf tubes containing 250μl of 5x molar excess Streptavidin-DyLight 633 prepared in 1xPBS (phosphate buffered saline). This step was performed in the dark for 35min on the tilt rocker (5rpm, two degrees) at room temperature. The samples were rinsed with 1xPBS and water to remove any physisorbed streptavidin molecules. Finally, a Nikon fluorescence microscope was used to measure the fluorescence intensity of each sample. For each concentration, three samples were prepared and ten intensity measurements were performed per sample. It is important to note that the intensity of bare silica/silicon wafer was subtracted from all the measurements. The results are shown in Figure B-4. It can be seen that as the concentration of the biotin reacting solution increases, the fluorescence intensity increases which is an indication of the increase in the biotin surface coverage. 212 Figure B-4 Fluorescence intensity versus reacting biotin solution concentration where each point represents three wafers. 213 Appendix B References 1. Armani, A.M., et al., Label-free, single-molecule detection with optical microcavities. science, 2007. 317(5839): p. 783-787. 2. Hunt, H.K., C. Soteropulos, and A.M. Armani, Bioconjugation strategies for microtoroidal optical resonators. Sensors, 2010. 10(10): p. 9317-9336. 3. Shi, C., S. Mehrabani, and A. Armani, Leveraging bimodal kinetics to improve detection specificity. Optics Letters, 2012. 37(10): p. 1643-1645. 4. Soteropulos, C.E., H.K. Hunt, and A.M. Armani, Determination of binding kinetics using whispering gallery mode microcavities. Applied physics letters, 2011. 99(10): p. 103703. 5. Cheema, M.I., et al., Simultaneous measurement of quality factor and wavelength shift by phase shift microcavity ring down spectroscopy. Optics express, 2012. 20(8): p. 9090-9098. 214 Appendix C Thulium cerium co-doped silica microtoroids Ultraviolet (UV) solid state microlasers have potential applications across many disciplines including biodetection, optical computing, communication and data storage. However, the devices developed so far have suffered from various drawbacks such as an incompatibility with silicon processing, costly material depositions and a dependence upon the rigorous requirements of non-linear techniques such as optical parametric oscillation. As explained in Chapter 5, upconversion of near-IR radiation to lower wavelengths using rare earth dopants in silica microtoroids offers a more direct approach. The fabrication process is compatible with silicon processing techniques, and does not require expensive material depositions. Therefore, the focus of this project was to develop an ultraviolet laser using rare earth doped silica microtoroids. Cerium (Ce) is a rare earth dopant that has energy levels required for UV emission [1-18]. Ultraviolet emission at ~380nm in cerium doped YAP (YAlO 3 ) crystal has been achieved upon excitation with an ~800nm femtosecond laser [19]. As noted in Chapter 5, pumping thulium doped silica microtoroids with 1064nm laser generates blue and near-IR emission around 800nm [20]. Therefore, in this project, thulium and cerium were co-doped into silica microtoroids, and the possibility of UV emission was investigated. Similar to Chapter 5, the sol-gel technique was used to incorporate the dopants within the silica microtoroid. Cerium was added in the form of Ce(NO 3 ) 3 .6H 2 O (Alfa Aesar 99.99%). Fourier transform infrared (FTIR) measurements on the fabricated films 215 confirmed the formation of a silica network in the presence of the two dopants (Figure C- 1). Figure C-1 Fourier Transform Infrared (FTIR) spectra of thermally grown silica, pure sol-gel silica and different Tm +3 and Ce +3 co-doped sol-gel silica. The quality factor and lasing emission of doped microtoroids were characterized similar to Chapter 5. A 1055-1070nm tunable laser (Newport, TLB6321) was used for the quality factor measurement as well as for pumping the co-doped devices. Representative quality factor measurement results are shown in Figure C-2. 216 Figure C-2 Transmission spectra for various co-doped silica sol-gel microtoroids. The red curve is the Lorentzian fit. 217 The blue and near-IR lasing from thulium upconversion was observed from all the co-doped samples and a representative spectra is shown in Figure C-3. However, UV lasing from cerium was not observed for any of the samples. It is important to note that even though the quality factor of the Tm +3 and Ce +3 co-doped microtoroids were lower than Tm doped samples, thulium lasing still occurred in the co-doped system. However, no emission from Cerium was detected. Figure C-3 A representative transmission spectra for one of the co-doped silica microtoroids. Silica microtoroids doped with 0.05at.% cerium were also characterized using a tunable laser 765-781nm (Newport, TLB6712). A representative quality factor measurement result is shown in Figure C-4. No UV emission was observed. 218 Figure C-4 A representative transmission of 0.05at.% Ce doped silica microtoroid. The major concern is related to the host medium, silica. As explained in Chapter 5, silica has a high phonon energy, which is not ideal for upconversion. This is especially important for cerium since the electronic transitions happen between the 4f and 5d shells, which makes it more susceptible to the host material, unlike thulium where all the electronic transitions occur within the 4f shell. In the future, integrating cerium in lower phonon energy host media and applying them as a thin film on silica microtoroids could facilitate upconversion of cerium [21]. 219 Appendix C References 1. Pinto, J.F. and L. Esterowitz, Distributed–feedback, tunable Ce 3+ -doped colquiriite lasers. Applied physics letters, 1997. 71(2): p. 205-207. 2. Sugar, J. and J. Reader, Ionization energies of the singly ionized rare earths. JOSA, 1965. 55(10): p. 1286-1290. 3. Wyart, J.-F. and P. Palmeri, Interpretation of the spectrum of Ce III. New energy levels and theoretical transition probabilities. Physica Scripta, 1998. 58(4): p. 368. 4. Ropp, R. and B. Carroll, High energy states of the trivalent rare earths. The Journal of Physical Chemistry, 1977. 81(8): p. 746-755. 5. Russell, H.N., R.B. King, and R. Lang, The Third Spectrum of Cerium (Ce III). Physical Review, 1937. 52(5): p. 456. 6. Shinde, K. and S. Dhoble, 5d–4f transition in new phosphate ‐based phosphors. Luminescence, 2012. 27(1): p. 69-73. 7. Weber, M., Optical spectra of Ce 3+ and Ce 3+ ‐sensitized fluorescence in YAlO3. Journal of Applied Physics, 1973. 44(7): p. 3205-3208. 8. Aitasalo, T., et al., Delayed luminescence of Ce 3+ doped Y 2 SiO 5 . Optical materials, 2004. 26(2): p. 107-112. 9. Jose, M. and A. Lakshmanan, Ce 3+ to Tb 3+ energy transfer in alkaline earth (Ba, Sr or Ca) sulphate phosphors. Optical Materials, 2004. 24(4): p. 651-659. 10. Carnall, W., et al., A systematic analysis of the spectra of the lanthanides doped into single crystal LaF3. The Journal of Chemical Physics, 1989. 90(7): p. 3443-3457. 11. Porter-Chapman, Y.D., et al., Scintillation and Luminescence Properties of Undoped and Cerium-Doped and. Nuclear Science, IEEE Transactions on, 2009. 56(3): p. 881-886. 12. Paulose, P., G. Jose, and N. Unnikrishnan, Energy transfer studies of Ce: Eu system in phosphate glasses. Journal of Non-Crystalline Solids, 2010. 356(2): p. 93-97. 13. Blasse, G. and A. Bril, Investigation of Some Ce 3+ ‐Activated Phosphors. The journal of chemical physics, 1967. 47(12): p. 5139-5145. 14. Ehrlich, D., P. Moulton, and R. Osgood, Optically pumped Ce: LaF 3 laser at 286 nm. Optics letters, 1980. 5(8): p. 339-341. 15. Subramanian, U., et al., Upconversion luminescence of cerium doped CoWO 4 nanomaterials. Journal of Luminescence, 2013. 134: p. 464-468. 16. Coutts, D.W. and A.J. McGonigle, Cerium-doped fluoride lasers. Quantum Electronics, IEEE Journal of, 2004. 40(10): p. 1430-1440. 220 17. Liu, H., et al. Low-threshold broadly tunable miniature cerium lasers. in Advanced Solid- State Photonics. 2006. Optical Society of America. 18. Wang, G., Q. Peng, and Y. Li, Luminescence tuning of upconversion nanocrystals. Chemistry-A European Journal, 2010. 16(16): p. 4923-4931. 19. Yang, L., et al., Three-photon-excited upconversion luminescence of Ce 3+ : YAP crystal by femtosecond laser irradiation. Optics express, 2006. 14(1): p. 243-247. 20. Mehrabani, S. and A.M. Armani, Blue upconversion laser based on thulium-doped silica m icrocavity. Optics letters, 2013. 38(21): p. 4346-4349. 21. De Queiroz, T. B., Ferrari, C. R., Ulbrich, D., Doyle, R., & De Camargo, A. S. S. Luminescence characteristics of YAP: Ce scintillator powders and composites. Optical Materials, 2010. 32(11): p. 1480-1484. 221 Appendix D Titania-silica hybrid microresonators Titanium dioxide (titania) is an appealing material for optical applications. Similar to silica, titania is transparent over a large range of wavelength from visible to near infrared. By adding titania into silica, the refractive index of the hybrid material can be tuned from ~1.45 to ~2.5 at optical communication wavelengths [1]. Previous research has shown the successful application of coating thin layer of silica sol-gel doped with titania on silica microtoroids to tune the behavior of light in the device [2]. The goal of this project is to study the differences between a coated device and a device that is completely made out of titania-silica sol-gel. In Chapter 5, it was shown that microdisks can be fabricated from dense silica sol-gel films. The success of this approach is due to the high selectivity of the XeF 2 gas to silicon over silica. In addition, both thermally grown silica and sol-gel silica have high absorption coefficient at CO 2 laser wavelength (10.6μm). Consequently, upon exposure to the CO 2 laser, silica microdisks melt down and form into microtoroids due to the surface tension. Successful fabrication of titania-silica hybrid microtoroids is highly dependent on the optimization of these two factors Titania sol-gel stock solution was prepared by mixing titanium-n-butoxide (precursor), benzoyl acetone (chelating agent) and ethanol with equal molar ratios. The mixture was stirred for three hours under a nitrogen atmosphere. It was then filtered through a 0.2μm PRFE filter. Silica sol-gel stock solution was prepared by mixing tetraethoxysilane (TEOS), ethanol, and hydrochloric acid (0.01M) with molar ratios of 222 1:3.91:2, respectively. The final solution was stirred at room temperature for two hours. The hybrid titania-silica sol-gel was then prepared by mixing the two stock solutions with molar ratio of silica: titania as 11%:89%. The hybrid sol-gel was spin-coated at 2000rpm on pre-cleaned silicon wafers. The samples were baked in tube furnace at 700°C for eight hours. Hybrid titania-silica sol-gel microdisks were fabricated following the procedure explained in Chapter 2 with two changes. First, wet etching using BOE was not successful. Therefore, instead of BOE, reactive ion etching (RIE) was used. The RIE process was carried out for four minutes under equal flow rates of O 2 and CF 4 (100mTorr, 70W). Second, during the XeF 2 etching, photoresist was left on the micropads. This was because the samples that had no photoresist were etched away; therefore, photoresist was left on to protect the micropads. After the XeF 2 etch, samples were rinsed with acetone, methanol, and isopropyl alcohol and blow dried with N 2 . Scanning electron microscope (SEM) images of the fabricated micropads and microdisks are shown in Figure D-1. 223 Figure D-1 Scanning electron microscope (SEM) Images of hybrid titania-silica fabricated micropads and microdisks [1]. 224 The quality factor of the hybrid titania-silica microdisk was tested using the testing setup discussed in Chapter 2. A 1270-1330nm tunable laser (Newport, TLB6324) was used as the laser source. Images from the top camera and side camera are shown in Figure D-2. Figure D-2 Top view and side view images of a hybrid microdisk (~80μm) next to a tapered optical fiber [1]. In Figure D-3, the broad scan of the device is shown. The highest quality factor is about ~5500 with an FSR of 2.57nm. In addition, a second set of resonant peaks with larger FSR of 3.97nm exists that shows a quality factor of ~2000. This second group corresponds to higher order radial modes resonating further from the periphery of the device. The low quality factors are mainly due to the scattering losses caused by the surface roughness on the edges of the device. 225 Figure D-3 Spectra of transmission through the tapered optical fiber coupling light into the hybrid microdisk (device scan) and the background transmission (taper scan) [1]. To smooth the rough edges of the microdisks and explore the possibility of forming hybrid titania-silica microtoroids, a CO 2 laser was shone onto the fabricated microdisks. Due to the high absorption of titania at 10.6μm, the CO 2 laser wavelength, the microdisks melted down. However, instead of forming microtoroids, they flowed downward to form a blanket-like structure. The scanning electron micrographs of the sample devices are shown in Figure D-4. 226 Figure D-4 Scanning electron micrographs of hybrid titania-silica microdisks after shining a CO 2 laser on them [1]. It is clear that the high titania content (~90%) of the hybrid films will change the material properties significantly from those of a pure silica system. Previous studies on surface tension of pure silica vs. silica-titania sol-gel in liquid phase, shows that silica- 227 titania has lower surface tension [3]. Also, it has been shown that the addition of titania to silica decreases the surface tension [4]. When surface tension is high, the material tends to consolidate into a globular form. In the case of only 11% silica, the film has much lower surface tension and as a result it does not reflow into a toroidal structure. It is important to note that film thickness and the relative concentration of titania to silica could also affect the reflow process which could be the focus of further studies. 228 Appendix D References 1. Mahalingam, H., Titania and hybrid titania – silica sol-gel thin films and their applications in integrated optical devices. 2014, University of Southern California. 2. Maker, A.J., B.A. Rose, and A.M. Armani, Tailoring the behavior of optical microcavities with high refractive index sol-gel coatings. Opt Lett, 2012. 37(14): p. 2844-6. 3. Ulatowska-Jarża, A., et al., Silica-based versus silica-titania sol-gel materials–comparison of the physical properties: surface tension, gelation time, refractive index and optical transmittance. Opt. Appl, 2009. 39: p. 211-220. 4. Hamer, F. and J. Hamer, The potter's dictionary of materials and techniques. 2004: University of Pennsylvania Press.
Abstract (if available)
Abstract
Ever since silica microtoroid optical resonators were developed in 2003, a significant amount of research has focused on utilizing their unique properties in various applications, ranging from fundamental physics research to chemical and biological detection. However, many of these applications require further design improvements for their optimum performance to be fully realized. Additionally, as the technology evolves, new applications are discovered. The focus of this thesis is to outline the development of such optimized microtoroids and their technological role. ❧ While biological detection has been the focal point of optical resonant cavity sensing, monitoring the environment can also benefit from these unique microsensors. Slight variations in environmental factors such as temperature, pressure, humidity and electromagnetic radiations such as ultraviolet, can significantly alter our lives. In addition, these factors play major roles in various industrial processes. Considering the rapid increase in the complexity of these processes, sensors that monitor the environment quickly, reliably, and sensitively will become more important. ❧ In this thesis, it is first shown that silica microtoroids provide an exceptional platform for monitoring different levels of ultraviolet radiation (UV) and relative humidity (RH) in the environment. The second part of this thesis discusses the possibility of utilizing silica microtoroids in creating various types of microlasers, which will improve the sensing performance metrics of silica microtoroids for environmental monitoring and biodetection.
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Creator
Mehrabani, Simin
(author)
Core Title
Development of hybrid microsensors for environmental monitoring and biodetection
School
Viterbi School of Engineering
Degree
Doctor of Philosophy
Degree Program
Chemical Engineering
Publication Date
03/10/2015
Defense Date
10/23/2014
Publisher
University of Southern California
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University of Southern California. Libraries
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Tag
biological microsensor,hybrid organic/inorganic sensor,initiated chemical vapor deposition (iCVD),OAI-PMH Harvest,optical microresonator,phase shift ring down cavity spectroscopy,poly (N-isopropylacrylamide),polymeric thin films,quantum dot,rare-earth elements,relative humidity microsensor,silica,sol-gel,solid state microlaser,thulium,titanium oxide,ultraviolet radiation microsensor,upconversion,whispering gallery mode resonator,zinc oxide
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Armani, Andrea M. (
committee chair
), Sahimi, Muhammad (
committee member
), Thompson, Barry C. (
committee member
), Wang, Pin (
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)
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mehraban@usc.edu,simin.mehrabani@gmail.com
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Tags
biological microsensor
hybrid organic/inorganic sensor
initiated chemical vapor deposition (iCVD)
optical microresonator
phase shift ring down cavity spectroscopy
poly (N-isopropylacrylamide)
polymeric thin films
quantum dot
rare-earth elements
relative humidity microsensor
silica
sol-gel
solid state microlaser
thulium
titanium oxide
ultraviolet radiation microsensor
upconversion
whispering gallery mode resonator
zinc oxide