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Development of hybrid optical microcavities for Plasmonic laser and improving biodetection
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Development of hybrid optical microcavities for Plasmonic laser and improving biodetection
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i DEVELOPMENT OF HYBRID OPTICAL MICROCAVITIES FOR PLASMONIC LASER AND IMPROVING BIODETECTION by Ce Shi 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 (MATERIALS SCIENCE) December 2014 Copyright 2014 ii Acknowledgments I would first like to express my deepest gratitude to my PhD advisor, Professor Andrea Armani, for admitting me and giving me a wonderful research environment, great mentoring for my research projects, and always supporting me for both of my research projects and personal life with many considerations throughout the five years. This thesis won’t exist here without her great help and supports. Through my PhD her solid and broad knowledge about photonics and materials science has helped me a lot and guided me in a right direction of research. When I felt confused or when I came across hurdles about my projects, I would like to talk with her. Discussions and suggestions from her can always spike a good idea to solve my problems. I was also extremely impressed by her patience when she showed me how to perform experiments about label-free biodetection by herself, and when many times she listened to my problems carefully and gave me suggestions and generous help. I still remember she told me that “Patience is a virtue that you must exhibit”. I believe the patience she has shown me will change my way dealing with things in the future and will be a fortune for me all my life. I am very thankful for her for understanding me and trusting me all the time. When I told her I was pregnant during my third year of PhD, I thought she would be mad, but she didn’t. She told me congratulations and I should feel excited because that was a good timing for a woman to be pregnant. She also asked help from my lab mates to prepare samples for me so that I can avoid chemicals, and I was aware of this iii a month later after it happened. She considered for me and prepared everything for me in advance. Through my PhD a couple of times I thought I wouldn’t be able to continue and finish, but I did it finally due to the endless help and supports from her. No matter what I say, I realize it won’t be enough to express how grateful I feel for her. I would like to thank all of my group members in the lab. It has been a great pleasure working with all of you. I would particularly like to thank Simin Mehrabani, Dr. Hongseok Choi, Soheil Soltani and Dr. Imran Cheema for their fruitful collaborations. Special thanks go to Simin Mehrabani for making samples for me during my pregnancy though she was already overwhelmed for her own projects. I owe my deepest thanks to you. I would like to thank Dr. Xiaomin Zhang for being my first mentor here and teaching me all the techniques I needed for my projects. You are such a great mentor! I would also like to thanks Maria Chistiakova, Ashley Maker, Mark Harrison, and Soheil Soltani for helpful discussions. Thank you to Victoria Sun for helping me teaching TA class and sharing so many laughers with me. I would like to thank my friends who I met at USC for sharing helpful discussions and hanging our together during weekends. Many thanks go to Liubing Huang, Xiaolu Ma, Xiaomin Zhang, Hongseok Choi, Lisa Lin, Mengyao Zhang, Yannai Wang, Simin Mehrabani, Hooman, Jing Qiu, Wangxue Deng and many others. iv Last but not the least; I would like to give my deepest gratitude to my family. The biggest thank you to my Mom and Dad who born me, raised up me, and taught me. I won’t get my PhD without their encouragements and supports. Thank you for always encouraging me and listening to me when I feel sad, and always sharing my pleasures when I feel excited. This thesis is for you, my dearest Mom and Dad. Thank you to my sister Ji Shi for all the warmness you provided to me. Thank you to my husband Biliang Hu for meeting me, marrying me, and showing me true love and warmness. I feel so lucky to have you in my life. The sweetest thank you goes to my daughter Alice. Thank you for being so sweet and nice. I love you so much and your happiness means a lot to me. v Table of Contents List of Tables ....................................................................................................................................... vii List of Figures ..................................................................................................................................... viii Abstract ................................................................................................................................................. xiii Chapter 1 Introduction ........................................................................................................................ 1 1.1 Motivation .......................................................................................................................... 1 1.2 Chapter Overview ............................................................................................................ 3 Chapter 1 References ................................................................................................................. 5 Chapter 2 Overview of Optical Microtoroidal Resonators ..................................................... 6 2.1 Introduction ........................................................................................................................... 6 2.2 Background ........................................................................................................................... 8 2.2.1 Quality Factor (Q) ................................................................................. 8 2.2.3Finesse (𝓕) ........................................................................................... 11 2.2.4Circulating Power ................................................................................ 11 2.3 Microtoroid Fabrication .................................................................................................. 12 2.3.2 Fabricate Microdisk By Xenon Difluoride (XeF 2 ) Etching ................ 13 2.3.3 Fabricate Microtoroid By a Carbon Dioxide Reflow ......................... 13 2.4 Device Testing Procedure ............................................................................................... 14 2.4.1 Optical Coupling between Taper and Microtoroid ............................. 14 2.4.2 Quality Factor (Q) Measurement ........................................................ 15 2.4.3 Lasing Data Measurement .................................................................. 17 2.4.4 Label-Free Biodetection ...................................................................... 19 Chapter 2 References ............................................................................................................... 22 Chapter 3 Finite Element Method (FEM) .................................................................................. 24 3.1 Introduction ......................................................................................................................... 24 3.2 Microtoroid Simulations ................................................................................................. 25 3.3 Nanoparticle and Thin Film Coating Simulations .................................................. 30 3.4 Plasmonic Interaction Simulations .............................................................................. 31 3.5 Conclusions ......................................................................................................................... 33 Chapter 3 References ............................................................................................................... 34 Chapter 4 Optical Microcavities With Gold Nanocomposite Thin Film Coating ........ 35 4.1 Introduction ......................................................................................................................... 35 4.2 Synthesis Procedure ......................................................................................................... 37 4.2.1 Synthesis of Thiol-Stabilized Gold Nanoparticles .............................. 37 4.2.2 Solutions of PMMA-Gold Nanoparticles, Characterization ............... 39 4.3.1 Applying PMMA-Gold thin film onto microtoroid ............................ 41 4.3.2 Characterization of PMMA-Gold Thin Film ...................................... 42 4.4 Results and Discussion .................................................................................................... 45 4.4.1 Comsol Simulation and Theoretical Q ................................................ 45 4.4.2 Q measurements Results ..................................................................... 47 4.4.3 Photon Lifetime .................................................................................. 50 vi 4.5 Conclusions ......................................................................................................................... 51 Chapter 4 References ............................................................................................................... 52 Chapter 5 Gold Nanorod Plasmonic Upconversion Microlaser ......................................... 54 5.1 Introduction ......................................................................................................................... 54 5.2 Motivation ............................................................................................................................ 55 5.3.2 Transfer of Gold Nanorods from Water into Toluene ........................ 59 5.3.3 Fluorescence Emissions of Gold Solutions ......................................... 60 5.4 Plasmonic Simulation ...................................................................................................... 64 5.4.1 Plasmonic Interaction between Gold Nanorods and Microtoroid ....... 64 5.5 Upconversion Microlaser Measurements .................................................................. 70 5.5.1 Typical Emission Spectra ................................................................... 70 5.5.2 Threshold Power for Upconversion Laser .......................................... 71 5.7.1 Layer-by-Layer Method ...................................................................... 80 5.7.2 Characterization of Toroidal Surface .................................................. 81 5.7.3 Optical Testing .................................................................................... 82 Chapter 5 References ............................................................................................................... 86 Chapter 6 Leveraging Bimodal Kinetics to Improve Detection Specificity ................... 88 6.1 Introduction ......................................................................................................................... 88 6.2 Motivation ............................................................................................................................ 88 6.3 Unspecific Biodetection .................................................................................................. 90 6.4 Surface Functionalization Method for Specific Biodetection ............................ 93 6.5 Results and Discussion .................................................................................................... 97 6.5.1 Bimodal Spectra for Two Species Biodetection ................................. 97 6.5.2 Discussion of Kinetics ........................................................................ 99 6.6 Conclusions ...................................................................................................................... 101 Chapter 6 References ............................................................................................................ 102 Chapter 7 Future Work .................................................................................................................. 104 Chapter 7 References ............................................................................................................ 107 Appendix A: Optimizing the Signal to Noise Ratio of Microcavity Sensors .............. 108 A1 Introduction ...................................................................................................................... 108 A2 Theoretical Simulation ................................................................................................. 110 A3 Experiments and Discussions ..................................................................................... 112 A4 Conclusions ...................................................................................................................... 117 Appendix A References ....................................................................................................... 118 vii List of Tables Table 3-1 The advantages (A)/ disadvantages (D) of FEM and FDTD ........................... 25 Table 4‐ 1 Summary of spectroscopic ellipsometry results and theoretical calculations .................................................................................................................................................................. 45 Table 4‐ 2 Summaries of Model and Experimental Fitting Parameters ................... 50 Table 4‐ 3 Conversion of experimental quality factors to photon lifetime .................... 50 viii List of Figures Figure 2- 1 Various types of the WGM optical resonators based on different geometries . ............................................................................................................................................ 7 Figure 2- 2 Overview of the fabrication process. a) Silica pad after photolithography and BOE etching, (b) Silica disk after XeF 2 etching, (c) Silica microtoroid after CO 2 laser reflow. .......................................................................................................................................... 12 Figure 2‐ 3 An example of a pulled single mode tapered optical fiber. We can clearly see the rainbow color on optical fiber. This is due to an optical filtering effect. .......... 15 Figure 2‐ 4 The schematic of a resonator testing set-ups. To measure the quality factor, the light from the laser is coupled into the resonator using a tapered optical fiber and is detected using a photo detector. The signal is recorded using a PCI digitizer/oscilloscope which is integrated into the computer. The laser wavelength is controlled using a GPIB card and function generator, also integrated into the computer. The coupling between the waveguide and resonator are visualized using top and side view machine vision systems. All data and images can be recorded using the computer. ....................................................................................................................................... 16 Figure 2‐ 5 Transmission spectra of microtoroid coated by spinning PMMA solution doped with 0.0224Mol/L gold nanoparticles. ........................................................................... 17 Figure 2‐ 6 The schematic of a lasing testing set-ups. To detect lasing, the side view camera is replaced by the fiber coupled spectrograph. The signal from the spectrograph goes to the computer for data acquisition. ....................................................... 19 Figure 2‐ 7 Schematic of testing set up for biosensor. A tapered fiber is used to couple laser into the device. A cover slide is placed above the wafer to form a chamber, where is filled with 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer. A syringe injects protein solutions into this chamber from a direction perpendicular to the wafer. ........................................................................................... 21 Figure 3‐ 1 Cross section of microtoroid is drawn in COMSOL Multiphysics in accordance with the major/minor diameter (D/d). The whole domain is divided into numerous triangle elements, and the area of boundary is assigned with denser meshes to get accurate details about mode distribution. ....................................................................... 27 Figure 3‐ 2 One example for the 115 th TE polarized mode operated at 1553nm wavelength, for the microtoroid with major/minor diameter as 40µm/3µm. (a) cross-sectional electric field intensity distribution of microtoroid; (b) the corresponding radial field intensity distribution as a function of radius. ........................ 28 Figure 3‐ 3 Cross section of microtoroid with nanocomposite thin film coating is drawn in COMSOL Multiphysics in accordance with the major/minor diameter (D/d). The thickness of nanocomposite film can be drawn based on experimental ellipsometry measurements. ............................................................................................................ 30 Figure 3‐ 4 FEM simulations of the optical field distribution. a) The normalized radial optical field intensity as a function of radius for silica (solid black line) and for ix 10% PMMA-nanoparticles nanocomposite film (red dashed line). .................................. 31 Figure 3‐ 5 Schematic of FEM simulations for gold nanorod coated microtoroids. ... 32 Figure 3‐ 6 COMSOL Multiphysics finite element method simulations of the transverse magnetic mode of the microtoroid interacting with a 3.5 AR gold nanorod. The operating wavelength is 780nm that matches the Plasmonic resonance of gold nanorods. ............................................................................................................................................... 33 Figure 4‐ 1 Schematic of synthesis for Thiol- stabilized gold nanoparticles. First a gold hydrosol is synthesized. Then, to replace the –OH on the surface with –SH, a small amount of 1-dodecanethiol is added, and the gold was transferred completely to toluene. ................................................................................................................................................... 38 Figure 4‐ 2 A Picture of reaction solution. The upper layer is toluene with purple color while the below layer is water without any color. This indicates that almost all of gold nanoparticles are transferred from water into toluene phase. ............................... 39 Figure 4‐ 3 UV-Vis spectra of gold nanoparticles stabilized by (black) –OH group, (blue) –SH group without PMMA, and (red) –SH group with PMMA solutions. ....... 40 Figure 4‐ 4 Gold concentrations for the solutions with the volume ratio at 0:100, 5:100, 10:100 and 20:100. In other words, gold solutions volume percentages are 0, 4.76, 9.09, and 16.67, respectively. It shows concentration changes almost linearly with gold volume percentage. Results are taken by the inductively coupled plasma-atomic emission spectrometry. ....................................................................................... 41 Figure 4‐ 5 a) Artistic rendering and b) optical image of the gold coated hybrid devices. Gold nanoparticles suspended in a PMMA solution are coated onto the microtoroid surface. The major diameter for the microtoroid is approximately 50µm. The gold nanoparticles are too small to visualize in this optical image. .... 42 Figure 4‐ 6 FEM simulations of the optical field distribution. a) The normalized radial optical field intensity as a function of radius for silica (solid black line) and for 10% PMMA-nanoparticles nanocomposite film (red dashed line). The zero point indicates the center of the minor diameter. This graph was determined from the optical field distribution for (b) silica microtoroid and (c) nanoparticles-polymer coated microtoroid with a 30nm thick film. The device size is 50(5) µm major (minor) diameter and operating wavelength is 635nm. ......................................................................... 47 Figure 4‐ 7 Transmission spectra of microtoroid coated by spinning PMMA solution doped with 0.0224Mol/L gold nanoparticles. ................................................... 48 Figure 5‐ 1 Typical UV-Vis spectra of the series of gold nanorods with longitudinal Plasmonic wavelength tuned from 650nm to 850nm. ............................................................ 60 Figure 5‐ 2 UV-Vis spectra of the gold nanorods in toluene and the nanorods in toluene with PMMA. ......................................................................................................................... 61 Figure 5‐ 3 Spectrofluorometer spectra of the nanorods and PMMA in toluene solution. The nanorods can be easily excited at both 760nm and 780nm, emitting at 550nm. ................................................................................................................................................... 62 Figure 5‐ 4 Dependence of emission wavelength on refractive index of solution. All solutions are excited at 780nm. ..................................................................................................... 62 x Figure 5‐ 5 Pictures of toroidal optical resonant cavities. a) Rendering of a toroidal resonant cavity coated with gold nanorods. b)/c)/d) Scanning electron microscopy images of a gold nanorod coated cavity at different magnifications. ............................... 63 Figure 5‐ 6 Schematics of the COMSOL Simulation investigating interaction between gold nanorod and optical field. ...................................................................................................... 65 Figure 5‐ 7 COMSOL Multiphysics finite element method simulations of the transverse magnetic mode of the microtoroid interacting with a 3.5 AR gold nanorod. The gold nanorod (outlined in white) is (a/c) parallel or (b/d) perpendicular with the boundary of the microtoroid. The surface Plasmon resonance of the nanoparticles is only excited when the optical resonance of the cavity overlaps with the optical absorption of the nanoparticles. ..................................................................................................... 66 Figure 5‐ 8 The dependence of the cavity resonance frequency on the refractive index of the coating. ...................................................................................................................................... 67 Figure 5‐ 9 As the coating refractive index increase, the maximum mode field inside the microtoroid decreases. ............................................................................................................... 68 Figure 5‐ 10 The dependence of the field size on the coating index. .............................. 68 Figure 5‐ 11 The dependence of the effective mode radius on the coating refractive index. ...................................................................................................................................................... 69 Figure 5- 12 Dependence of Plasmonic enhancements on coating index. ...................... 70 Figure 5- 13 Typical lasing spectra of the nanorods coated microcavities pumped at different powers. ................................................................................................................................. 71 Figure 5- 14 Lasing spectra and (inset) threshold data of the gold nanorod coated microtoroid cavity. The gold nanorods are excited by the evanescent field from microtoroid and lase at 581.5nm. The wavelength difference of the hybrid device from solution is caused by the slight refractive index change between the solution and the solid thin film. A threshold power as low as 20 µw is achieved. ................................ 73 Figure 5- 15 The dependence of quality actor and lasing threshold power on the nanoparticles dilution factor. .......................................................................................................... 75 Figure 5- 16 Typical Raman lasing (a) spectra and (b) threshold power for gold nanorods coated microtoroid. ......................................................................................................... 77 Figure 5- 17 Raman spectra from gold nanorods coated microtoroid. The ratio of intensities among different peaks can be tuned by varying the coupling. ....................... 79 Figure 5- 18 A schematic showing the Au NRs coating through polymer electrostatic attractions. ............................................................................................................................................. 81 Figure 5- 19 a) SEM of Au NRs on silica microtoroids and b) Atomic force microscope (AFM) image of Au NRs on a silica wafer. ....................................................... 81 Figure 5- 20 A typical transmission spectra at a) 780nm, and b) 633nm. ....................... 82 Figure 5- 21 A typical emission spectra from bare silica microtoroid and Au coated microtoroid. Both were pumped by a tunable 780nm laser. ................................................ 83 Figure 5- 22 Threshold powers of emissions at 420nm, 520nm, and 600nm changing with different Au concentrations on the surface. ..................................................................... 84 Figure 5- 23 Efficiency of emissions at 420nm, 520nm, and 600nm changing with different Au concentrations on the surface. ............................................................................... 84 xi Figure 6- 1 Background signal when buffer is flowed into the chamber including microtoroids immersed in buffer at the flowrate of 100ml/min. We changed the minor diameter of microtoroids to be (a) 6µm and (c) 8µm, which results in shifts of (b) 0.003nm and less than (d) 0.001nm, respectively. .................................................................. 91 Figure 6- 2 Resonant shifts when injecting streptavidin into chamber. We performed experiments using different modes of the same microtoroid. The Q of these modes are 2E6, 4E6, and 6E6, respectively. We have detected streptavidin solutions with different concentrations. Though shifts become stronger when concentration increases with all four concentrations (1pM, 10fM, 500aM, and 10aM), difference is bigger at smaller concentration range. ........................................................................................................... 92 Figure 6- 3 Resonant shifts when injecting streptavidin into chamber. We performed experiments using different frequencies of function generator from 400mVPP to 1000mVPP using a same microtoroid. We have detected streptavidin solutions with different concentrations.. The resonant shifts are bigger at higher frequency of function generator. ............................................................................................................................. 93 Figure 6- 4 Schematic of biotinylation process on silica microtoroids. ........................... 94 Figure 6- 5 Schematic of the detection process. (a) The surface of the silica microtoroid is biotin-functionalized. The free streptavidin in solution binds to the microtoroid first. Once the free streptavidin begins to dissociate, the streptavidin-polybeads, which contain numerous streptavidin molecules per bead, can bind. (b) A scanning electron micrograph of a biotin functionalized microtoroid with major (minor) diameter at 105µm (5 µm). ................................................................................. 96 Figure 6- 6 A representative transmission spectrum. The Q factor is calculated to be 3.27×10 6 . ............................................................................................................................................... 98 Figure 6- 7 Detection based on K d . The resonant wavelength shifts as the solution containing the streptavidin and the streptavidin polybeads is injected. Two clearly identifiable peaks are observed. ..................................................................................................... 99 Figure 6- 8 Calculation of K d1 by the slope (above picture) and calculation of K d2 (below picture). ................................................................................................................................ 100 Figure 7- 1 Schematic of enhanced label-free biodetection owing to Plasmonic enhancement of noble metals. (a) Rendering of Ag Nanoparticles coated microtoroid, and (b) is schematic of enhanced biodetection. It is anticipated that resonance shifts due to attachment of protein will get enhanced significantly. .......................................... 105 Figure A- 1 Several ways for label-free biodetection using microcavities, including (a) resonant wavelength shift method [6], resonant peak split method [12], and heterodyne and beat note method [7] ........................................................................................ 109 Figure A- 2 Study of noise levels versus different Q both theoretically and experimentally. (a)Monte Carlo simulations (MC) of individual noise sources sL-Laser wavelength instability.( b)Total wavelength noise, theoretically (solid line) and experimentally (dots); (c)A representative experimental histogram of counts versus minima shift; (d)Total quality factor noise, σQ. ..................................................... 113 Figure A- 3 Experimental quality factors of Microtoroids with various major xii diameters in water. The minor diameter is fixed at 6±0.5mm. ........................................ 114 Figure A- 4 Resonant shift with time when NaCl (black line) and Water (red line) are flowed into chamber containing microtoroids respectively. Solution starts to flow into the chamber since the initial point and stops at 20mins. .................................................... 115 Figure A- 5 Signal analysis both theoretically and experimentally. .............................. 116 Figure A- 6 Signal/Noise analysis both theoretically and experimentally. .................. 117 xiii Abstract High quality factor microcavities can serve as a good platform for microlaser, label free biosensor, and fundamental physics study due to its high circulating power. This thesis mainly investigates possibilities for developing Plasmonic nanoparticles and polymer thin film coating microcavities, and demonstrating the optical interaction between optical filed and Plasmonic nanoparticles. A new type of upconversion microlaser based on two photon upconversion of gold nanorods was invented. Performances such as threshold power of the new type laser were demonstrated and optimized. Besides, this thesis has also demonstrated improved performances of label-free biosensor based on microcavities by combining the kinetics constants or varying quality factors of microcavities which were used for detection. In this thesis, it is first demonstrated the possibilities of permanently coating gold nanoparticles and polydimethylmethacrylate (PMMA, M.W. 35,000) thin film on microcavities with quality factor over than 10 7 . The quality factor was studies both theoretically and experimentally, proving materials loss was the main loss mechanisms. Then low threshold Plasmonic upconversion microlaser is demonstrated where Plasmonic resonance of gold nanorods overlaps with optical resonance of microcavities. The Plasmonic interaction between optical field and gold nanorods are investigated theoretically using 3-D FDTD (Finite Domain Time Domain) simulations. Finally this thesis shows improved performances of label-free biosensor based on resonant shift method of microcavitie 1 Chapter 1 Introduction 1.1 Motivation Ultra-high Quality factor (Q) whispering gallery mode optical microcavities have been used in numerous applications ranging from fundamental physics investigations to telecommunications, owing to their long photon lifetime which results in high circulating optical fields[1-2]. Over the past decade, hybrid microcavities formed by integrating microcavities with functional materials, such as biological molecules, nanoparticles, or polymers, has drawn great attention and paved the way for biosensors and studying the fundamental material properties [3-4]. However, despite numerous studies on these devices, there has been limited research to improve whispering gallery mode biosensors by combining them with Plasmonic resonance of noble nanoparticles. Optical resonators are an ideal platform to integrate with a Plasmonic gain medium and to build a microlaser due to their enhanced circulating power. While microcavities can be fabricated in much geometry, in the present thesis, the silica microtoroid resonator is used as it can be fabricated on a chip in batch, which makes it suitable for handling and testing in most telecommunications applications. The hybrid microtoroid consists of a passive silica cavity integrated with various functional materials which can result in numerous applications in biosensing and studying physics. For example, organic/inorganic hybrid microtoroids can be created by coating microtoroids with a thin film of polymer, which exhibits many advantages over pure silica devices. For example, this type of device has enabled the 2 development of an a thermal or temperature insensitive resonant cavity. In addition, the polymer coating can also behave as a host matrix for nanoparticles. The interaction of the optical field with the coated nanocomposite makes it possible to excite the Plasmonic modes of nanoparticles that can enhance the optical field of the nanoparticles. Therefore, improved biosensors with significant larger signal-to-noise ratios are expected. In addition, lasers can be made from either noble metal nanoparticles or from nanoparticles coated with fluorescent dye molecules. Combining the Finite Element Method (FEM) simulations and the optical device experiments, we studied the interaction between the optical mode and both gold nanoparticles and gold nanorods. We have also demonstrated a novel Plasmonic microlaser with low threshold based on two-photon emission of gold nanorods. In the biodetection realm, we functionalized the surface of silica microtoroid cavities with biotin to perform label free detection with high accuracy. Conventionally, detection is performed by monitoring the resonant frequency of the cavity as molecules bind. However, in the present work, we more thoroughly analyzed the kinetics buried in the detection signal. Specifically, by discriminating between different dissociation constants and mass transport values, we were able to distinguish between streptavidin and avidin- coated beads. 3 1.2 Chapter Overview The organization of this thesis is as follows: Chapter 2 gives the background about optical microcavities and whispering gallery mode microcavities in general. The characteristics for the optical resonators, such as quality factor and mode volume, are introduced. The detailed fabrication procedures to make the microtoroid optical resonators are explained as well as the importance of each fabrication step. Testing procedures, such as quality factor measurements, label free biodetection methods, and lasing measurements, are explained. Chapter 3 explains how to simulate the optical resonators in both 2-D and 3-D using COMSOL Multiphysics FEM. It shows how to determine the optical field distribution, radial intensity profile, resonant frequency, different modes and how to determine the interaction between optical mode and gold nanoparticles. It also shows how the polymer thickness affects the Plasmonic interaction between optical field and nanoparticles. Chapter 4 investigates optical microcavities with a Thiol -functionalized gold nanoparticles polymer thin film coating. The theoretical calculations are initially performed to determine how the gold nanoparticles concentration affects the quality factor, based on the hypothesis of the dominant loss mechanism being material loss. Then, we experimentally verify the theoretical Q calculations by coating the resonator with polydimethylmethacrylate (PMMA, M.W. 35,000) and different nanoparticles concentrations. 4 Chapter 5 demonstrates an optical microcavity with gold nanorods and PMMA polymer thin film coating. A theoretical study of the interaction between the cavity’s optical field and the gold nanorods is performed. We also experimentally develop a method for coating high quality factor toroidal optical cavities with gold nanorods, forming a photonic-Plasmonic laser. Chapter 6 demonstrates a method to improve specificity of label free biodetection by combining resonant shift detection and analysis of kinetics of dissociation. Using microtoroids functionalized with biotin, the dissociation constant of the biotin and the two different streptavidin complexes (free and polystyrene bead) is investigated. The two complexes are identified from a mixture based on the different kinetics constants. Chapter 7 discusses potential future directions for research, including topics in biodetection and device development. This could serve as a roadmap for current or future students. Appendix A presents the results of collaboration with Dr. Imran Cheema, an EE PhD student at McGill University, which investigated the optimum parameters of microtoroid to improve the S/N (signal/noise) of biodetection. It is revealed both theoretically and experimentally that the signal of biodetection decreases as the major diameter increases, while noise decreases as the major diameter increase as well. Therefore, it is anticipated that an optimum device size can be found to get the highest S/N. 5 Chapter 1 References 1. H. K. Hunt, A. M. Armani, Nanoscale, Label-free biological and chemical sensors, 2, 1544 (2010). 2. K. J. Vahala, Nature, Optical microcavities, 424, 839 (2003). 3. A. M. Armani, R. P. Kulkarni, S. E. Fraser, R. C. Flagan, K. J. Vahala, Science, Label-free, single-molecule detection with optical microcavities, 317, 783 (2007). 4. T. Aoki et al., Nature, Observation of strong coupling between one atom and a monolithic microresonator, 443, 671 (2006). 6 Chapter 2 Overview of Optical Microtoroidal Resonators 2.1 Introduction Compared to the confined modes of a Fabry-Perot or a distributed feedback resonator, a Whispering Gallery Mode (WGM) microcavity has its distinct feature where light is bounced back at the surface of the device due to total internal reflection. In the past decades, WGM microcavities have been extensively studied for lasing, biodetection and telecommunication due to their low loss and ease of fabrication [1-3]. The concept of the term “whispering gallery mode” came from the Whispering Gallery in St. Paul’s Cathedral in London, and was first theorized in the form of an acoustical wave by Rayleigh [4-5]. In this gallery, a whisper at one side of the gallery can be heard all along the inside perimeter of the dome. Light behaves in the same manner in WGM type cavities, orbiting along the perimeter. Unlike waveguides, optical resonators can only confine light of wavelengths that are integral multiples of the device circumference. Those wavelengths are called resonant wavelengths. These cavities can be made in numerous geometries and from many different materials [6-10]. Figure 2-1 gives an overview of these devices. After a decade of research on transient liquid droplets induced by surface induced tension, a solid-state version of the spherically shaped cavity was successfully made by melting the tip of a fiber to create a silica microsphere, and a quality factor as high as 8×10 9 [9]. The quality factor is an indicator of the photon lifetime inside the cavity. As a result, this 7 device enabled research areas such as micro-laser development, biosensors, and non-linear physics. More recently, as a result of advances in fabrication technology, additional numerous geometries have been developed. The most widely used are rotationally symmetric structures such as cylinders, toroids, and disks, which have been shown to support very high-Q whispering-gallery (WG) modes whose field intensity is mainly confined near the device-air interface. Resonator Type Features Resonator Type Features Microsphere Wedged Microdisk Microtoroid CaF2 Toroid Chemically Etched Microdisk Flat Microdisk/ Microring Q~8×10 9 Micron-Sized, Lefevre-Seguin, V., Opt. Mater., 1999 Q>10 5 ~10 7 Micron-Sized, T. J. Kippenberg,Keryy Vahala, Appl. Phy. Lett. 2003 Q>10 8 Micron-Sized, D. K. Armani, Kerry Vahala, Nature, 2013 Q>2×10 10 mm-Sized, Lute Maleki, Phys. Rev. A, 2004 Q>10 4 ~10 6 Micron-Sized, Baba, T. et al, IEEE Photon. Technol.Lett. 1997,Heinrich Kurz et al, Opt. Lett. 2004 Q>10 8 mm-Sized, Hansuek Lee, Kerry Vahala, Nature Photonics, 2012 Figure 2-1 Various types of the WGM optical resonators based on different geometries 8 Though silica microspheres exhibit an ultra-high (nearly 9 billion) Q-factors, yet have a very dense spectrum of multiple-degenerate WG modes, and is not compatible with on-chip integration with other components. Other devices, such as the wedged silica microdisks and the silicon nitride flat microdisks, are also fabricated on a chip but have significantly lower quality factors which limit their applications. The highest Q devices, the CaF 2 toroidal resonators, are large (mm diameter), which limits their on a chip integration applications. WGM microcavities made of other materials such as Si 3 N 4 or Silicon on insulator (SOI) have also been developed in the past few decades. However, their Q is much lower compared to the silica microtoroid due to material absorption and carrier losses. Based on these facts, in this thesis we choose to focus on the silica microtoroid. 2.2 Background 2.2.1 Quality Factor (Q) When wavelengths of lights are an integral multiple of the device circumference, the light is confined within the periphery of microcavity and considered on-resonance. The cavity geometry, material, and environment determine the resonant wavelength of the microcavity. The length of time that the photon is confined within the cavity is characterized by the photon lifetime (τ) or quality factor (Q). A higher Q implies a longer photon lifetime [11-13]. 9 The quality factor of dielectric cavities is governed by a series of loss mechanisms, both intrinsic and extrinsic to the microtoroid. These losses can be summarized by the following expression: Q !"! !! =Q !"# !! +Q !! !! +Q !"# !! +Q !"#$ !! +Q !"#$ !! (2.1) where Q rad is quality factor determined by the radiation loss, Q ss is quality factor determined by the surface scattering loss, Q mat is quality factor determined the material loss, Q cont is quality factor determined the contamination loss, and Q coup is quality factor determined by the coupling loss. The first four terms are intrinsic to the cavity (Q 0 ), and the fifth is extrinsic to the cavity (Q ext ). Depending on the device structure, the primary loss mechanism in air can be either Q mat or Q ss , and it is possible to calculate these terms. However, for a microtoroid in water, Q mat is typically the primary loss. Q mat is the quality factor determined by the material loss[14]. 𝑄 !"# = !!! !"" !! !"" (2.2) Where λ is the wavelength, n eff is the effective refractive index, α eff is the effective material loss. In a hybrid system, to calculate n eff and α eff, all loss components need to be considered by multiplying the value of each component by its optical mode percentage. Q SS is the quality factor determined by surface scattering induced by roughness of microcavities. Q SS is comprised of surface scattering and internal scattering. Q ss and Q is can be expressed by[14]: 𝑄 !! = ! !!! !! ! ! !! !"" ! ! ! ! ! ! (2.3) 10 where K defines the internal reflection condition, σ and B are the surface roughness of the cavity, n eff is the effective refractive index, λ is the wavelength, and R is the radius of the cavity. 𝑄 !" = !!! ! !! ! ! ! !"! ! ! !"" ! (2.4) where T is the melting temperature, p, κ and β are the Pockels coefficient, Boltzman constant, and isothermic compressibility at room temperature. 2.2.2 Mode Volume Microtoroids confine light both temporally in the form of Q and spatially in the form of the optical mode volume (V). A smaller mode volume means a denser optical field confinement, making it more suitable for fundamental research or device development. It is normally described as below: 𝑉 !"#$ = ! ! ! ! ! ! ! ! !"# ! (2.5) Where V Mode , 𝜀 𝑟 and 𝐸 represent the mode volume, the permittivity and the electric field, respectively. An approximate analytic formula for the nonlinear mode volume has been found by Braginsky et al. [12]: 𝑉≈ 3.4𝜋 !.! ( ! !!" ) ! 𝑙 !! ! 𝑙−𝑚+1 (2.6) Where λ, n, D, l, and m are the wavelength, refractive index, cavity diameter, azimuthal quantum number and magnetic quantum number.For microspheres when l=m (fundamental mode), V∝𝐷 !! ! ! ! ! ! . 11 2.2.3Finesse (𝓕) Finesse (ℱ) is the ratio of the cavity mode spacing to the cavity bandwidth. This definition factors in both the cavity loss (Q) and the cavity mode spacing (FSR) to obtain a dimensionless single parameter that characterizes the ability to resolve the cavity resonance structure. ℱ= !! !"# ! ! = ! !"# (2.7) 2.2.4Circulating Power The circulating power (P circ ) in the cavity is proportional to the quality factor, device diameter and the input power. It is the reason that higher Q devices with smaller diameter have lower lasing thresholds. The circulating power is theoretically described by: 𝑃 !"#! =𝑃 !" 𝑄 ! ! ! !" ! !!! ! (2.8) Where 𝜅 is the coupling coefficient, 𝜆 is the wavelength, R is the device radius, P in is the input power, Q is the quality factor, and n is the effective refractive index. The coupling coefficient can be determined from the transmission spectrum (T) using the below relation: 𝑇= !!! ! !!! ! (2.9) 12 2.3 Microtoroid Fabrication When the microtoroid was invented in 2002, one of the significant advances was the ability to fabricate it in arrays using conventional lithographic methods [6]. To fabricate microtoroids, three simple steps are used: photolithography and buffered oxide etchant (BOE) etching, xenon difluoride (XeF 2 ) etching and carbon dioxide (CO 2 ) laser reflow, as is shown in Figure 2-2. Figure 2- 1 Overview of the fabrication process. a) Silica pad after photolithography and BOE etching, (b) Silica disk after XeF 2 etching, (c) Silica microtoroid after CO 2 laser reflow. 2.3.1 Define Micropads by Patterning Oxide Layer The silicon wafer with a 2µm oxide layer was placed in a hexamethyldisilazane (HMDS) exposure set up for 2 minutes to ensure the adhesion between the oxide layer and the photoresist. S1813 photoresist (Shipley) was coated on the wafer using a 4000rpm spin speed for 1minutes using a spin coater. The sample was exposed with a circular pattern mask with intensity of 4mJ/cm 2 for 20secs with Karl Suss MJB 3 photomask aligner. Afterwards we use MF-321 (Shipley) to develop the sample for 30s. This process defines a clear circular pattern. After a DI water rinse, followed by N 2 gas drying, we finally wet etch the sample with improved buffered oxide etchant (BOE) for 19 minutes. To ensure the oxide layer was etched completely, the sample is 13 monitored carefully and checked with optical microscopy. The sample was finally cleaned with acetone, methanol and isopropyl-alcohol to ensure its clear surface. 2.3.2 Fabricate Microdisk By Xenon Difluoride (XeF 2 ) Etching The silica is undercut using XeF 2 , an isotropic silicon etchant. The xenon difluoride is highly selective to silicon and it is isotropic with fast etching rate. So the silica will not be disturbed. To achieve a smooth etch, the number of chips put into the chamber should be controlled (Normally 8-12 chips depending on the size of chips): too many chips results in insufficient exposure of the silicon to XeF 2 , while too few chips can cause rapid etching and a waste of the XeF 2 source. By controlling how much silicon is undercut, the major and minor diameter of the microtoroid can be adjusted. 2.3.3 Fabricate Microtoroid By a Carbon Dioxide Reflow In the final step to make a silica microtoroid, a carbon dioxide (CO 2 ) laser is used to reflow or melt the microdisk, forming a microtoroid. The wavelength of the CO 2 laser (10.6µm) is absorbed by silica while it is transparent to silicon. During reflow, edge of silica pad is melted and a toroid is formed. The toroid is attached to the silicon pillar by the silica membrane. 14 2.4 Device Testing Procedure 2.4.1 Optical Coupling between Taper and Microtoroid Various coupling devices have been developed during the last decade such as prism couplers, side polished fiber couplers, angle polished fiber tips, pedestal antiresonant reflecting waveguides, and tapered fiber couplers. Among these, the tapered fiber was the most successful coupler to ideally couple light into device and afterwards couple the light out [15]. Using a tapered fiber, the coupling loss could be negligible, resulting in minimal extrinsic loss. We use a hydrogen torch to pull the fiber and monitor the whole pulling process using a camera. To pull a taper, we first strip the cladding layer of a bare fiber, clean the stripped part using methanol, and then place the fiber to the fiber holder. We lit the hydrogen torch, pull the taper using a motorized stage, and simultaneously monitor using a camera. When the desirable thickness is achieved, we stop the pulling process. An example of a tapered fiber is shown in figure 2-3. The rainbow color is from light diffraction of the thin fiber (around 700nm). More details about taper pulling set up and procedure is available through a video [16]. 15 Figure 2- 2 An example of a pulled single mode tapered optical fiber. We can clearly see the rainbow color on optical fiber. This is due to an optical filtering effect. 2.4.2 Quality Factor (Q) Measurement There are two methods to determine the Q of a resonant cavity. One is a ring down method based on the fact that the quality factor is the photon lifetime times the resonant frequency. The second is the linewidth method based on measuring the resonance spectra and using the expression: Q=𝜆/Δ𝜆 where λ is wavelength on resonance, and Δλ is the full width at half maximum (FWHM) of the spectra. We use the second method in this thesis to test the quality factor[17]. Using the same testing set-up, we also perform lasing and biodetection experiments (Fig 2-4). 16 The quality factor of the device is measured by coupling light from a tunable narrow linewidth laser (Velocity series, Newport) into the cavity using a tapered optical fiber waveguide. Tapered fiber waveguides are a low loss (high efficiency) method for coupling light into and out of optical cavities. The tapered waveguide is aligned with the cavity using a high precision 3-axis nano-positioning stage and is monitored on top and side view machine vision system simultaneously. This approach allows the coupling between the resonator and the waveguide to be precisely controlled, and all coupling regimes (under-coupled through over coupled) can be achieved. Figure 2- 3 The schematic of a resonator testing set-ups. To measure the quality factor, the light from the laser is coupled into the resonator using a tapered optical fiber and is detected using a photo detector. The signal is recorded using a PCI digitizer/oscilloscope which is integrated into the computer. The laser wavelength is controlled using a GPIB card and function generator, also integrated into the computer. The coupling between the waveguide and resonator are visualized using top and side view machine vision systems. All data and images can be recorded using the computer. To determine the Q, the transmission spectrum is recorded using a high-speed digitizer/oscilloscope (National Instruments) in the under-coupled regime. During the quality factor measurements, the laser scan rate and range are controlled using a 17 function generator and are optimized to minimize any nonlinear effects (e.g. thermal broadening) that might distort the shape of the resonance. The loaded Q is calculated by fitting the spectra to a Lorentz and using the expression Q=𝜆/Δ𝜆 whereΔ𝜆 is the full-width-half-maximum determined from the fit. It should be noted that the resonant wavelength of the cavity is dependent on the specific properties of that cavity, such as the refractive index of the cavity (silica), the refractive index of the polymer-nanoparticles coating (if one is present), the environment, and the geometry. As a result, each cavity has a unique resonant wavelength. Therefore, figure 2-5 is a representative spectra of a microtoroid coated by a PMMA film doped with 0.0224Mol/L gold nanoparticles at 983nm[18]. Figure 2- 4 Transmission spectra of microtoroid coated by spinning PMMA solution doped with 0.0224Mol/L gold nanoparticles. 2.4.3 Lasing Data Measurement To characterize the lasing behavior, the microtoroids and tapered fiber were first aligned to the optimum position. We verify the alignment position is good when we 18 got the highest Q of fundamental mode. Through broad scan, a single device has different modes within a free spectra range. Though fundamental mode has highest Q, in our experiments we find it is not necessarily the best mode to excite lasing. In fact, the mode that has most significant thermal broadening and mechanical vibration is the best mode to get high efficiency lasing. The reason is microtoroid is on resonance with longer time fractions compared to other modes. After we align the microtoroid and taper, the side view camera is replaced with a fiber coupled spectrograph (Andor) (Figure 2-6), and the fiber is aligned with the microtoroid [19]. The position of the spectrograph fiber is optimized using the input tunable laser signal as a reference. We calibrate the offset of the spectrograph’s grating based on the input tunable laser’s wavelength. This calibration procedure is expected based on Andor’s protocols. The testing set-up is enclosed in a black-out curtain to reduce background noise. A background measurement is performed before beginning the experiment and is used to normalize the threshold measurements. The lasing threshold is determined by changing the input power to the toroid using an optical attenuator. The maximum detectable signal on the spectrograph is 65,000 counts. 19 Figure 2- 5 The schematic of a lasing testing set-ups. To detect lasing, the side view camera is replaced by the fiber coupled spectrograph. The signal from the spectrograph goes to the computer for data acquisition. The input power is the power which is coupled into the resonator. This power is distinct from several other power values, such as the circulating power in the resonator or the power from the laser to the waveguide-cavity system. The input power takes into account the amount of the laser power which is coupled into the resonator. For example, at critical coupling when 100% of the power is coupled into the resonator, the coupled input power would equal the laser power. However, in the majority of our experiments, approximately 50% of the power from the laser was coupled into the cavity; therefore, the output power was similarly decreased. 2.4.4 Label-Free Biodetection To perform biodetection experiments, the microtoroid was first glued on a cylinder sample holder, and then aligned with the taper to the optimum position. We verify the alignment position is good when we got the highest Q of fundamental mode. 20 Two pieces of thick glasses were glued at the end of the holder in advance, and one piece of cover glass was glued onto these thick glasses and formed a chamber [20]. (2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer was injected using a syringe by hand and the chamber was immersed in the HEPES buffer thoroughly. Then the broad scan and fine scan were performed again to obtain the fundamental mode in buffer. The reason we chose fundamental mode is because it has the highest Q, and the most efficient interaction with its environment. Additionally, we can accurately model the fundamental mode. The microtoroid was kept on resonance of the fundamental mode throughout the experiments. Biomolecules dissolved in buffer were injected using a syringe pump. As molecules attach onto the toroidal surface, the resonant wavelength of microtoroid began to shift. We use a customized Labview program to record the resonant shifts with time. It also recorded the minimum peak depth to void those shift caused by coupling change or coupling lost. One thing worth mentioning, the coupling might be lost completely during recording data and therefore testing has to be started over. When this happens, we carefully flush the chamber with HEPEs buffer thoroughly and wait till the resonant wavelength becomes stable before starting over. 21 Figure 2- 6 Schematic of testing set up for biosensor. A tapered fiber is used to couple laser into the device. A cover slide is placed above the wafer to form a chamber, where is filled with 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer. A syringe injects protein solutions into this chamber from a direction perpendicular to the wafer. 22 Chapter 2 References 1. A. M. Armani, R. P. Kulkarni, S. E. Fraser, R. C. Flagan, K. J. Vahala, Science, Label-free, single-molecule detection with optical microcavities, 317, 783 (2007). 2. B. Min, T. J. Kippenberg, K. J. Vahala, Opt Lett, Compact, fiber-compatible, cascaded Raman laser, 28, 1507 (2003). 3. F. Blom, D. Van Dijk, H. Hoekstra, A. Driessen, T. J. Popma, Applied physics letters, Experimental study of integrated-optics microcavity resonators: Toward an all-optical switching device, 71, 747 (1997). 4. L. Rayleigh, The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, CXII. The problem of the whispering gallery, 20, 1001 (1910). 5. L. Rayleigh, The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, IX. Further applications of Bessel's functions of high order to the Whispering Gallery and allied problems, 27, 100 (1914). 6. D. K. Armani, T. J. Kippenberg, S. M. Spillane, K. J. Vahala, Nature, Ultra-high-Q toroid microcavity on a chip, 421, 925 (2003). 7. T. Kippenberg, S. Spillane, D. Armani, K. Vahala, Applied physics letters, Fabrication and coupling to planar high-< equation> Q</equation> silica disk microcavities, 83, 797 (2003). 8. U. Mohideen, W. Hobson, S. Pearton, F. Ren, R. Slusher, Applied physics letters, GaAs/AlGaAs microdisk lasers, 64, 1911 (1994). 9. L. Collot, V. Lefevre-Seguin, M. Brune, J. Raimond, S. Haroche, EPL (Europhysics Letters), Very high-Q whispering-gallery mode resonances observed on fused silica microspheres, 23, 327 (1993). 10. H. Lee et al., Nature Photonics, Chemically etched ultrahigh-Q wedge-resonator on a silicon chip, 6, 369 (2012). 11. M. L. Gorodetsky, A. A. Savchenkov, V. S. Ilchenko, Optics Letters, Ultimate Q of optical microsphere resonators, 21, 453 (1996). 12. V. Braginsky, M. Gorodetsky, V. Ilchenko, Physics Letters A, Quality-factor and nonlinear properties of optical whispering-gallery modes, 137, 393 23 (1989). 13. S. Spillane, T. Kippenberg, K. Vahala, Nature, Ultralow-threshold Raman laser using a spherical dielectric microcavity, 415, 621 (2002). 14. M. L. Gorodetsky, A. D. Pryamikov, V. S. Ilchenko, JOSA B, Rayleigh scattering in high-Q microspheres, 17, 1051 (2000). 15. J. Knight, G. Cheung, F. Jacques, T. Birks, Optics Letters, Phase-matched excitation of whispering-gallery-mode resonances by a fiber taper, 22, 1129 (1997). 16. Armani Research Group, USC http://www.youtube.com/watch?v=XEF9q_thbOg, (2011). 17. X. Zhang, H. S. Choi, A. M. Armani, Applied physics letters, Ultimate quality factor of silica microtoroid resonant cavities, 96, 153304 (2010). 18. C. Shi, H. Seok Choi, A. M. Armani, Applied physics letters, Optical microcavities with a thiol-functionalized gold nanoparticle polymer thin film coating, 100, 013305 (2012). 19. M. V. Chistiakova, A. M. Armani, Optics Letters, Cascaded Raman microlaser in air and buffer, 37, 4068 (2012). 20. A. M. Armani, K. J. Vahala, Optics Letters, Heavy water detection using ultra-high-Q microcavities, 31, 1896 (2006). 24 Chapter 3 Finite Element Method (FEM) 3.1 Introduction Though analytic calculations are the preferred way to investigate the properties of any system, it is not always possible to develop a fully analytical expression. A substitution to a mathematical calculation is using Finite Element Method (FEM). In this approach, the whole domain is divided into small elements. Each individual element, which is typically a triangular area in two dimensions and pyramidal in three dimensions, is governed by the equation to be solved in the whole domain and is related to all neighboring elements through the governing equation of the problem to be solved, such as Helmholtz’s equation for Eigenvalue/ Eigenfunction problems in electromagnetics. Therefore, each element is related to its neighboring elements and results in a matrix form. This matrix form then is solved to obtain the problem solution to the whole domain. FEM can be utilized to solve problems from heat transfer or mass transfer to optical field distribution. FEM has a number of advantages compared to other methods simulations such as Finite Difference Time Domain (FDTD), especially when solving high-Q resonators. The advantages/disadvantages of FEM and FDTD are listed in Table 3-1 [1]. In this thesis, Finite Element Method simulation is leveraged to simulate optical field distributions in silica and nanocomposite hybrid microtoroid [2]. It is also used to investigate the Plasmon interaction between gold nanoparticles and the optical field [3]. 25 FDTD (Finite-difference time-domain method) FEM (Finite-Element Method) D: Hard to model curvilinear functions without either choosing a nonstandard grid or very densely gridding then entire computation region to minimize the “staircase effect” A: Very easily allow the elements to match the local geometry boundaries. D: Need to use larger amount of memory compared to FEM A: The ability to vary the element size over the computational region results in a much smaller amount of data that must be stored for data accuracy. Therefore, FEM has a lower requirement for computer memory. D: High computational time A: The computational time is dramatically lower than FDTD. A: Fully explicit, and can handle most types of electromagnetic problems, including current sources and nonlinearities. D: The programmed electromagnetics tools are not directly applicable to solve the resonant spectrum of azimuthally-symmetric derived which do represent the correct problem. A: The core algorithms are relatively simple, allowing easy implementation and parallelization. A: Can be used to calculate the radiation loss of rotationally- symmetric structures with high accuracy. A: Besides electromagnetic problems, FEM is commonly used to simulate physical systems, from heat-flow to structural mechanics. Table 3-1 The advantages (A)/ disadvantages (D) of FEM and FDTD 3.2 Microtoroid Simulations As the microtoroids have axisymmetric structure, the complexity can be reduced to a two-dimensional problem with rotational symmetry. Simulations of the bare microtoroids are made based on Mark Oxborrow’ s model [4].First of all, the cross section of the microtoroid is drawn in COMSOL Multiphysics in accordance with the major/minor diameter (D/d). The silica, which is assumed to be isotropic, 26 was set with a refractive index based on literature. The operating wavelengths can also be changed under this category by controlling the azimuthally mode order number “M”. M is related to wavelength based on the following expression for circular WGM optical microcavities: !"!" ! =M (3.1) Where n is the refractive index of the material for optical resonator, R is radius of the optical resonator and λ is operating wavelength. 27 Figure 3- 1 Cross section of microtoroid is drawn in COMSOL Multiphysics in accordance with the major/minor diameter (D/d). The whole domain is divided into numerous triangle elements, and the area of boundary is assigned with denser meshes to get accurate details about mode distribution. The whole domain is then divided into small elements of triangular areas. To get results with accurate details, boundary between periphery of microtoroid and air is assigned with denser mesh elements (Figure 3-1). 28 Figure 3- 2 One example for the 115 th TE polarized mode operated at 1553nm wavelength, for the microtoroid with major/minor diameter as 40µm/3µm. (a) cross-sectional electric field intensity distribution of microtoroid; (b) the corresponding radial field intensity distribution as a function of radius. (a) (b) 29 To consider the effect of radiation loss of a microcavity that is due to the curved cavity boundary in the azimuthally direction coupled with an infinite outer boundary, an artificial boundary was set as a perfectly-matched layer (PML)[5]. PML possesses the properties of an infinite boundary and appears transparent and absorbs all incident optical energy, such that there is minimal energy reflected back into the domain of interest. One example of cross-sectional electric field intensity is shown in Figure 3-2 (a). The result is for the 115 th TE polarized mode operated at 1553nm wavelength, for the microtoroid surrounded by free space with the major/minor diameter set as 40µm/3µm.The corresponding radial field distribution as a function of the radius also can be determined as can be seen Figure 3-2 (b). It can be seen that most of the optical field is confined within the periphery, while only a small amount evanesces into the environment. The n, l, and m represent the radial, angular, and azimuthal mode numbers, respectively. 30 3.3 Nanoparticle and Thin Film Coating Simulations To simulate nanocomposite coated optical microcavities, a thin film with certain thickness is added onto the toroidal surface (Figure 3-3). To study the effect of nanocomposite films on the optical field distribution, gold nanoparticles with poly methyl methacrylate (PMMA) was treated as an integral. The refractive index of silica is taken from literature while the refractive indices for the PMMA-nanoparticles thin films are experimentally determined by ellipsometry. The resonant frequencies or wavelengths are controlled by the azimuthally mode M. Figure 3- 3 Cross section of microtoroid with nanocomposite thin film coating is drawn in COMSOL Multiphysics in accordance with the major/minor diameter (D/d). The thickness of nanocomposite film can be drawn based on experimental ellipsometry measurements. 31 Optical field of silica microtoroid and nanocomposite thin film coated microtoroid are compared. One example of radial field intensity distribution as a function of radius is shown in Figure 3-4 (a). The corresponding cross-section field distributions are shown in Figure 3-4 (b-c). It can be seen that gold nanocomposite film has affected the optical field and a larger fraction of the optical field intensity evanesces out of the microtoroid. Figure 3- 4 FEM simulations of the optical field distribution. a) The normalized radial optical field intensity as a function of radius for silica (solid black line) and for 10% PMMA-nanoparticles nanocomposite film (red dashed line). 3.4 Plasmonic Interaction Simulations 32 To thoroughly study Plasmonic interaction between nanoparticles and optical modes, we use a 3-D COMSOL Multiphysics finite element method to model the Plasmonic enhancement of the circulating optical field. Because of the asymmetry of the nanorods and the toroidal cavity and the wide range of length-scales, this specific structure is particularly complex to model [6]. Figure 3- 5 Schematic of FEM simulations for gold nanorod coated microtoroids. The gold nanoparticles are placed on the central part of a slice of the resonator with an arclength of approximately λ/2 with mirrored faces, where λ is the resonant wavelength (Figure 3-5). One example of a cross- section field intensity distribution is shown in Figure 3-6. It is noticeable that the optical field around gold nanorods are enhanced significantly due to Plasmonic interaction. 33 Figure 3- 6 COMSOL Multiphysics finite element method simulations of the transverse magnetic mode of the microtoroid interacting with a 3.5 AR gold nanorod. The operating wavelength is 780nm that matches the Plasmonic resonance of gold nanorods. 3.5 Conclusions In summary, it has been shown how to simulate optical microtoroids and study optical field distributions. Starting from the simple model, the more complicated hybrid system of a silica and gold nanocomposite can be simulated as shown above. We can also leverage 3-D COMSOL simulation to study the Plasmonic interaction between gold nanorods and optical mode thoroughly. 34 Chapter 3 References 1. S. M. Spillane, California Institute of Technology (2004). 2. C. Shi, H. Seok Choi, A. M. Armani, Applied physics letters, Optical microcavities with a thiol-functionalized gold nanoparticle polymer thin film coating, 100, 013305 (2012). 3. C. Shi, S. Soltani, A. M. Armani, Nano letters, Gold nanorod plasmonic upconversion microlaser, (2013). 4. M. Oxborrow, in Lasers and Applications in Science and Engineering. (International Society for Optics and Photonics, 2007), pp. 64520J-64520J-12. 5. Z. S. Sacks, D. M. Kingsland, R. Lee, J.-F. Lee, Antennas and Propagation, IEEE Transactions on, A perfectly matched anisotropic absorber for use as an absorbing boundary condition, 43, 1460 (1995). 6. A. Kaplan et al., arXiv preprint arXiv:1305.0555, Finite element simulation of a perturbed axial-symmetric whispering-gallery mode and its use for intensity enhancement with a nanoparticle coupled to a microtoroid, (2013). 35 Chapter 4 Optical Microcavities With Gold Nanocomposite Thin Film Coating 4.1 Introduction Ultra-high-quality factor (Q) whispering gallery mode optical resonators have been used extensively to study non-linear physics and biophotonics and have been a key element in communications systems [1-2]. The ubiquitous nature of these devices arises from the long photon lifetime or Q, which results in a high circulating optical field inside the cavity. Similar to other types of optical resonators, the optical field is only partially confined within the device and evanesces into the environment; therefore, it strongly interacts with both the cavity and the surface of the cavity. By coating a silica device with a polymer film, it is possible to form a hybrid organic-inorganic device, which exhibits many advantages over pure silica devices. For example, they can be used to stabilize and remove many of the non-linear effects that typically plague pure silica devices [3]. They can also be used to study the fundamental behavior of the polymer film. However, all previous work using hybrid devices has focused on combining silica or silicon devices with simple homopolymers [4]. If the polymer film is used as a matrix for nanoparticles, it will enable numerous investigations into the interactions between microcavities and nanoparticles [5]. Additionally, unlike nanoparticles, which are simply physio-adsorbed to the surface, the system will be stable for significant periods of time, allowing for storage of the devices. 36 One of the key challenges in fabricating such a platform device is it is critical to maintaining a high quality factor and a strong optical field overlap with the embedded nanoparticles [6]. In the present study, different concentrations of gold nanoparticles in polymethylmethacrylate (PMMA, Sigma Aldrich, M.W. 35,000) thin films are coated on a toroidal whispering gallery mode resonant cavity device, and the impact of the hybrid film on the device performance (cavity Q factor) is experimentally determined and compared to simulation results [7]. 37 4.2 Synthesis Procedure 4.2.1 Synthesis of Thiol-Stabilized Gold Nanoparticles Based on previous research results, polymethylmethacrylate (PMMA) is chosen as the host for the gold nanoparticles [8]. Though PMMA can be dissolved in ethanol or methanol, after spin coating PMMA in ethanol/methanol solution onto toroidal surface, visible defects were noticed on microtoroids, and the Q was dropped below 100 thousand. And only PMMA in toluene can form a smooth thin film on silica surface after spin coating by spinner. It has been shown that Q factors as high as 1x10 7 can be achieved with PMMA thin film coatings. Although PMMA can be easily dissolved in toluene, the standard synthesis for gold nanoparticles occurs in water [9]. Therefore, after synthesizing the –OH group stabilized gold nanoparticles in water, it is necessary to transfer them into toluene phase. This can be achieved by adding mixture of toluene and a small amount of 1-dodecanethiol (Sigma Aldrich, 98% purity) [10]. Then the gold nanoparticles were transferred completely from water into toluene and stabilized by a Thiol group (Figure 4-1). Synthesis of Gold Nanoparticles in water: the gold nanoparticles are synthesized by combining Hydrogen Tetrachloroaurate (HAuCl 4 , Sigma Aldrich) with a freshly prepared sodium borohydride (NaBH 4 , Sigma Aldrich) aqueous solution. While stirring, add 0.2 mL of 0.11M NaBH 4 drop-wise into a beaker of 10 mL of 2 mM HAuCl 4 Solution. The gold nanoparticles form as the NaBH 4 reduces the Au (III). The resulting nanoparticles solution consists of gold nanospheres suspended in water. 38 Figure 4- 1 Schematic of synthesis for Thiol- stabilized gold nanoparticles. First a gold hydrosol is synthesized. Then, to replace the –OH on the surface with –SH, a small amount of 1-dodecanethiol is added, and the gold was transferred completely to toluene. Thiol Functionalization of Gold Nanoparticles: The gold nanoparticles are functionalized with a thiol group by adding 10ml toluene mixed with 10 mg of 1-dodecanethiol into the prepared solution in the last step. Next, 2mL of concentrated hydrochloric acid was added immediately. After 5min of stirring, two miscible layers formed The upper level is toluene with gold nanospheres in it, while the bottom layer is water without nanospheres. We can tell based on the color that almost all gold nanospheres are stabilized by thiol groups and transferred into the toluene (Figure 4-2). A series of freshly made toluene solutions containing gold nanoparticles were mixed 1% weight PMMA solution at different volume ratios (0:100, 5:100, 10:100, and 30:100). Therefore, the concentrations of gold nanorods vary in these series of solutions. 39 Figure 4- 2 A Picture of reaction solution. The upper layer is toluene with purple color while the below layer is water without any color. This indicates that almost all of gold nanoparticles are transferred from water into toluene phase. 4.2.2 Solutions of PMMA-Gold Nanoparticles, Characterization Material Absorption Spectra: All solutions of gold nanoparticles functionalized by –OH group and –SH group were characterized using UV-Vis. As can be seen in the spectra in Figure 4-3, there is a clearly identifiable peak at 530nm for –OH stabilized gold nanospheres and 520nm for -SH stabilized gold spheres before mixing with PMMA, which corresponds to a well-known absorption wavelength of gold particles at radius of 17nm~20nm. After mixing with PMMA solution at ratio 2:10, the peak shows a red shift to around 524nm, which is the result of the particles being transferred to a higher refractive index media. It is important to note that the solution still has only a single peak, indicating that the spheres did not aggregate. All spectra are normalized by their intensity at 450nm. 40 Figure 4- 3 UV-Vis spectra of gold nanoparticles stabilized by (black) –OH group, (blue) –SH group without PMMA, and (red) –SH group with PMMA solutions. To convert the volume percentage into concentrations, inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements were taken in Professor Lowell D. Stott’s lab in the Earth Science department at USC. ICP-AES, also referred to as inductively coupled plasma optical emission spectrometry (ICP-OES), is an analytical technique used for the detection of trace metals [11]. It is a type of emission spectroscopy that uses the inductively coupled plasma to produce excited atoms and ions that emit electromagnetic radiation at wavelengths characteristic of a particular element. The intensity of this emission is indicative of the concentration of the element within the sample. As shown in Figure 4-4, the relationship between the volume ratio and the ICP measured concentration is extremely linear, indicating that the solutions were well mixed and the nanoparticles were evenly distributed throughout the solution. 41 Figure 4- 4 Gold concentrations for the solutions with the volume ratio at 0:100, 5:100, 10:100 and 20:100. In other words, gold solutions volume percentages are 0, 4.76, 9.09, and 16.67, respectively. It shows concentration changes almost linearly with gold volume percentage. Results are taken by the inductively coupled plasma-atomic emission spectrometry. 4.3 Applying And Characterization of PMMA-Gold Film 4.3.1 Applying PMMA-Gold thin film onto microtoroid The polymer- nanoparticle solution is spun coated onto the toroidal surface at 4000rpm for 1min and the film is thermally reflowed in a gravity oven at 115 °C for 30 min (Figure 4- ). The temperature was set to be slightly above the glass transition temperature of the polymer (T g ). This thermal reflow reduces imperfections in the polymer film, thereby decreasing scattering losses. 42 Figure 4- 5 a) Artistic rendering and b) optical image of the gold coated hybrid devices. Gold nanoparticles suspended in a PMMA solution are coated onto the microtoroid surface. The major diameter for the microtoroid is approximately 50µm. The gold nanoparticles are too small to visualize in this optical image. 4.3.2 Characterization of PMMA-Gold Thin Film A series of Gold nanoparticles and PMMA solution were spun coated onto microtoroids. To compare the thickness and refractive index of the series of thin film, a series of PMMA-nanoparticles films were spun on silicon wafers at 4000 rpm for 60 seconds. The refractive index and thickness of the PMMA-Gold nanoparticles thin film was determined using spectroscopic ellipsometry (Table 4-1). The thickness and refractive index were checked at different wavelengths. As can be seen, the thickness of the film decreases slightly with gold nanoparticles concentration and the refractive index increases with gold nanoparticles concentration, as expected. To theoretically calculate the refractive index of the thin film, we used an effective media model for the refractive index of the nanoparticles- polymer system [12]. This assumption was verified by comparing a theoretical calculation of the refractive index based on an effective media model with ellipsometry measurements. The following paragraphs explain the calculations with a comparison to the 43 ellipsometry measurements at the end. To further study the effective refractive index of the nanoparticles- polymer film and compare with the experimental data, we only need to address the question of gold volume ratio. One example calculation is shown below; however, all calculations at 633nm were performed using the same procedure and the values are included in table SI. For this example, we will use the 0.0990mol/L gold nanoparticles in 1.0% weight PMMA solution at 633nm. The refractive index is calculated for 15nm and 20nm radius nanoparticles. The refractive index of the 0.0990 mol/L gold nanoparticles in a PMMA film was calculated at 633nm. The effective medium approximation allows the effective dielectric function to be written as: ε !"" = ε ! [1+ !"[(!!! ! )/(!!!! ! )] !!![(!!! ! )/(!!!! ! ) ]; (4.1) where ε !"" is the effective dielectric constant of the medium, ε is the dielectric constant of the gold nanoparticles, ε !""# is the dielectric constant of the matrix, and f is the volume fraction of the gold nanoparticles in the film. By using the Maxwell-Garnett mixing rule, the electromagnetic material properties of the nanoparticles- polymer film can be calculated by breaking it down into its components, namely the dielectric function of the PMMA and the dielectric function of the gold nanoparticles. 44 Specifically, the gold nanosphere’s dielectric function can be written as: ε ω,r = ε !"#$ + ! ! ! ! ! !!!! ! − ! ! ! ! ! !!!(! ! !!! ! /!) (4.2) where ε !"#$, ω ! ,ω,γ ! ,A,v ! , r represent the complex permittivity of gold bulk material, the plasmonic resonance frequency, the frequency of optical wave, the damping constant in the free-electron Drude model, the proportionality factor of the order of unit, the Fermi velocity of gold, and the radius of the gold nanoparticles, respectively. To determine the refractive index of the film, we only need the real parts in the equation. Therefore, equation 2 becomes: n ω,r =n !"#$ + ! ! ! ! ! !! ! ! − ! ! ! ! ! !(! ! !!! ! /!) ! (4.3) Based on previously published results of gold [13-14], n !"#$ is 0.193 at the wavelength of 635nm, ω ! is 9.03eV, A is 1.4 of the order of unity, v ! = 1.4× 10 ! cm/s and γ ! is 0.052eV. At 635nm, we calculated the refractive index for 15nm and 20nm radius nanoparticles to be 1.9527 and 1.2730 respectively. Through simple unit conversion, the f for 0.0990mol/L solution is 11.6%. Plugging the calculated f and ε into equation 1, the refractive indices for the two different polymer-nanoparticle films are 1.5336 and 1.4873 for the 15nm and 20nm radius nanoparticle-polymer films, respectively. The results of ellipsometry and theoretical calculation are compared in the following table. The good correspondence further justifies our hypothesis that gold nanorods are well dispersed in the thin film and we can treat the film as an effective media. 45 Table 4‐ 1 Summary of spectroscopic ellipsometry results and theoretical calculations 4.4 Results and Discussion 4.4.1 Comsol Simulation and Theoretical Q The quality factor (Q) can be described by a series of loss mechanisms, Q mat , Q ss , Q rad , Q cont , and Q coupl , which relate to the material, surface scattering, radiation, contamination and coupling losses of the cavity, respectively. In previous work, it was shown that in hybrid devices the material loss is the dominant factor, yielding Q o ~Q mat . Therefore, the quality factor can be described as: Q mat =2πn eff /λα eff (4.4 ) where n eff and α eff represent the effective refractive index and effective optical absorption coefficient. In this hybrid system, these are expressed as n eff = βn silica +γn gold-polymer + δn air , and α eff = βα silica +γα gold-polymer + δα air , where β, γ, and δ represent the percentage of the optical field in silica, gold-PMMA nanocomposite, and air, respectively. The sum of β, γ, and δ should be 1. According to Beer-Lambert law, α gold =εc gold , where c gold is the concentration of the gold nanoparticles. Therefore, through a simple substitution, we can easily draw the conclusion that Q mat =A*c gold k , where k should be equal to -1 and A is a constant which is proportional 46 to 2πn/λ. It is important to note that in the present work, the polymer-nanoparticles film is treated as an effective media. This assumption has been verified by comparing ellipsometric experimental data with theoretical calculations based on an effective media model of the refractive index at 633nm. To calculate a theoretical Q factor, it is first necessary to determine β, γ, and δ by modeling the optical field distribution using COMSOL Multiphysics [15]. In the present set of simulations, the device geometry is held constant at 50(5) µm major (minor) diameter. The film thicknesses held fixed at 35nm for 0 and 5% PMMA-nanoparticles solutions, 30nm for 10% PMMA-nanoparticles solutions and 25nm for 30% PMMA-nanoparticles solutions, based on the values measured by ellipsometry. The refractive index of silica is taken from literature while the refractive indices for the PMMA-nanoparticles thin films are experimentally determined by ellipsometry. The resonant frequencies/ wavelengths are controlled by the azimuthally mode number (M) in the cavity and the simulations are performed at the resonant wavelengths of the cavity. The percentage of the optical field in each region is determined by Power In (Silica/Nanocomposite/Air) / Power Tot , where the Power In and Power Tot represent the percentage of the optical power in each region and sum of the optical power in all of regions, respectively. As can be seen in Figure 4-6, the presence of the PMMA-nanoparticles film modifies the location of the optical field. Combining the values for the optical field intensity distributions (β, γ, δ) with UV-Vis absorption measurements and refractive index measurements, a theoretical Q can be determined. 47 These are plotted in Figure 4-6 to enable direct comparison with the experimental values. Figure 4- 6 FEM simulations of the optical field distribution. a) The normalized radial optical field intensity as a function of radius for silica (solid black line) and for 10% PMMA-nanoparticles nanocomposite film (red dashed line). The zero point indicates the center of the minor diameter. This graph was determined from the optical field distribution for (b) silica microtoroid and (c) nanoparticles-polymer coated microtoroid with a 30nm thick film. The device size is 50(5) µm major (minor) diameter and operating wavelength is 635nm. 4.4.2 Q measurements Results To experimentally measure the quality factor of the devices, low loss tapered optical fiber waveguides are used to couple light from a series of narrow-linewidth (300 kHz) tunable CW lasers operating at 633nm, 765nm, and 980nm to the optical cavity At these wavelengths, the spectral overlap between the Plasmon resonance of the nanoparticles and the polymer-coated microtoroid is negligible, as the Plasmon resonance is at approximately 530nm. The alignment of the waveguide to the resonant cavity is monitored with top- and side-view cameras and controlled with a nm-resolution motorized stage. The resonances are recorded on a high speed digitizer 48 (NI), and the Q is calculated by dividing the resonant wavelength (λ) by the resonant linewidth (δλ) [16-17]. All spectra are taken in under-coupled regime with less than 5µW of input power to minimize thermal broadening. It should be noted that the resonant wavelength of the cavity is dependent on the specific properties of that cavity, such as the refractive index of the cavity (silica), the refractive index of the polymer-nanoparticles coating, and the geometry. As a result, each cavity has a unique resonant wavelength. Therefore, figure 4-7 is a representative spectra of a microtoroid coated by a PMMA solution doped with 0.0224Mol/L gold nanoparticles at 983nm. Figure 4‐ 7 Transmission spectra of microtoroid coated by spinning PMMA solution doped with 0.0224Mol/L gold nanoparticles. Figure 4-8 shows the experimentally measured quality factors for the PMMA-nanoparticles coated toroidal resonant cavities across the range of nanoparticles concentrations. As can be observed, the quality factor decreases as the concentration of gold nanoparticles increases, indicating that the optical field is 49 strongly interacting with the nanoparticles. However, at these gold nanoparticles concentrations, the Q factors are above 10 6 at all wavelengths studied. The theoretical and the experimental Q values shown in Figure 4 are fit to an equation of the form y=ax b . The parameters (a, b) determined from this fit are in table 4-2. Figure 4-8 Experimental (solid squares) and theoretical (hollow circles) quality factor of hybrid devices as a function of gold concentrations in PMMA film at a) 633nm, b) 765nm and c) 980nm. 50 As can be seen from both the graphs and the table, there is clearly excellent agreement between the theoretical prediction and the experimental values. Therefore, the Q factor is material, and not surface scattering, limited. This strong agreement justifies our primary series of hypothesis: 1) that the effective media model is correct for this system and 2) that the Q is limited by material loss (Q tot ~Q mat ), therefore scattering from the nanoparticles plays a minor role. 633 3.3538 -0.7564 1.7234 -0.9145 765 2.7995 -0.7902 1.3222 -0.9847 980 2.7490 -0.8033 1.9110 -0.8867 Table 4‐ 2 Summaries of Model and Experimental Fitting Parameters 4.4.3 Photon Lifetime The conversion of the quality factors shown in figure 4-8 in the main paper to the photon lifetime τ in the cavity is: Q=ωτ= !"! ! τ. (4) The results are listed in Table 4-3. Gold Concentration (mol/L) Photon Lifetime(ns) 633nm 765nm 980nm 0 6.41 6.53 9.51 0.0224 1.91 2.65 2.94 0.0433 0.89 1.03 1.42 0.0990 0.60 0.68 0.95 Table 4‐ 3 Conversion of experimental quality factors to photon lifetime a (x 10 5 ) b a (x10 5 ) b Model Experiment Wavelength (nm) 51 4.5 Conclusions In summary, we have experimentally and theoretically investigated the impact of PMMA-nanoparticles coatings on the quality factor of ultra-high-Q devices. We have demonstrated Q factors above 10 6 for several different concentrations of gold nanoparticles and over a wide range of wavelengths. The measured Q values are in good agreement with the theoretically predicted ones based on a material-limited Q model using an effective media model for the nanoparticles-polymer coatings. The ability to combine gold nanoparticles-PMMA thin films with ultra-high-Q devices will impact a wide range of disciplines; for example, developing improved LSPR sensors and understanding nonlinear optics in hybrid organic-inorganic systems. 52 Chapter 4 References 1. L. Collot, V. Lefevre-Seguin, M. Brune, J. Raimond, S. Haroche, EPL (Europhysics Letters), Very high-Q whispering-gallery mode resonances observed on fused silica microspheres, 23, 327 (1993). 2. A. M. Armani, R. P. Kulkarni, S. E. Fraser, R. C. Flagan, K. J. Vahala, Science, Label-free, single-molecule detection with optical microcavities, 317, 783 (2007). 3. H. S. Choi, D. Neiroukh, H. K. Hunt, A. M. Armani, Langmuir, Thermo-optic coefficient of polyisobutylene ultrathin films measured with integrated photonic devices, 28, 849. 4. K. Guarini et al., Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, Process integration of self-assembled polymer templates into silicon nanofabrication, 20, 2788 (2002). 5. P. K. Ho, D. Stephen, R. H. Friend, N. Tessler, Science, All-polymer optoelectronic devices, 285, 233 (1999). 6. R. R. K. Chang, A. J. Campillo, Optical processes in microcavities. (World scientific, 1996), vol. 3. 7. C. Shi, H. Seok Choi, A. M. Armani, Applied physics letters, Optical microcavities with a thiol-functionalized gold nanoparticle polymer thin film coating, 100, 013305 (2012). 8. H. S. Choi, X. M. Zhang, A. M. Armani, Optics Letters, Hybrid silica-polymer ultra-high-Q microresonators, 35, 459 (2010). 9. S. Chen, K. Kimura, Langmuir, Synthesis and characterization of carboxylate-modified gold nanoparticle powders dispersible in water, 15, 1075 (1999). 10. J. Yang, J. Y. Lee, T. Deivaraj, H.-P. Too, Langmuir, An improved procedure for preparing smaller and nearly monodispersed thiol-stabilized platinum nanoparticles, 19, 10361 (2003). 53 11. Y. S. Chung, R. M. Barnes, J. Anal. At. Spectrom., Determination of gold, platinum, palladium and silver in geological samples by inductively coupled plasma atomic emission spectrometry after poly (dithiocarbamate) resin pre-treatment, 3, 1079 (1988). 12. Y. Pei, F. Yao, P. Ni, X. Sun, Journal of Modern Optics, Refractive index of silver nanoparticles dispersed in polyvinyl pyrrolidone nanocomposite, 57, 872 (2010). 13. S. Zhang et al., Physical review letters, Experimental demonstration of near-infrared negative-index metamaterials, 95, 137404 (2005). 14. M. Thompson, Philosophical Magazine, II. The energy spectrum of ejected atoms during the high energy sputtering of gold, 18, 377 (1968). 15. M. Oxborrow, in Lasers and Applications in Science and Engineering. (International Society for Optics and Photonics, 2007), pp. 64520J-64520J-12. 16. X. Zhang, H. S. Choi, A. M. Armani, Applied physics letters, Ultimate quality factor of silica microtoroid resonant cavities, 96, 153304 (2010). 17. S. M. Spillane, California Institute of Technology (2004). 54 Chapter 5 Gold Nanorod Plasmonic Upconversion Microlaser 5.1 Introduction High optical field intensities build up inside microtoroids owing to their ultra-high quality factors, making them an ideal platform for Plasmonic-photonic interactions with noble metals and a suitable pump source for microlasers. In this work, a microlaser based on hybrid silica microtoroids coated with gold nanorods is theoretically modeled and experimentally demonstrated. Theoretically, we used 3-D COMSOL Multiphysics and modeled the interaction between the optical mode of the microtoroids and the surface Plasmonic resonance of gold nanorods, both on and off resonance. To thoroughly study the role that the polymer layer plays in the Plasmonic laser system, we perform a series of finite element method simulations in which the polymer layer thickness and refractive index is varied, and its effect on the Plasmonic resonance is quantified. Experimentally, we demonstrated a visible laser at 580nm from hybrid microtoroids with a 20µW-threshold and an approximately 1nm linewidth. We have also varied the gold nanorod concentration on the microtoroids surface, and studied its effect on the quality factor and the threshold power in order to get the optimum concentration for lasing [1]. 55 5.2 Motivation Rare-earth metal-doped glasses have enabled a wide range of optical technologies, including low threshold integrated lasers and high transmission optical fibers [2-4]. However, unlike many semiconductors or even noble metals, these materials are not widely available, and as such, alternatives to rare earth materials are being actively sought. The challenge in developing an alternative is finding a material, which has the same optical stability, environmental robustness, and optical properties of the rare-earth metals. For example, while many inorganic dyes emit at similar wavelengths, their emission properties slowly degrade over time . One emerging alternative is based on gold nanoparticles . Gold nanoparticles can be synthesized in many geometries and sizes, including spheres, rods, prisms and stars . As a result of this variety, it is possible to tune the optical properties and absorption of these particles from the visible through the near-IR. Additionally, they have also exhibited many interesting nonlinear phenomena, such as two-photon-based emission . One reason for the increased interest in gold nanoparticles is their inherent stability against environmental changes due to their minimal reactivity (e.g. low oxidation). Because of this, their optical properties do not change over time. Therefore, they are being studied for a wide range of emerging applications, including Plasmonic solar cells, gene delivery, lasers, and sensors . However, while advances in many of these fields have progressed to the point of transitioning to industry, in the area of laser development, several key technical hurdles remain. 56 The motivation for using gold nanoparticles as a laser gain medium is the large and easily tunable absorption cross section [12-13]. This parameter governs the threshold of an optically-pumped laser, and, therefore, is a critical consideration in the design of a laser. The absorption spectrum is determined by the geometry of the nanorods and contains two peak wavelengths corresponding to the transverse and longitudinal modes of the nanoparticles [14]. By changing the growth conditions, the specific wavelengths of these peaks can be easily tuned. Additionally, previous work has shown that gold nanorods undergo two photon emission [15-16]. In this non-linear process, the nanorod absorbs two photons and emits one photon of higher energy or lower wavelength. As such, by optimizing the nanorod geometry such that the absorption peaks match those of the pump laser, it is possible to develop an optically pumped visible laser that does not rely on a rare earth element. 57 Previous research has demonstrated that nanorod-based two-photon emission is possible by illuminating a cover slip coated with nanoparticles with a high power laser [16]. While this approach demonstrates the fundamental principle, the emission is very broad, covering hundreds of nanometers. Therefore, this approach is not feasible for a true on-chip microlaser design. One approach for improving the excitation efficiency is to directly intercalate the nanoparticles with the beam path or to create a fiber laser. However, because the melting temperature of gold is significantly below the melting temperature of silica, this combination is not possible. Additionally, simply dip coating a fiber in a nanoparticles solution will not give a sufficiently dense coverage of the nanoparticles to allow lasing at a reasonable threshold. An alternative is to create an optical microcavity-based laser. Whispering gallery mode microcavities confine light at the device/air boundary, resulting in large circulating intensities which can strongly interact with and transfer energy to a nanoparticles [17]. Recent work has demonstrated a method to stably attach spherical metal nanoparticles to the surface of high quality factor devices without degrading the device performance [18]. It is critical to align the resonant wavelength of the cavity with the Plasmonic resonance of the nanoparticles. If these two values are not matched, the energy from the optical cavity will not be efficiently transferred to the nanoparticles. However, commercially available tunable laser sources are not available at the appropriate pump wavelength for nanospheres. Therefore, in order to achieve a strong overlap between the excitation source and the nanoparticles 58 absorption wavelength, nanorods must be used. In the present work, a Plasmonic laser based on the combination of gold nanorods with high Q silica microtoroids is theoretically modeled and experimentally demonstrated. The gold nanorods embedded in Polymethylmethacrylate (35k molecular weight, PMMA, Sigma-Aldrich) were coated on toroidal surface. The optical field acts as a pump source and gold nanorods behave as a gain medium. Low threshold lasing from gold nanorods are achieved. The circulating intensity is directly proportional to the photon lifetime within the cavity or the quality factor (Q) of the device. As such, high Q devices are ideally suited for this application. 5.3 Synthesis Procedure 5.3.1 Synthesis of Gold Nanorods in Water First gold nanorods (NRs) are synthesized in water via the seed mediated method [19]. The seed solution is formed by mixing 5ml CTAB (0.2M), 5ml HAuCl 4 (0.5mM) and 600µl of NaBH 4 (0.01M) together. The stir bar is taken out using a magnetic stick after reacting for 2min. The gold seeds are then left unstirred on the hotplate. Afterwards, 5ml CTAB (0.2M), various amounts of AgNO 3 (8mM, 50, 75, 100 and 125µl respectively), and 5ml HAuCl 4 (1mM) are combined serially. Then 70µl of L-Ascorbic Acid (0.788mM) is introduced to the growth solution. The solution turned to be colorless after adding L-Ascorbic acid. Finally, 12µl of the above made gold seed solution is added, and the solution is left on a hotplate set at 59 45°C for 3hrs. The freshly made gold nanorods are centrifuged at 10,000 rpm for 10min and washed by DI water twice to purify the solution. The freshly made gold nanorods are characterized via UV-Vis Spectrometer. The Plasmonic resonance can be tuned by changing the amounts of AgNO 3 added (Figure 5-1). 5.3.2 Transfer of Gold Nanorods from Water into Toluene The freshly made gold nanorods are re-suspended in 5ml DI water. Then, 10mg of mPEG-thiol (M.W. 5K, Laysan Bio) in degassed water is added to the solution and sonicated for 30s to mix thoroughly and react for 2hrs on a shaker. Finally the –SH functionalized nanorods are centrifuged and washed by methanol twice and re-suspended in toluene. The –SH functionalized gold nanorods are characterized using a spectrofluorometr. A distinct peak at wavelength of 780nm is noticed, and red shifts for about 30nm after mixing with PMMA due to change of environmental refractive index (Figure 5-2). 60 Figure 5‐ 1 Typical UV-Vis spectra of the series of gold nanorods with longitudinal Plasmonic wavelength tuned from 650nm to 850nm. 5.3.3 Fluorescence Emissions of Gold Solutions The –SH functionalized gold nanorods are characterized using a spectrofluorometry. Taking gold nanorods with absorption at 780nm as an example, these solutions with PMMA were put into spectrofluorometr and got pumped at 780nm, 760nm, 650nm, and 630nm, respectively. 61 Figure 5- 2 UV-Vis spectra of the gold nanorods in toluene and the nanorods in toluene with PMMA. To characterize the dependence of plasmonic resonance of the nanorods on the refractive index of solvents, a series of measurements were performed with the nanorods suspended in water (n=1.333), methanol (n=1.316) and toluene/PMMA solution (n=1.486). For reference, the refractive index of PMMA at 780nm is 1.485. The typical fluorescence spectra are shown in Figure 5-3 and wavelength shift results are plotted in Figure 5-4. As can be observed, the emission wavelength clearly red-shifts as the refractive index increases. 62 Figure 5- 3 Spectrofluorometer spectra of the nanorods and PMMA in toluene solution. The nanorods can be easily excited at both 760nm and 780nm, emitting at 550nm. Figure 5- 4 Dependence of emission wavelength on refractive index of solution. All solutions are excited at 780nm. 63 5.3.4 Apply PMMA-Gold Film and Characterize using Scanning Electron Microscope (SEM) Polymethylmethacrylate (35k molecular weight, PMMA, Sigma-Aldrich) is added to the nanorod solution to reach 0.5% weight. Finally, the nanorod-PMMA film is deposited on the surface of the device by spin coating at 4000rpm for 1min. The nanorod-PMMA coated microtoroid is annealed in a gravity oven at 150°C for 2hrs to remove any residual solvent and thermally reflow the polymer film. Based on previous work, when these synthesis parameters (polymer and nanoparticles concentrations) are used, polymer films of approximately 30nm are achieved, as measured with multi-wavelength ellipsometry. Based on these film thickness and the SEM images, it is expected that all of the nanorods would be parallel to the surface. Typical artistic renderings and SEM images of hybrid toroids are shown Figure 5-5. 55 Figure 5- 5 Pictures of toroidal optical resonant cavities. a) Rendering of a toroidal resonant cavity coated with gold nanorods. b)/c)/d) Scanning electron microscopy images of a gold nanorod coated cavity at different magnifications. 64 5.4 Plasmonic Simulation 5.4.1 Plasmonic Interaction between Gold Nanorods and Microtoroid As microtoroids have an axisymmetric structure, the complexity can be reduced to a two-dimensional problem with rotational symmetry. However, gold nanorods coated on the microtoroid surface break the symmetry, and we have to utilize 3D COMSOL Multiphysics to study the Plasmonic interaction between gold nanorods and optical modes. Because of the length scale difference between microtoroids and nanorods, this specific structure is particularly complex to model. Recently, this challenge was solved by simplifying a microtoroid structure and only modeling a single slice. Based on the model detailed in Kaplan et al [20], we have developed a 3D COMSOL Multiphysics finite element method model and simulated both the transverse electric and transverse magnetic modes of our system, both on and away from the Plasmonic resonance of the nanoparticles (765nm and 1550nm, respectively). To model the Plasmonic enhancement of the circulating optical field, we cut a slice of the microtoroid with an arclength of approximately λ/2, where λ is the resonant wavelength. The two sliced sides are defined to be perfectly matched layers while the other sides are set to be open. The gold nanoparticles are placed on the central part of a slice of the resonator. The detailed structure is shown in the following figure 5-6. 65 Figure 5- 6 Schematics of the COMSOL Simulation investigating interaction between gold nanorod and optical field. To achieve an acceptable accuracy, the sizes of the mesh elements are chosen to be 0.5, 50 and 3nm near the gold nanoparticles, near the optical mode and in the thin polymer layer. For the rest of the simulation region, including the air around the WGM device and the regions of the WGM device far from the fundamental mode, the mesh element size varied from 3.6 to 360nm. One example of an enhanced optical field induced by the Plasmonic signal of gold nanorods is shown in Figure 5-7. When microtoroids are operated at 780nm wavelength, which matches the p\Plasmonic wavelength of the gold nanorods, the optical field around nanorods is enhanced significantly. In contrast, when the microtoroids are operated at 1550nm, which does not match with the Plasmonic wavelength, the optical field is not enhanced, but simply scattered. 66 Figure 5- 7 COMSOL Multiphysics finite element method simulations of the transverse magnetic mode of the microtoroid interacting with a 3.5 AR gold nanorod. The gold nanorod (outlined in white) is (a/c) parallel or (b/d) perpendicular with the boundary of the microtoroid. The surface Plasmon resonance of the nanoparticles is only excited when the optical resonance of the cavity overlaps with the optical absorption of the nanoparticles. 5.4.2 Effect of Polymer Thin Film on the Device To study the effect of polymer on the optical interaction between nanorods and the microtoroid, and to justify that the lasing shown in the following text was not caused by the impact of polymer, we performed a series of simulations by varying the refractive index of polymer. 67 By increasing the refractive index of the polymer thin film, the eigenvalue of the cavity decreases (Figure 5-8). In other words, as the refractive index of the polymer layer increases, the mode moves towards the outside of the microtoroid. Consequently, the resonance wavelength increases. This result is expected based on fundamental resonator physics and provides one method of validating that our model is working correctly. Figure 5- 8 The dependence of the cavity resonance frequency on the refractive index of the coating. As the refractive index of the coating increases, the maximum field inside the toroid decreases (Figure 5-9). This trend indicates that the mode is pushed towards the outside of the microtoroid where it interacts more strongly with the scattering point (nanoparticles). As a result, the Q drops and the field enhancement decreases. 68 Figure 5- 9 As the coating refractive index increase, the maximum mode field inside the microtoroid decreases. While the intensity of the field is dependent on the film refractive index, the radius of the maximum field inside the WGM device is independent of the refractive index of the polymer layer (Figure 5-10). This result indicates that the thin layer around the device acts as a perturbation and the field pattern inside the device is not significantly changed. Figure 5- 10 The dependence of the field size on the coating index. 69 As shown in Figure 5-11, by increasing the refractive index of the polymer layer from 1 to 1.486, effective mode radius increases by a negligible amount (24.8nm). Given the R/R (change in radius/radius) is less than 0.09%, we can assume that the effective mode radius is not modified by the polymer layer, and the 24nm thick film has almost no effect on the dynamics of the resonance. However, if a thicker layer was used, the behavior would be different. Figure 5- 11 The dependence of the effective mode radius on the coating refractive index. By increasing the refractive index of the polymer layer, the evanescent tail of the optical field of the microcavity is able to interact with the scattering points (gold nanoparticles). Therefore, the Q factor decreases. This decrease in Q results in a corresponding decrease in field enhancement. By analyzing the field intensity at the surface of the nanoparticles, the relationship between Plasmonic field enhancement and film refractive index can be determined (Figure 5-12). This relationship shows that in order to achieve maximum enhancement, the coating with the lowest refractive index should be used. 70 Figure 5- 12 Dependence of Plasmonic enhancements on coating index. The results from our simulations direct us towards the fact that presence of the thin layer of PMMA does not modify the mode shape and resonance field enhancement and it could be assumed as a constant perturbation to the whole experiment and therefore the analysis could be done without taking the thin PMMA layer into account. 5.5 Upconversion Microlaser Measurements 5.5.1 Typical Emission Spectra The lasing behavior is characterized using the testing set-up introduced in introduction part. Lasing spectra from a nanorods coated microcavity at different coupled powers are shown in Figure 5-13. The split emission peak centered at 581.5nm is slightly red-shifted from the spectrofluorometry measurement, most likely due to the small change in effective refractive index. Fitting the lasing line to a 71 dual-peak Gaussian, the full-width-half-max (FWHM) values of the laser lines are 0.83nm and 1.00nm. These linewidths are significantly narrower than the previous result produced by coating a cover slip by several orders of magnitude. Figure 5- 13 Typical lasing spectra of the nanorods coated microcavities pumped at different powers. 5.5.2 Threshold Power for Upconversion Laser The lasing threshold measurement is presented in the inset picture of Figure 5-14. The coupled power is defined as the output power from fiber times the coupling efficiency. This power is distinct from several other power values, such as the circulating power in the resonator or the power from the laser to the 72 waveguide-cavity system. The input power takes into account the amount of the laser power, which is coupled into the resonator. For example, at critical coupling when 100% of the power is coupled into the resonator, the coupled input power would equal the laser power. However, in the majority of our experiments, approximately 50% of the power from the laser was coupled into the cavity; therefore, the coupled power was similarly decreased. The hybrid device achieves a threshold as low as 20µW, which is an order of magnitude improvement over the previous approach. While this improvement is not as significant as the change in FWHM, the decrease in threshold is related to both the concentration of the nanorods as well as the circulating intensity. The circulating power (P circ ) in the cavity is proportional to the quality factor, device diameter and the input power. It is the reason that higher Q devices with smaller diameter have lower lasing thresholds. The circulating power is theoretically described by: 𝑃 !"#! =𝑃 !" 𝑄 ! ! ! !" ! !!! ! (5-1) where 𝜅 is the coupling coefficient, 𝜆 is the wavelength, R is the device radius, P in is the input power, Q is the quality factor, and n is the effective refractive index. The coupling coefficient can be determined from the transmission spectrum (T) using the below relation: 𝑇= !!! ! !!! ! (5-2) At 20µW, with the previously defined Q and diameter, the circulating optical power in the cavity is approximately 1.3W, which corresponds to a circulating optical intensity of approximately 60MW/cm 2 . While the circulating intensity has 73 significantly increased over the previous approach, the concentration of the gain medium has decreased; as such, these changes nearly balance. In the process of achieving Plasmonic lasing, a series of optimization experiments were performed during which the concentration of the nanorods in the initial solution was varied. Figure 5- 14 Lasing spectra and (inset) threshold data of the gold nanorod coated microtoroid cavity. The gold nanorods are excited by the evanescent field from microtoroid and lase at 581.5nm. The wavelength difference of the hybrid device from solution is caused by the slight refractive index change between the solution and the solid thin film. A threshold power as low as 20 µw is achieved. 5.5.3 Optimum Gold Concentration for Upconversion Laser Effect of nanoparticles on quality factor: The quality factor is a sum of all of the losses in the resonator, including the scattering loss, absorption loss, radiation loss and coupling loss. In previous work using spherical nanoparticles coated cavities, it was demonstrated that the absorption loss is dominant. The expression for Q mat is: Q mat = 2πn/λα, where n and α are the effective refractive index and effective absorption or material loss, which incorporate the refractive index and material loss of 74 the silica, the nanoparticles-polymer coating and air. The optical loss can be determined from the UV-Vis measurements. In the present work, the same trend was observed, with Q decreasing in proportion to 1/α. Dependence of threshold on nanorod density: The Plasmonic laser behaves analogously to a dye laser. Therefore, similar equations describing the threshold can be applied. Specifically, the lasing threshold (γ(λ)) can be described by ( ) ( ) ( ) ( ) ( ) 1 2 t t abs t abs emis n Q n n n n π λ σ λ γλ σ λ σ λ + == + (5-3) Where n is the refractive index, n 1 is the number of excited Plasmonic particles (on-resonance) and n t is the total number of Plasmonic particles per unit volume, σabs(λ) and σemis(λ) are the absorption and emission cross sections, and Q is the quality factor of the cavity. Therefore, an increase in the number of excited particles can decrease the threshold. However, because an increase in the number of Plasmonic particles decreases the Q, by increasing the material loss, there is an optimum balance between high particle concentration and high Q factor. In Figure 5-15, this balance is clearly evident. The Q factors of the devices ranged from 7E5 to 8E7. However, lasing was only observed in the middle three concentrations, due to the balance between Q and concentration. 75 Figure 5- 15 The dependence of quality actor and lasing threshold power on the nanoparticles dilution factor. 5.6 Raman Lasing from Gold Nanorods Coated Toroid Besides lasing emitted by gold nanorods, enhanced Raman lasing from silica microtoroids are noticed and recorded as well. Taking gold nanorods with Plasmon resonance at 780nm as an example, gold nanocomposite coated microtoroids are 76 made based on the method stated above. Quality factor is checked using the testing set up as 1E7. When pumping at 780nm, we get Raman lasing at 810nm with threshold as low as 7µW as shown in Figure 5-16. 77 Figure 5- 16 Typical Raman lasing (a) spectra and (b) threshold power for gold nanorods coated microtoroid. 78 Cascaded Raman lasing can be achieved by changing coupled modes and optimizing the coupling, and Raman lasing with different wavelengths has been recorded (Figure 5-17). Interestingly, the ratio of intensities among those peaks can be tuned by changing coupling. 79 Figure 5- 17 Raman spectra from gold nanorods coated microtoroid. The ratio of intensities among different peaks can be tuned by varying the coupling. 80 5.7 Gold Nanorods Coated Toroid through Layer-by-Layer Method 5.7.1 Layer-by-Layer Method Another way to apply gold nanorods onto silica microtoroids evenly is by polymer layer-by-layer method [21]. After the gold nanorods were successfully achieved in solution, it was applied to Silica device surface by electrostatic forces. First of all, poly (vinylpyrrolidone) (PVP) was added into the gold solution, and reacted overnight under gentle stir. It formed a thin film of PVP on the surface of gold nanorods, which was negatively charged. Then the solution was centrifuged at 4000rpm for 30mins, deposing the suspension liquid, the residue was dissolved in water again, which was stored for use in the next step. Afterwards, the silica chip was treated by oxygen plasma and a monolayer of hydroxyl formed on the surface. The chip was immersed in Poly (diallydimethylammonium chloride) (PDDA) solution for 30 min, making a positively charged surface. Finally the chip was put inside the gold and PVP solution for 30min, and theoretically gold nanorods would be successfully attached onto the surface. A scheme for Au NRs coating is shown in Figure 5-18. 81 Figure 5- 18 A schematic showing the Au NRs coating through polymer electrostatic attractions. 5.7.2 Characterization of Toroidal Surface The gold nanorods are dispersed evenly and densely on microtoroids surface. This is verified by high resolution Scanning Electron Microscopy (SEM) and fluorescent microscope, shown in Figure 5-19. Figure 5- 19 a) SEM of Au NRs on silica microtoroids and b) Atomic force microscope (AFM) image of Au NRs on a silica wafer. 3µm 82 5.7.3 Optical Testing The optical performance (Q factor) of the hybrid device was characterized at 780nm and 633nm using a narrow-linewidth (300 kHz) tunable CW laser (Figure 5-20). The quality factor or Q of the cavity is calculated based on Lorentz fitting of peaks where Q=𝜆/Δ𝜆, where Δ𝜆 is the full-width-half-maximum determined from the fit. Quality factor is as high as 1E6 at 780nm, while it is 3E6 at 633nm. Figure 5- 20 A typical transmission spectra at a) 780nm, and b) 633nm. The emitted light was measured using a fiber-coupled spectrograph, which was positioned adjacent to the microtoroid. Bare microtoroid was checked as a control experiment. Both of bare microtoroid and Au NRs coated microtoroid were checked with 5mW of 780nm laser coupled into the devices. The spectrograph picked up signal of emission from 200nm to 900nm shown in Figure 5-21. Both bare microtoroid and Au NRs coated microtoroid showed corresponding peaks at multiple positions. However except for enhancement of silica fluorescence, no exclusive lasing for Au NRs coated microtoroid was noticed. It is estimated that gold nanorods has a weak interaction with evanesced field of silica due to the spacing of multi-layers polymer. 83 Figure 5- 21 A typical emission spectra from bare silica microtoroid and Au coated microtoroid. Both were pumped by a tunable 780nm laser. To dig out the origin of those silica fluorescence peaks, and the enhancement caused by gold nanorods coating, different gold concentrations coated microtoroids were investigated. Threshold of three peaks (420nm, 520nm, and 600nm) were checked under spectrograph. The coupled power was controllably changed for various concentrated Au NRs on the surface (Figure 5-22 and Figure 5-23). It is noticed that though Q decreased with Au NRs concentration increasing on the surface, silica fluorescence intensity and efficiency became stronger. This justifies our hypothesis that these emissions originate from fluorescence of silica device, and gold nanorods coating can enhance the detection due to increased scattering into the detector. 84 Figure 5- 22 Threshold powers of emissions at 420nm, 520nm, and 600nm changing with different Au concentrations on the surface. Figure 5- 23 Efficiency of emissions at 420nm, 520nm, and 600nm changing with different Au concentrations on the surface. 85 5.8 Conclusions In summary, we have theoretically and experimentally demonstrated a visible laser based on a nanorod coated toroidal microcavity. By embedding the gold nanorods within a conformal polymer coating on the surface of the microtoroid, we have fabricated hybrid devices with Q factors above 10 million. Through judicious selection of the pump wavelength, the high intensity circulating field within the optical cavity efficiently excites the Plasmonic modes within the nanoparticles, enabling two photon emissions from the nanorods. This combination results in a 20µW-threshold laser with an approximate 1nm linewidth. This simple approach for device fabrication is not limited to toroidal cavities and can be translated to other optical devices; for example, the deposition of metal nanoparticles on photonic crystals. It will impact researchers over a wide range of disciplines beyond laser development, such as improved signal to noise for label-free protein detection and the study of photonic-Plasmonic interactions. 86 Chapter 5 References 1. C. Shi, S. Soltani, A. M. Armani, Nano letters, Gold nanorod plasmonic upconversion microlaser, (2013). 2. H.-S. Hsu, C. Cai, A. M. Armani, Optics Express, Ultra-low-threshold Er: Yb sol-gel microlaser on silicon, 17, 23265 (2009). 3. S. Poole, D. N. Payne, M. E. Fermann, Electronics Letters, Fabrication of low-loss optical fibres containing rare-earth ions, 21, 737 (1985). 4. M. J. Digonnet, Rare-Earth-Doped Fiber Lasers and Amplifiers, Revised and Expanded. (CRC press, 2002). 5. P. Blánquez et al., Water Research, Mechanism of textile metal dye biotransformation by Trametes versicolor, 38, 2166 (2004). 6. S. Kühn, U. Håkanson, L. Rogobete, V. Sandoghdar, Physical review letters, Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna, 97, 017402 (2006). 7. T. K. Sau, C. J. Murphy, Journal of the American Chemical Society, Room temperature, high-yield synthesis of multiple shapes of gold nanoparticles in aqueous solution, 126, 8648 (2004). 8. M. Zavelani-Rossi et al., Applied physics letters, Near-field second-harmonic generation in single gold nanoparticles, 92, 093119 (2008). 9. H. Wang et al., P Natl Acad Sci USA, In vitro and in vivo two-photon luminescence imaging of single gold nanorods, 102, 15752 (2005). 10. A. K. Salem, P. C. Searson, K. W. Leong, Nature materials, Multifunctional nanorods for gene delivery, 2, 668 (2003). 11. J. H. Lee, J. H. Park, J. S. Kim, D. Y. Lee, K. Cho, Organic Electronics, High efficiency polymer solar cells with wet deposited plasmonic gold nanodots, 10, 416 (2009). 12. M. B. Cortie, A. M. McDonagh, Chemical Reviews, Synthesis and Optical Properties of Hybrid and Alloy Plasmonic Nanoparticles, 111, 3713 (2011). 13. Y. Liu, E. N. Mills, R. J. Composto, Journal of Materials Chemistry, Tuning optical properties of gold nanorods in polymer films through thermal 87 reshaping, 19, 2704 (2009). 14. S. Link, M. A. El-Sayed, Annual Reviw of Physical Chemistry, Optical properties and ultrafast dynamics of metallic nanocrystals, 54, 331 (2003). 15. N. J. Durr et al., Nano Letters, Two-Photon Luminescence Imaging of Cancer Cells Using Molecularly Targeted Gold Nanorods, 7, 941 (2007). 16. H. Wang et al., Proceedings of the National Academy of Sciences, In vitro and in vivo two-photon luminescence imaging of single gold nanorods, 102, 15752 (2005). 17. A. B. Matsko, V. S. Ilchenko, IEEE Journal of Selected Topics in Quantum Electronics, Optical Resonators with Whispering-Gallery Modes-Part I: Basics, 12, 3 (2006). 18. C. Shi, H. Seok Choi, A. M. Armani, Applied physics letters, Optical microcavities with a thiol-functionalized gold nanoparticle polymer thin film coating, 100, 013305 (2012). 19. N. R. Jana, L. Gearheart, C. J. Murphy, The Journal of Physical Chemistry B, Wet chemical synthesis of high aspect ratio cylindrical gold nanorods, 105, 4065 (2001). 20. A. Kaplan et al., arXiv preprint arXiv:1305.0555, Finite element simulation of a perturbed axial-symmetric whispering-gallery mode and its use for intensity enhancement with a nanoparticle coupled to a microtoroid, (2013). 21. N. Malikova, I. Pastoriza-Santos, M. Schierhorn, N. A. Kotov, L. M. Liz-Marzán, Langmuir, Layer-by-layer assembled mixed spherical and planar gold nanoparticles: control of interparticle interactions, 18, 3694 (2002). 88 Chapter 6 Leveraging Bimodal Kinetics to Improve Detection Specificity 6.1 Introduction Optical microcavities are high sensitivity transducers, capable of detecting single nanoparticles and molecules [1-2]. However, the specificity of detection is dependent on the availability of an appropriate targeting moiety with no cross-reactivity [3-4]. In the present work, an alternative approach is shown by combining surface functionalization as well as analyzing kinetics constants [5]. Namely, using biotin-functionalized toroidal microcavities, the dissociation constant of the biotin and the two different streptavidin complexes (free and polystyrene beads) is determined. Based on the differences in the affinity and in the mass transport, the two complexes are identified from a mixture. By leveraging information on the binding site, improved specificity can be achieved. 6.2 Motivation Label-free biosensors based on whispering gallery mode optical resonators have numerous applications ranging from defense to fundamental science explorations [6-8]. For example, previous research using this type of sensor devices has demonstrated the detection of molecules in complex environments and the characterization of binding constants [9-10]. One of the reasons for the widespread use of these types of biosensors is the combination of sensitivity and specificity, which improves the accuracy of the sensor signal. The sensitivity is 89 inherent to the microcavity sensor while the specificity is typically the result of a surface chemistry or the attachment of a recognition element to the surface of the device. Whispering gallery mode resonators confine light of specific frequencies, also known as resonant frequencies, in circular orbits at the periphery of the device [11]. The resonant frequency is determined by the material and geometrical properties of the cavity, among other parameters. Therefore, when a molecule binds to the surface of the cavity, the resonant frequency shifts. Because these devices can have long photon lifetimes (>100 ns) or high quality factors (Q>100 million), they are extremely sensitive to small perturbations. In classic microcavity detection experiments, the presence of a molecule is identified based solely on a frequency shift [6-8]. This approach assumes that the interaction between a given recognition element and its analyte is truly unique; however, this ideal is rarely achieved [12]. Part of the challenge of creating an ideal recognition element lies in how the binding site identifies the target analyte. Although a binding site incorporates both morphology and chemical interactions to correctly identify its target, frequently, incorrect molecules can also bind in the same site, albeit for shorter time periods. For example, a smaller molecule can fit into the binding pocket of a larger molecule. One way to distinguish between the correct and the incorrect molecule is to measure the dissociation constant (K d ) of the binding reaction, which describes the affinity between the binding site and the target molecule [12]. 90 Using resonant cavities, this metric can be calculated by measuring the resonant shift (Δλ), and then linearizing the dissociation phase according to the following equation [13]: ln (λ(t)/λ 0 )=-K d t (1) where λ 0 is the wavelength at the beginning of each dissociation phase, and λ(t) is the wavelength as a function of time. In the present work, biotin-functionalized silica microtoroid resonant cavity biosensors are used to detect free streptavidin and streptavidin-labeled polystyrene beads (streptavidin-polybeads). By measuring the dissociation constant of the biotin-streptavidin binding reaction, we distinguish between the two analytes (Fig ure 6-5). The presence of the polybead changes the K d of the biotin-streptavidin interaction, allowing the identification of those species. Because both species bind to biotin, it would be extremely difficult to perform this detection using the conventional resonant cavity detection technique. 6.3 Unspecific Biodetection Microtoroids have specific resonant wavelengths determined by their diameter, wavelength, environment, etc. The resonant wavelength shifts when its environment (such as refractive index, temperature, humidity, pressure, etc.) changes. Therefore when protein dissolved in buffer flows into the chamber including microtoroids filled with buffer, the resonant wavelength red shifts due to both refractive index changes caused by unspecific attachment of protein, and temperature/ pressure changes caused by flow. To irradiate the impact of flow, two options can be considered. The first option is recording spectra of both buffer and protein in buffer flowing into the chamber, and getting buffer signal deducted from 91 the protein solution can irradiate the impact of buffer. The second option is to adjust the flow rate, distance between injection needle and microtoroids, and size (D/d) of microtoroids, to minimize the signal from buffer so that it is negligible when analyzing detection signal. Figure 6-1 shows the background signal of HEPES buffer flowing into buffer using microtoroids with different sizes. We changed the minor diameters of microtoroids at 8 µm and 6µm, and we tried to keep other parameters including flow rate set at 100µl/min and distance between the needle and microtoroids the same. We found that when the minor diameter is 8µm the shift is about 0.003nm, while when the minor diameter is 6µm, the background shift is less than 0.001nm. Based on these findings, we fabricated our microtoroids with minor diameter at about 6µm in further biodetection studies. Figure 6- 1 Background signal when buffer is flowed into the chamber including microtoroids immersed in buffer at the flowrate of 100ml/min. We changed the minor diameter of microtoroids to be (a) 6µm and (c) 8µm, which results in shifts of (b) 0.003nm and less than (d) 0.001nm, respectively. (a) (c) (b) (d) 92 A set of experiments is performed to further study the mechanisms by changing quality factor of microtoroids and changing the frequencies of function generator. After putting microtoroids into buffer, we search different optical modes of microtoroids, which exhibit diferent quality factors. We tested three modes with Q factors of 2E6, 4E6, and 6E6, respectively. As the Q increases, the optical mode becomes more intense, and therefore resutls in stronger signals when reacting with protein. We have deteced streptavidin solutions with different concentrations. Though shifts become stronger when concentration increases with all four concentrations (1pM, 10fM, 500aM, and 10aM), difference is bigger at small er concentration range. The results are summarized in Figure 6-2. Figure 6- 2 Resonant shifts when injecting streptavidin into chamber. We performed experiments using different modes of the same microtoroid. The Q of these modes are 2E6, 4E6, and 6E6, respectively. We have detected streptavidin solutions with different concentrations. Though shifts become stronger when concentration increases with all four concentrations (1pM, 10fM, 500aM, and 10aM), difference is bigger at smaller concentration range. 93 We have changed the frequency of the function generator from 400 mVPP to 1000 mVPP with 200 mVPP intervals with frequency fixed at 100Hz to delve into the effect of function generator. The resonant shifts are bigger at higher frequency of function generator. These experiments are performed at four different concentrations as well to justify t his effect is universal for all kinds of solution concentrations. The results are summarized in Figure 6-3. Figure 6- 3 Resonant shifts when injecting streptavidin into chamber. We performed experiments using different frequencies of function generator from 400mVPP to 1000mVPP using a same microtoroid. We have detected streptavidin solutions with different concentrations.. The resonant shifts are bigger at higher frequency of function generator. 6.4 Surface Functionalization Method for Specific Biodetection The biotin was covalently attached to the surface of the microtoroid using a previously detailed process [14]. The silica surface was terminated with hydroxyl groups using an oxygen plasma treatment. Next, the hydroxylated microtoroids are 94 aminated through chemical vapor deposition of 3-aminopropyltrimethoxysilane (APTMS). Finally, the incubation of devices in a solution of NHS-Biotin (N-hydroxysuccinimide Biotin) in dimethylsulfoxide (DMSO) results in the formation of stable amide bonds on the devices. The final devices show uniform surface coverage, with no micron-scale damage defined by fluorescence microscopy. The schematic of biotinylation is shown in Figure 6-4. A scanning electron micrograph of a biotin functionalized microtoroid is shown in Figure 6-5 (b). Figure 6- 4 Schematic of biotinylation process on silica microtoroids. The Biotinylated microtoroid is utilized to detect a mixture solution of streptavidin and streptavidin functionalized polysterene beads. By combining the detection spectr a and dissociation kinetics, the specificity of label-free detection is improved. The schematics for detection is shown in Figure 6-5 (a). When both streptavidin and streptavidin functionalized polysterene beads flow into the chamber, streptavidin first binds onto the microtoroids surface 95 due to the mass transport. During balance of association and dissociation of streptavidin, several streptavidin species got dissociated from the binding sites, and the left vacancies were taken over by streptavidin functionalized polysterene beads. By analyzing the dissociation kinetics from the first blue shift spectra and second blue shift spectra, the different dissociation kinetics justify that two different species are existing. 96 Figure 6- 5 Schematic of the detection process. (a) The surface of the silica microtoroid is biotin-functionalized. The free streptavidin in solution binds to the microtoroid first. Once the free streptavidin begins to dissociate, the streptavidin-polybeads, which contain numerous streptavidin molecules per bead, can bind. (b) A scanning electron micrograph of a biotin functionalized microtoroid with major (minor) diameter at 105µm (5 µm). 97 6.5 Results and Discussion 6.5.1 Bimodal Spectra for Two Species Biodetection To characterize the optical performance or Q factor of the biosensor devices, tapered optical fibers are used to couple light from a narrow-linewidth tunable CW laser centered at 765nm into the microcavities [15-16]. Tapered optical fibers are low-loss waveguides which can work both in air and in buffer [17]. To align the sensor with the waveguide, the microtoroid is mounted on a nm-resolution stage and monitored by both a top-view and a side-view camera simultaneously. All measurements are performed in HEPEs buffer to ensure the conformational stability of the streptavidin and biotin. The quality factors of the microcavities were measured by scanning the laser across the resonant wavelength and measuring the linewidth (Δλ). The scan speed is optimized to eliminate non-linear distortion of the linewidth. The Q factor was calculated from the expression Q= λ/ Δλ and was greater than 10 6 in HEPEs buffer in all experiments. A representative transmission spectrum is shown in Figure 6-6. 98 Figure 6- 6 A representative transmission spectrum. The Q factor is calculated to be 3.27×10 6 . A syringe pump (0.1µm diameter, Bangs Laboratories, Inc)is used to inject the 100 fM protein solutions composed of streptavidin functionalized polystyrene beads and free streptavidin at a continuous flow rate of 100µL/min for 2 min before stopping. A custom LabVIEW program records the resonant wavelength position every 0.5 seconds for 5 minutes, including 2 minutes during injecting (association phase) and 3 minutes after injection (equilibrium and dissociation phase). All experiments were repeated multiple times, and the results shown in Figure 3 are representative data sets. Upon immediate inspection, this data is significantly different from the conventional resonant cavity detection experiments. Namely, the sensing signal contains two distinctly different resonant frequency shifts for a single injection. The appearance of the dual peaks or bimodal behavior is the result of a combination of effects, including the difference in the mass transport and association/dissociation constants of the two materials. 99 Figure 6- 7 Detection based on K d . The resonant wavelength shifts as the solution containing the streptavidin and the streptavidin polybeads is injected. Two clearly identifiable peaks are observed. 6.5.2 Discussion of Kinetics Using equation 1, the dissociation constants (K d ) of the pair of reactions can be calculated. The values of K d for the two dissociations are calculated based on the slope of each phase separately. K d for the first dissociation (K d1 ) is 2.72×10 -7 , while it is 8.94×10 -8 for the second dissociation (K d2 ) as is shown in Figure 6-8 top and bottom. It is important to note that a lower K d indicates a stronger interaction [13, 18]. Because the streptavidin polybeads have multiple streptavidin molecules per polystyrene bead, this peak is the streptavidin-polybead complex. 100 Figure 6- 8 Calculation of K d1 by the slope (above picture) and calculation of K d2 (below picture). In addition, because there are a finite number of binding sites on the cavity surface, until the streptavidin dissociates, the streptavidin-polybead cannot bind (Figure 6-5). In fact, given the cross-sectional area of the polystyrene bead, numerous streptavidin molecules must dissociate before such a binding event can occur. This dependence on binding site availability, termed steric hindrance or binding site occlusion, is frequently observed in biosensor surface chemistry. An additional piece of information which is somewhat hidden in this data is the molecule transit time and the association constant (K a ) [19-20]. Recent work studying fluid flow around optical resonant cavities has theorized that diffusion is responsible for the transport of molecules across the boundary layer [21]. The thickness of this boundary layer is related to the toroid geometry, with the minor diameter playing a significant role. Based on these calculations and the relative diffusion coefficients of streptavidin and 101 streptavidin-polybead, it is straightforward to determine that the streptavidin should bind to the surface of the cavity before the streptavidin-polybeads binds. The presented data set supports this hypothesis. 6.6 Conclusions In summary, by leveraging the information in a binding site, we have demonstrated a method to distinguish between two molecules, using the same device and a single surface chemistry. Namely, using biotin-functionalized microtoroid resonators, we identified streptavidin and streptavidin-polybeads from a mixture by measuring their K d values. This simple approach will allow researchers to use a single antibody to detect multiple analytes or to better confirm the presence of the specific analyte. As such, the combination of frequency shift and K d measurement will increase the reliability of resonant cavity-based biosensors, thus impacting both healthcare and defense industries [6-8]. 102 Chapter 6 References 1. A. M. Armani, R. P. Kulkarni, S. E. Fraser, R. C. Flagan, K. J. Vahala, Science, Label-free, single-molecule detection with optical microcavities, 317, 783 (2007). 2. F. Vollmer, S. Arnold, Nature methods, Whispering-gallery-mode biosensing: label-free detection down to single molecules, 5, 591 (2008). 3. H. K. Hunt, A. M. Armani, Nanoscale, Label-free biological and chemical sensors, 2, 1544 (2010). 4. F. Vollmer, L. Yang, Nanophotonics, Review Label-free detection with high-Q microcavities: a review of biosensing mechanisms for integrated devices, 1, 267 (2012). 5. C. Shi, S. Mehrabani, A. Armani, Optics Letters, Leveraging bimodal kinetics to improve detection specificity, 37, 1643 (2012). 6. A. L. Washburn, R. C. Bailey, Analyst, Photonics-on-a-Chip: Integrated Waveguides as Enabling Detection Elements for Lab-on-a-Chip Biosensing Applications, 136, 227 (2011). 7. H. K. Hunt, A. M. Armani, Nanoscale, Label-Free Biological and Chemical Sensors, 2, 1544 (2010). 8. D. Erickson, S. Mandal, A. H. J. Yang, B. Cordovez, Microfluid Nanofluid, Nanobiosensors: optofluidic, electrical and mechanical approaches to biomolecular detection at the nanoscale, 4, 33 (2008). 9. M. S. Luchansky, A. L. Washburn, M. S. McClellan, R. C. Bailey, Lab on a Chip, Sensitive on-chip detection of a protein biomarker in human serum and plasma over an extended dynamic range using silicon photonic microring resonators and sub-micron beads, 11, 2042 (2011). 10. C. Soteropulos, H. Hunt, A. M. Armani, Applied Physics Letters, Determination of binding kinetics using whispering gallery mode microcavities, 99, 103703 (2011). 11. A. B. Matsko, V. S. Ilchenko, IEEE Journal of Selected Topics in Quantum Electronics, Optical Resonators with Whispering-Gallery Modes-Part I: Basics, 12, 3 (2006). 103 12. W. E. Paul, Fundamental Immunology. (Lippincott, Williams & Wilkins, Philadelphia, ed. 5, 2003), pp. 1700. 13. S. Zhao, W. M. Reichert, Langmuir, Influence of Biotin Lipid Surface-Density and Accessibility on Avidin Binding to the Tip of an Optical Fiber Sensor, 8, 2785 (1992). 14. H. K. Hunt, C. Soteropulos, A. M. Armani, Sensors, Bioconjugation to Optical Microresonators, 10, 9317 (2010). 15. D. Armani, T. Kippenberg, S. Spillane, K. Vahala, Nature, Ultra-high-Q toroid microcavity on a chip, 421, 925 (2003). 16. X. Zhang, H. S. Choi, A. M. Armani, Applied Physics Letters, Ultimate quality factor of silica microtoroid resonant cavities, 96, 153304 (2010). 17. A. M. Armani, D. K. Armani, B. Min, K. J. Vahala, S. M. Spillane, Applied Physics Letters, Ultra-high-Q microcavity operation in H2O and D2O, 87, (2005). 18. N. M. Green, Method Enzymol, Avidin and Streptavidin, 184, 51 (1990). 19. D. G. Myszka, Curr Opin Biotech, Kinetic analysis of macromolecular interactions using surface plasmon resonance biosensors, 8, 50 (1997). 20. P. Schuck, A. P. Minton, Anal Biochem, Analysis of mass transport-limited binding kinetics in evanescent wave biosensors, 240, 262 (1996). 21. J. M. Gamba, R. C. Flagan, Applied Physics Letters, Flow-enhanced transient response in whispering gallery mode biosensors 99, 253705 (2011). 104 Chapter 7 Future Work Whispering Gallery Mode (WGM) microtoroid has been utilized in the field of label free protein detection with high sensitivity due to its high circulating power. Meanwhile the Plasmonic interaction between microtoroid and noble metals can significantly enhance the signal of resonant shift caused by enhanced optical field [1]. It is anticipated that when noble metal is on Plasmonic resonance, the enhanced optical field can increase the signal significantly (Figure 7-1). While it has been recently studied theoretically that when resonance of microtoroid overlap with Plasmon resonance of noble metal, the signal of protein detection can be optimized to maximum, the investigation of experimental data is not keeping face due to the lack of appropriate laser sources. For most of the noble metals such as Au, Ag, or Pt which can exhibit Surface Plasmon phenomena, their Plasmon resonances lie in visible range. For example it is about 300nm for Pt nanoparticles, 400nm for Ag nanoparticles, and 500nm for Au nanoparticles. Though fruitful achievements on utilizing microresonators for biodetection have been obtained, almost all of these experiments are performed at wavelength in the range of 600nm-1000nm. The hinder that limits a combination of noble metal Plasmon and microresonators experimentally is that a tunable narrow line-width laser with blue wavelength is rarely owned. Only with an appropriate wavelength laser, namely a blue laser, can the enhanced protein detection through Plasmon interaction got applied on an experimental investigation. 105 Figure 7- 1 Schematic of enhanced label-free biodetection owing to Plasmonic enhancement of noble metals. (a) Rendering of Ag Nanoparticles coated microtoroid, and (b) is schematic of enhanced biodetection. It is anticipated that resonance shifts due to attachment of protein will get enhanced significantly. Ag Plasmonic Enhanced protein detection We have successfully incorporated Au nanoparticles and nanorods onto toroid and studied the Plasmon interaction both experimentally and theoretically. Though Au nanorod has a Plasmon resonance at a visible range that can be tuned from 550nm to 800nm, it is not a good choice for enhanced protein detection. The reason is to be able to stabilize and not immobile in water, the Au nanorod has to be embedded in silica sol-gel, which requires to be treated at high temperature for 2 days. After that treatment, the nanorod is reshaped to nanospheres, and its Plasmon resonance shifts to 500nm. Unfortunately there is no tunable narrow line-width laser at 500nm available commercially so far. However if using the same technique to apply Ag nanoparticles onto the microresonator, we’ll be able to tune the Plasmon resonance of Ag to blue light [2], and a blue tunable narrow line-width laser is invented and commercially sold recently. By testing protein through Ag coated microtoroid, the signal can be significantly enhanced. Moreover compared to mixing protein with metal 106 nanoparticles, only a small amount of protein is needed and the protein can stay in good performance without having to get contaminated with other label molecules or particles. LSPR (Localized Surface Plasmonic Resonance) Based Label-Free Biodetection Plasmonic nanoparticles have been endowed to enhance signal of label-free biodetection both theoretically and experimentally. This has been achieved by combining nanoparticles with waveguide, photonic crystal or microcavities [3-5]. Taking nanoparticles coating microtoroids as an example, a tapered fiber is used to couple tunable laser into hybrid microtoroids, and a resonant wavelength is recorded in a high speed digitizer. When biotin attaches to the surface of hybrid microtoroids, resonant wavelength shows shifts. The existing Plasmonic nanoparticles can enhance the shift due to attachments; therefore increase the sensitivity and accuracy of label-free biodetection. Herein, we are planning to detect protein attachment by monitoring change of spectra caused by LSPR of gold nanorods instead of using resonant peak of microcavities. LSPR has a strong dependence of polarization of incident light [6]. Namely the spectra and photoluminescence can be illuminated only by certain polarized light. By putting a polarization controller on the taper before coupling position, the polarization of light in taper can be varied, therefore TE/TM fundamental mode of microcavities can be chosen to be excited. By comparing between TE/TM signals, we can acquire information about LSPR spectra and paves a way for further applications of biodetection. 107 Chapter 7 References 1. V. Dantham, S. Holler, V. Kolchenko, Z. Wan, S. Arnold, Applied physics letters, Taking whispering gallery-mode single virus detection and sizing to the limit, 101, 043704 (2012). 2. A. J. Haes, R. P. Van Duyne, Journal of the American Chemical Society, A nanoscale optical biosensor: sensitivity and selectivity of an approach based on the localized surface plasmon resonance spectroscopy of triangular silver nanoparticles, 124, 10596 (2002). 3. K. Lodewijks, W. Van Roy, G. Borghs, L. Lagae, P. Van Dorpe, Nano letters, Boosting the figure-of-merit of LSPR-based refractive index sensing by phase-sensitive measurements, 12, 1655 (2012). 4. C. Ropers et al., Nano letters, Grating-coupling of surface plasmons onto metallic tips: a nanoconfined light source, 7, 2784 (2007). 5. Y. Wu, F. Vollmer, in Cavity-Enhanced Spectroscopy and Sensing. (Springer, 2014), pp. 323-349. 6. T. Ming et al., Nano letters, Strong polarization dependence of plasmon-enhanced fluorescence on single gold nanorods, 9, 3896 (2009). 108 Appendix A: Optimizing the Signal to Noise Ratio of Microcavity Sensors A1 Introduction Over the last few years, optical microcavities have been utilized in a wide range of biological and chemical sensing applications, ranging from the detection of cytokines in serum to single virus and single nanoparticle detection [1-4]. One reason for pursuing microcavity-based sensors over alternative devices is their high quality factors (Q) or long photon lifetimes and the high built-up power [5]. The sensing signals can be obtained in various ways including resonant wavelength shift method [3, 6], heterodyne and beat note method [7], and resonant peak split method [8]. Typically, the sensing experiments involve measuring a change, in the resonant wavelength, δλ, or the quality factor, δQ, of the cavity, in response to a refractive index or optical absorption change [9]. Because the signal to noise ratio (SNR) governs the overall performances of biosensor, it is important to obtain a high SNR for solution with a low concentration, especially for single molecule detection. The signal can be enhanced by localizing the optical field in a smaller volume through Plasmon confinement or by adding a photonic crystal structure around the microcavity [10-11]. To decrease the noise level, 109 high quality factor of microcavities are utilized where the optical field is confined in microcavities for a longer period of time. High quality factor microcavities are widely used in label-free biodetection also because they possess a narrow linewidth and therefore a tiny resonant shift for example caused by a single molecule attachment can be detected. Figure A- 1 Several ways for label-free biodetection using microcavities, including (a) resonant wavelength shift method [6], resonant peak split method [12], and heterodyne and beat note method [7] The Q of a microcavity influences not only the signal, i.e., changes in the resonant wavelength, δλ, or the quality factor, δQ, but also the noise in both of the signals. Herein we work on different Quality factors and study their S/N respectively 110 both theoretically and experimentally, and try to figure out the optimum parameter for S/N of biodetection. The experiments were in the same trend as theoretical results, and interestingly the optimum Q for S/N is not the highest Q, but slightly below the highest Q. A2 Theoretical Simulation The simulations for the signals (Δλ and ΔQ) are based upon a finite element model of a microtoroidal cavity immersed in distilled (DI) water [4]. The constants, such as the refractive index of silica, water and the absorption of silica, water at 765nm, used in the simulations are either experimentally determined or taken from the published literature [13]. The changes in wavelength (Δλ) and in quality factor (ΔQ) are determined for the fundamental TM mode of the cavity. The signals (ΔQ) are the cavity response to a refractive index (RI) change of 5 10 -4 introduced into the DI water. These changes are recorded as a function of the Q of the cavity in the DI water (i.e., the Q before introducing the RI change). The Q is varied by changing the major diameter (D) of the cavity while keeping the minor diameter constant (d = 6um) [16]. The modeling results are shown in Figure A-2 (a), and (c). From the results, it can be seen that the signal (ΔQ) decreases with increase in the Q of the cavity. On the other hand, the signal ΔQ, increases with increase in the Q of the cavity until it reaches a maximum point, at which it starts decreasing. The primary reason for this behavior is that the overall Q of the cavity is limited by the absorption of the medium after a critical cavity size (here around major diameter of 75µm). 111 To develop an accurate model of the noise sources in the present system, it is necessary to examine all of the components (equipment specifications) individually. By doing this analysis, we conclude factors of the voltages due to the RIN, detector Shot and Johnson noise [14] in noise analysis. The voltage due to the quantization noise is estimated by using the analysis presented in [15]. The wavelength instability of the tunable laser is assumed to be of order of one linewidth (1fm) [16]. We use these noise voltages as input to the Monte Carlo simulations where we generate one thousand random vectors of each noise corresponding to the estimated standard deviations (i.e., previously mentioned estimated voltages). These noise voltages are then superimposed on the Lorentz resonant peak of the desired Q (i.e., generating one thousand Lorentz peaks with the superimposed noise). There are various noise sources in the microcavity sensors that affect both of the change in wavelength, ∆λ, and the change in quality factor, ∆Q, measurements. These include noise due to the (i) Laser intensity fluctuations (relative intensity noise, RIN), (ii) Wavelength instability of the laser, (iii) Wavelength non-repeatability in fine wavelength scan of the laser with an external function generator, (iv) Thermal fluctuations in the resonator, (v) Temperature variations of the flowcell, (vi) Detector (Shot and Johnson), (vii) Quantization in the digitizer/oscilloscope (i.e., No. of bits), and the (viii) Fluctuations in coupled power (e.g., tapered fiber jitter). The noise level of different quality factor devices is shown in figure A-2 (a). 112 The simulations were performed by Dr. Imran Cheema in Mcgill University (Canada). And we will show the comparison between theoretical and experimental results in the following sections. A3 Experiments and Discussions To experimentally study the optimum parameter for S/N, a series of microtoroids with major diameters ranging from 67µm to 121µm, and minor diameters set at 6 0.5µm are fabricated using the method stated in above chapters [10]. On a resonator testing set up, the microtoroids and a tapered fiber were immersed in a chamber filled with milli Q water. The coupling and wavelength of tunable laser was fixed at toroidal fundamental TE/TM mode. Without injecting NaCl solution and before immersed in water, the minimal point of the resonant peak is recorded using a custom Labview program for 3min. The recorded minima shift in a single interval time is then fitted to the Gaussian distribution to find the standard deviation. A typical experimental histogram of counts versus minima shift is shown in Figure A-2 (c). To reduce the effect of the taper jitter, the measurements are made with the taper in contact with the cavity. Additionally, to maximize the Q of the cavity, the noise measurements are performed in air. A summary result of noise varying with Q is shown in Figure A-2 (b). The modeling and experimental results were compared in Figure A-2 (b) and both of them share the same trend, though there are certain deviations in the exact values. However, if the assumption of the wavelength instability is modeled from 1fm to 19fm, then the 113 modeling and the experimental results are in good agreement. The results indicate that the total wavelength noise (Δλ) decreases with the increase in the Q of the cavity, for a Q > 10 6 , the wavelength instability of the laser starts becoming the dominant noise mechanism, and the noise due to the fine wavelength scan, a type of wavelength instability noise, is possibly in the range of 19f m. Figure A- 2 Study of noise levels versus different Q both theoretically and experimentally. (a)Monte Carlo simulations (MC) of individual noise sources sL-Laser wavelength instability.( b)Total wavelength noise, theoretically (solid line) and experimentally (dots); (c)A representative experimental histogram of counts versus minima shift; (d)Total quality factor noise, σQ. Since Q affects both signal and noise in biodetection experiments, we vary the values of Q to pursue an optimum condition for S/N and we did this by changing the major diameter of microtoroids here. The relationships between Q in water and the corresponding D were shown in Figure S-3. It is experimentally shown that Q increases from 2E5 to 2E7 as the D increases from 60µm to 121µm. However when 114 the diameter reaches certain size (121µm), dominant loss transitions from scattering loss to materials loss caused by water absorption, beyond which range Q should stay stable as diameter continues to be bigger. It decreases slightly after 121µm in this set of data mainly due to experimental limitations where both CO 2 reflow for toroids fabrication and tapered fiber-coupling become more difficult. Figure A- 3 Experimental quality factors of Microtoroids with various major diameters in water. The minor diameter is fixed at 6±0.5mm. Resonant shifts due to environmental refractive index changes for microtoroids are characterized experimentally on a resonator testing set up. Briefly, the microtoroid is immersed in a chamber filled with the Milli Q water. The initial resonant wavelength and the Q of the cavity are determined. The salt solution is then injected continuously by a syringe pump at 100µL/min for 20min until the resonant wavelength keeps stable with injecting. A typical resonant shift versus time spectra was shown in Figure A-3. The spectra first red shifts since initial injection, afterwards 115 reach a plateau, and finally blue shift after stop injection. A control experiment was performed under same conditions except pure milli Q water was chosen to flow into the chamber. The ending point difference between salt injection and water injection was defined as Δλ due to change of refractive index. Figure A- 4 Resonant shift with time when NaCl (black line) and Water (red line) are flowed into chamber containing microtoroids respectively. Solution starts to flow into the chamber since the initial point and stops at 20mins. While the change in the resonant wavelength, ∆λ, is tracked and recorded continuously, the change in quality factor, ∆Q is measured at the end of the experiment. The experimental results for ∆λ and ∆Q are shown in Figure A-5 (b), and Figure A-5 (d). The corresponding theoretical results from simulation are shown in Figure A-5 (a) and Figure A-5 (c). It is clear that they follow the same trend. The 116 exact values are different between theoretical and experimental. The main reason is the initial quality factor differences in water which may be caused by doped silica used in fabrication and coupling loss during experimental testing. Figure A- 5 Signal analysis both theoretically and experimentally. After studying noise and signal separately for microtoroids with different sizes, we now consider the S/N by combining the obtained data both theoretically and experimentally. The results were shown in Figure A-6. Both the theory and the experimental results exhibit threshold behavior in the S/N point, demonstrating that the highest Q is not necessarily the best condition for detection. There is a difference in the precise threshold value, which again is caused by the initial Q difference in water. 117 Figure A- 6 Signal/Noise analysis both theoretically and experimentally. A4 Conclusions In conclusion, we demonstrate that operating a microcavity sensor at its highest possible Q factor does not mean an optimal sensor performance for boththe wavelength shift and the quality factor shift measurements. In a sensing experiment, instead of picking the dimensions of a cavity (e.g., microtoroid, microsphere, Ring resonators, etc) randomly, one should pick an optimally sized cavity for an optimal Q to achieve the maximum signal to noise ratio. We believe that the present work will provide a starting point for achieving the ultrasensitive microcavity sensors. 118 Appendix A References 1. M. S. Luchansky, R. C. Bailey, Analytical chemistry, Silicon photonic microring resonators for quantitative cytokine detection and T-cell secretion analysis, 82, 1975 (2010). 2. F. Vollmer, S. Arnold, D. Keng, Proceedings of the National Academy of Sciences, Single virus detection from the reactive shift of a whispering-gallery mode, 105, 20701 (2008). 3. A. M. Armani, R. P. Kulkarni, S. E. Fraser, R. C. Flagan, K. J. Vahala, Science, Label-free, single-molecule detection with optical microcavities, 317, 783 (2007). 4. A. M. Armani, K. J. Vahala, Optics Letters, Heavy water detection using ultra-high-Q microcavities, 31, 1896 (2006). 5. M. S. Luchansky, R. C. Bailey, Analytical chemistry, High-Q optical sensors for chemical and biological analysis, 84, 793 (2011). 6. C. E. Soteropulos, H. K. Hunt, A. M. Armani, Appl Phys Lett, Determination of binding kinetics using whispering gallery mode microcavities, 99, 103703. 7. A. J. Maker, A. M. Armani, Applied physics letters, Heterodyned toroidal microlaser sensor, 103, 123302 (2013). 8. W. Kim, Ş. K. Özdemir, J. Zhu, L. He, L. Yang, Applied physics letters, Demonstration of mode splitting in an optical microcavity in aqueous environment, 97, 071111 (2010). 9. R. McKendry et al., Proceedings of the National Academy of Sciences, Multiple label-free biodetection and quantitative DNA-binding assays on a nanomechanical cantilever array, 99, 9783 (2002). 10. S. Shopova, R. Rajmangal, S. Holler, S. Arnold, Applied physics letters, Plasmonic enhancement of a whispering-gallery-mode biosensor for single nanoparticle detection, 98, 243104 (2011). 11. F. Vollmer, L. Yang, Nanophotonics, Review Label-free detection with high-Q microcavities: a review of biosensing mechanisms for integrated devices, 1, 267 (2012). 12. J. Zhu et al., Nature Photonics, On-chip single nanoparticle detection and 119 sizing by mode splitting in an ultrahigh-Q microresonator, 4, 46 (2010). 13. T. Toyoda, M. Yabe, Journal of Physics D: Applied Physics, The temperature dependence of the refractive indices of fused silica and crystal quartz, 16, L97 (1983). 14. A. Yariv, P. Yeh, Photonics: Optical Electronics in Modern Communications (The Oxford Series in Electrical and Computer Engineering). (Oxford University Press, Inc., 2006). 15. B. P. Lathi, Modern Digital and Analog Communication Systems 3e Osece. (Oxford university press, 1998). 16. J. D. Swaim, J. Knittel, W. P. Bowen, Applied physics letters, Detection limits in whispering gallery biosensors with plasmonic enhancement, 99, 243109 (2011).
Abstract (if available)
Abstract
High quality factor microcavities can serve as a good platform for microlaser, label free biosensor, and fundamental physics study due to its high circulating power. This thesis mainly investigates possibilities for developing Plasmonic nanoparticles and polymer thin film coating microcavities, and demonstrating the optical interaction between optical filed and Plasmonic nanoparticles. A new type of upconversion microlaser based on two photon upconversion of gold nanorods was invented. Performances such as threshold power of the new type laser were demonstrated and optimized. Besides, this thesis has also demonstrated improved performances of label-free biosensor based on microcavities by combining the kinetics constants or varying quality factors of microcavities which were used for detection. ❧ In this thesis, it is first demonstrated the possibilities of permanently coating gold nanoparticles and polydimethylmethacrylate (PMMA, M.W. 35,000) thin film on microcavities with quality factor over than 10⁷. The quality factor was studies both theoretically and experimentally, proving materials loss was the main loss mechanisms. Then low threshold Plasmonic upconversion microlaser is demonstrated where Plasmonic resonance of gold nanorods overlaps with optical resonance of microcavities. The Plasmonic interaction between optical field and gold nanorods are investigated theoretically using 3-D FDTD (Finite Domain Time Domain) simulations. Finally this thesis shows improved performances of label-free biosensor based on resonant shift method of microcavities.
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Development of hybrid optical microcavities for Plasmonic laser and improving biodetection
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