University of Southern California Dissertations and Theses

Metasurface-integrated black phosphorus photodetectors

(USC Thesis Other)

Metasurface-integrated black phosphorus photodetectors

PDF

Download a page range

Download transcript

Copy asset link

Request this asset

Transcript (if available)

Content Copyright 2023 Max Ray Lien
METASURFACE-INTEGRATED BLACK PHOSPHORUS PHOTODETECTORS
by
Max Ray Lien
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
(ELECTRICAL ENGINEERING - ELECTROPHYSICS)
December 2023

ii
Acknowledgements
I would like to express my heartfelt gratitude to all those who have supported and guided me
throughout this arduous yet incredible journey of pursuing my doctorate. It would not have been
possible without the unwavering encouragement, assistance, and inspiration of numerous
individuals, to whom I am deeply indebted.
Foremost, I extend my sincere appreciation to my advisors, Michelle L. Povinelli and Han Wang,
for their invaluable guidance and expertise. Their insightful feedback and dedication to my
academic growth have been instrumental in shaping the direction of this dissertation.
I am profoundly thankful to my colleagues and fellow researchers, at UW-Madison, USC, and
JPL, for their knowledge, shared insights, and collaborative spirit. Their diverse perspectives
have enriched my understanding and expanded the horizons of my work.
To my family, whose unwavering support has been my bedrock throughout this journey, I offer
my deepest gratitude. Your belief in my abilities and understanding during these challenging
times have been a constant source of motivation.
And to my friends who stood by me since the screening exam: Your presence and shared laughter
provided a much-needed balance when discovering the truths of academia.

iii
Table of Contents
Acknowledgements ......................................................................................................................... ii
List of Tables .................................................................................................................................. iv
List of Figures ..................................................................................................................................v
List of Publications ........................................................................................................................ xi
Abstract ......................................................................................................................................... xii
Chapter 1: Introduction ....................................................................................................................1
Chapter 2: Background on Black Phosphorus .................................................................................3
Chapter 3: Characterization of Black Phosphorus ...........................................................................5
Chapter 4: Integrated Metasurfaces for Black Phosphorus Photodetectors .....................................8

Chapter 5: Resonant Grating-Enhanced Black Phosphorus Photoconductors .................................9
Chapter 6: Black Phosphorus Molybdenum Disulfide Photodiodes
with Broadband Metasurface Gratings ........................................................................16
Chapter 7: Metalens-Integrated Black Phosphorus Photodetector ................................................26
Chapter 8: Conclusion, Outlook, and Future Research Directions ................................................38
References ......................................................................................................................................40
Appendix A: Fellowship Acknowledgement .................................................................................47

iv
List of Tables
Table 1. Structural parameters of each device in Figure 6-3. ........................................................22
Table 2. Responsivity R and specific detectivity D* for Device 1 and Device 2 with and
without the metasurface grating. ....................................................................................................25

v
List of Figures
Figure 1. Atomic structure of black phosphorus. Diagram showing two layers of the puckered
black phosphorus crystal structure. The interlayer spacing is approximately 5.3 Å. ......................3
Figure 2. Characterization of a black phosphorus sample. (a) Atomic force microscope line
scan measurement of a BP sample. An image of this BP sample is shown in the inset. The
direction of the line scan is represented with the dashed red line in the inset, as well as BP’s
orthogonal axes. (b) Measured absorption at 3.4 µm of the sample in (a) with a fit line. The BP
rests on an Al2O3-on-Au substrate. Absorption was approximated with A(α) = 1 - R - T (T = 0).
α is the incident light’s linear polarization angle measured from the AC axis, which is depicted
in the inset of (a). (c) Polarization-resolved Raman spectra of the BP sample shown in (a). 532
nm incident light was polarized along the AC or ZZ direction........................................................6
Figure 5-1. Metal-insulator-metal gratings with black phosphorus. (a) Metal-insulator-metal
(MIM) grating structure with a black phosphorus (BP) absorber layer and its structural
parameters. A is the grating period, L is the grating length, TB is the thickness of the BP, and
TD is the thickness of the Al2O3. (b) Scanning electron microscope image of four distinct MIM
gratings each with A = 1000 nm on a TB = 33 nm and TD = 80 nm sample. The inset is a
zoomed-in image of the grating with L = 650 nm. (c) Measured absorption spectra of the
L = 520 nm MIM grating at linear polarization angles α = 0° and α = 90° and the same sample
without a grating at α = 0°. The inset is the maximum absorption value plotted as a function

vi
of α. (d) Measured absorption spectra (α = 0°) of the four gratings in (b). (e) Measured and
simulated absorption at α = 0° of the L = 520 nm MIM and the same sample with no MIM. (f)
Measured and simulated wavelength λMax at which absorption is maximized for the gratings in
(d). The dashed line is a linear function fitted with simulated data points. ................................... 11
Figure 5-2. Metasurface grating-integrated black phosphorus photoconductor. (a) Scanning
electron microscope image of two BP photoconductors. One photoconductor has an integrated
metasurface grating, and the other photoconductor has no grating. (b) Current-voltage
measurement of the integrated-grating photoconductor: IOn is the current when the device is
illuminated, IDark is the dark current, and IPH = IOn - IDark. (c) IPH and responsivity at three
different device biases as a function of incident power. ................................................................13
Figure 5-3. Room-temperature responsivity enhancement in a grating-enhanced black
phosphorus photoconductor. Polarization-resolved photodetector responsivity for a
photoconductor with its grating’s resonance centered around the testing wavelength λ = 3.37
µm and a photoconductor with no resonant grating. α is angle of the incident light’s linear
polarization, and φ is angle between α = 0° and the fitted line. ....................................................15
Figure 6-1. Black phosphorus photodiode with an integrated metasurface grating. (a) Device
schematic of a black phosphorus (BP) molybdenum disulfide photodiode structure, in which
TBP and TMo are the thicknesses of the BP and MoS2 layers respectively. (b) Scanning electron
microscope (SEM) image of a BP-MoS2 photodiode. The angle between BP’s orthogonal axes
is denoted with θ. (c) Absorption spectra (A = 1 - R - T, T = 0) of the device in (b) measured

vii
with linearly polarized light along θ = 0° (AC axis) and θ = 90° (ZZ axis). The inset is the
Raman spectra with linearly polarized 532 nm light along θ = 0° and θ = 90°. (d) Device
schematic of a BP-MoS2 photodiode after integrating the metasurface grating with period A
and width W. (e) SEM image of the same device in (b) after integrating the metasurface. The
inset shows a zoomed-in view of the grating. (f) Simulated and measured absorption spectra
of the device in (b) at θ = 0° and θ = 90° after integrating the metasurface. The inset is the
simulated intensity profile of |H|2
at λ = 3 µm. .............................................................................18
Figure 6-2. Multi-resonator metasurface gratings. (a) Metasurface-integrated BP-MoS2
photodiode with a unit cell that has two gratings: one grating has a width W1 and the other has
a width W2. (b) BP-MoS2 photodiode with a triple-resonator grating. Like the double
resonator system, one unit cell includes three gratings with widths W1, W2, and W3.
(c) Measured and simulated absorption spectra (A = 1 - R - T, T = 0) of a double-resonator
metasurface grating. (d) Measured and simulated absorption of a triple-resonator metasurface
grating. ...........................................................................................................................................20
Figure 6-3. Absorption spectra of metasurface gratings with varying geometries. (a) Measured
and (b) simulated absorption spectra (A = 1 - R - T, T = 0) of three single-resonator devices
(MIM 1-3). The structural parameters for all devices in this Figure are included in the text as
Table 1. (c) Measured and (d) simulated absorption spectra of three double-resonator devices
(MIM 4-6). (e) Measured and (f) simulated absorption spectra of two triple-resonator devices
(MIM 7-8). ....................................................................................................................................21

viii
Figure 6-4. Room temperature performance of two black phosphorus photodiodes. Device 1
has an integrated metasurface with a single resonance, and Device 2 has an integrated
metasurface with three resonances. (a) Measured absorption spectra (A = 1 - R - T, T = 0) of
Device 1. Device 1 has same the structural parameters as MIM 1 in Table 1. The testing
wavelengths λ1 = 3.39 and λ2 = 3.88 µm are shown with vertical lines. (b) Dark current as a
function of applied voltage bias VDS in Device 1. (c) Measured current in Device 1 with
linearly polarized λ1 = 3.39 µm illumination (IOn) both before and after integration of the
metasurface, and dark current (IOff). The inset is the polarization-resolved responsivity at λ1 =
3.39 µm and VDS = -0.5 V after integrating the metasurface (θ is defined in Figure 6-1).
(d) IOn in Device 1 measured with 30 µW of linearly polarized λ2 = 3.88 µm illumination
before and after the integration of the metasurface, and IOff. The inset is the polarizationresolved responsivity at λ2 = 3.88 µm and VDS = -0.5 V after integrating the metasurface. (e)
Measured absorption spectra of Device 2. Device 2 has the same structural parameters as
MIM 7 in Table 1. (f) Dark current as a function of applied voltage bias VDS in Device 2.
(g) IOff and IOn in Device 2 with 30 µW of linearly polarized λ1 = 3.39 µm light before and
after integrating the metasurface. (h) IOff and IOn in Device 2 with 30 µW of linearly polarized
λ2 = 3.88 µm light before and after integrating the metasurface. The insets of (g) and (h) are
the polarization-resolved responsivity after integrating the metasurface at λ1 = 3.39 and
λ2 = 3.88 µm, respectively, and VDS = -0.5 V. ...............................................................................23
Figure 7-1. Concept a metalens-integrated black phosphorus photodiode. Incident light is
focused by the metalens with area Alens into a photodiode with a smaller area Adect, thereby
increasing the impinging power density on the detector. ...............................................................27

ix
Figure 7-2. Design of a silicon metalens for focusing mid-wave infrared light. (a) Nanorod
unit cell with height h, diameter d, and gap spacing g. (b) Simulated phase shift (normalized to
2π) and transmittance at 3 and 4 µm for a nanorod unit cell with variable d and fixed h = 2.5
µm and g = 0.3 µm. (c) Metalens design map for focusing mid-wave infrared light through a
f = 300 µm thick substrate. Only the center part of the full metalens design is shown. Each
marker represents one nanorod, and the size and color of a marker represents the
corresponding the nanorod’s diameter d. ......................................................................................29
Figure 7-3. Metalens characterization. (a) Scanning electron microscope image of a silicon
metalens. The inset shows the constituent nanorods. (b) Setup for characterizing the focal spot
of a metalens. (c) Measured xz-plane image of the metalens’ focal spot at λ = 3050 nm. (d)
Measured and simulated normalized intensity profile of the focal spot at z = 300 µm and λ =
3050 nm. ........................................................................................................................................31
Figure 7-4. Measured and simulated focal spots. The measured and simulated position and
chromatic trend of the focal spots are in good agreement at five testing wavelengths (3050,
3390, 3900, 4220, and 4665 nm). The dashed lines indicate the silicon-air interface. ..................32
Figure 7-5. Black phosphorus photodiodes with integrated metalenses. (a) Photodiode with an
integrated metalens. (b) Photodiode with no integrated metalens. (c) Measured absorption
spectrum (A = 1 - R - T) of a black phosphorus molybdenum disulfide photodiode along its

x
orthogonal axes. The inset is a scanning electron microscopy image of the metalens-integrated
photodiode......................................................................................................................................33
Figure 7-6. Metalens-enhanced mid-wave infrared photodetection. (a) Measured current as a
function of voltage bias for the photodiode with no integrated metalens: IOff is the dark current
and IOn is the current under λ = 3.39 or 3.88 µm illumination. (b) IOff and IOn for a
photodetector with an integrated metalens: IOn exceeds that of the photodetector with no
integrated metalens. (c) Extracted photocurrent IPH = IOn - IOff and responsivity at λ = 3.39 µm
for both the metalens-enhanced and unenhanced photodetectors. (d) Extracted photocurrent
IPH and responsivity at λ = 3.88 µm for both the metalens-integrated and unintegrated
photodetectors. ..............................................................................................................................34

xi
List of Publications
Certain figures and text within this dissertation have been reproduced in part from the following
publications with copyright permission.
Reproduced in part with permission from Lien MR, Wang N, Wu J, Soibel A, Gunapala SD,
Wang H, Povinelli ML. Resonant grating-enhanced black phosphorus mid-wave infrared
photodetector. Nano Letters. 2022;22(21):8704-10. Copyright 2022 American Chemical Society.
Reproduced in part with permission from Nano Letters, submitted for publication. Unpublished
work copyright 2023 American Chemical Society: Lien MR, Wang N, Guadagnini S, Soibel A,
Gunapala SD, Wang H, Povinelli ML. Black phosphorus molybdenum disulfide mid-wave
infrared photodiodes with broadband absorption-increasing metasurfaces.
Reproduced in part with permission from Advanced Optical Materials, in press. 2023 John Wiley
& Sons, Inc.: Lien MR, Wenger T, Wang N, Wang H, Gunapala SD, Povinelli ML. An AllSilicon Metalens Integrated with a Mid-Wave Infrared Black Phosphorus Photodiode.

xii
Abstract
Thin-film black phosphorus has emerged as a promising material for room temperature midwave infrared photodetection. Among its direct bandgap in the mid-wave infrared, black
phosphorus exhibits high carrier mobility, substrate compatibility, and bandgap tunability.
However, many of these attractive optoelectronic properties are only observable at smaller film
thicknesses, which ultimately inhibits photodetector absorption. To address this challenge, we
propose to integrate black phosphorus photodetectors with metasurfaces that boost absorption
and performance in the mid-wave infrared. This dissertation begins by providing a background
on black phosphorus and integrated metasurface gratings and lenses. Then, we discuss our
experimental results from integrating metasurface gratings and lenses with black phosphorus
photodetectors. We show that these integrated metasurfaces enhance the room temperature
performance of black phosphorus photoconductors and photodiodes in the mid-wave infrared.
Finally, we summarize our results and discuss future research directions.

1
Chapter 1: Introduction
Spectral imaging in the mid-wave infrared (MWIR, 3 - 5 µm) has emerged as a highly effective
tool for large scale environmental studies and is central to Earth and planetary science
missions.1,2 By analyzing rich spectral data from a scene, scientists are able to extrapolate
information about soil composition, plant health, and biodiversity.1,3 For these applications,
imaging spectrometers have been traditionally designed for integration with large platforms such
as airplanes. In recent years, compact imaging spectrometers have been integrated with lowermass platforms such as SmallSats.4
In 1993, NASA’s first airborne imaging spectrometer AVIRIS
was integrated with a U2 reconnaissance airplane.1
In 2015, the SWIS imaging spectrometer was
successfully launched on a SmallSat platform.4
The reduced mass and footprint of smaller
platforms such as CubeSats and SmallSats are economically desirable for orbital launch vehicles.
However, it is challenging to miniaturize imaging spectrometers for platforms that limit size,
weight, and power (SWaP) without sacrificing performance.
Typical MWIR detectors and focal plane arrays (FPAs), which generate noise at ambient
temperatures and have a fixed cut-off wavelength, rely on cooling systems for operation.5
However, the large mass and power consumption of cooling systems make it challenging to
integrate MWIR imaging systems with small platforms. FPAs based on thin-film black
phosphorus (BP) and its arsenic alloy BP-As could accelerate the miniaturization of imaging
spectrometers without sacrificing performance. Specifically, thin-film BP’s unique material
properties could be used to enable uncooled MWIR photodetectors with high performance at
room temperature and dynamic cut-off wavelength tunability.

2
BP offers attractive material properties that can be leveraged for miniaturizing imaging
spectrometers while introducing novel detector capabilities. Specifically, it features a high
photoresponse in the MWIR, room temperature operation, and a dynamically tunable optical
bandgap.6,7 Multiple studies have shown that uncooled BP detectors exceed or match the
performance of commercially available cooled detectors.8,9 But in order to take advantage of
many of these properties, the BP layer thickness must be kept thin, which ultimately limits
photodetector absorption and performance. To address this challenge, we identified, fabricated,
and characterized two nanostructured metamaterials (metasurfaces) to boost absorption in BP
photodetectors. The nanostructures, which are a metasurface grating and lens, can be easily
integrated with BP detector architectures and are scalable to FPAs. To demonstrate the boost in
absorption and ease of integration, we integrated thin-film BP photodetectors with metasurfaces
and showed that the nanostructures increase performance in the MWIR.
This dissertation details the results from integrating absorption-boosting metasurfaces with BP
photodetectors. First, a background on the remarkable optoelectronic properties of BP is
provided, which is followed by a summary of its experimental material characterization. Then,
we describe the integration of a metasurface grating and lens with BP photodetectors and show
that they boost performance at room temperature. Finally, future research directions and outlooks
are discussed.

3
Chapter 2: Background on Black Phosphorus
BP belongs in the class of 2D layered materials, which are single layers of atoms that are stacked
and held together by van der Waals forces.10 2D materials such as graphene, hexagonal boron
nitride, and transition metal dichalcogenides have attracted significant interest due to their
electronic, optical, and mechanical properties.10,11 In recent years, BP has been investigated as a
materials system for MWIR photodetection.12 Figure 1 depicts the anisotropic layered crystalline
structure of BP, which exhibits an anisotropic and puckered atomic structure.13
Figure 1. Atomic structure of black phosphorus. Diagram showing two layers of the puckered black
phosphorus crystal structure. The interlayer spacing is approximately 5.3 Å.
BP enables high responsivity MWIR photodetection at room temperature because of its direct
bandgap (300 meV, 4 µm) and high carrier mobility (~1000 cm2
V−1 s−1).6,11 Previously reported
MWIR BP photodetectors have exhibited specific detectivity (D*) values of 2.4 × 1010 cm Hz1/2
W−1 at room temperature, which is an order of magnitude higher than commercially available
MWIR detectors operating at room temperature.9,14 The high performance of BP photodetectors
at room temperature, along with its material properties, holds significant promise for
miniaturizing MWIR imaging spectrometers.

4
A unique property of BP is its dynamically and statically tunable bandgap. It has been shown that
bandgap of BP can be dynamically tuned with vertical electric fields (~50 - 300 meV, 2.5 - 4 µm)
and statically tuned with the number of layers (~300 - 2200 meV, 4 - 5.6 µm) or percent of
arsenic alloying (~ 150 - 300 meV, 4 - 8 µm).7,14-16 Because tuning a photodetector’s bandgap
changes the range of absorbed wavelengths, BP devices introduce new functionalities that are not
available in conventional MWIR systems. For instance, dynamic bandgap tuning facilitates
spectral imaging because taking the differential signal from two wavelength ranges yields
spectral information.17 Additionally, we anticipate that a tunable bandgap will enable BP
detectors to exhibit dynamic absorptive spectral features.18,19

In addition to its tunable bandgap, the anisotropy of BP can significantly enhance the science
capabilities of MWIR photodetectors. The low crystalline symmetry of BP, which is evident
from the orthogonal armchair (AC) and zigzag (ZZ) axes in Figure 1, creates polarizationdependent absorption and photocurrent collection in the MWIR.9,13 The AC axis exhibits high
absorption and mobility; and the ZZ axis exhibits lower absorption and mobility than the AC
axis.6,13 These characteristics could be used to support science missions that require
spectropolarimetric measurements.9,20,21

5
Chapter 3: Characterization of Black Phosphorus
Each BP device discussed in this dissertation was fabricated with samples that originated from a
bulk crystal. This section details the methods used to create and characterize samples of BP that
are intended for use as photodetector absorber layers.
Bulk BP crystals were procured from HQ Graphene and were exfoliated onto
polydimethylsiloxane (PDMS) stamps. We performed the exfoliation in an argon-purged glove
box that is free of oxygen and water vapor in order to minimize oxidation of BP.22 Then, BP
flakes with desired thicknesses were identified with optical microscope images.23 Once suitable
BP flakes were identified, we transferred them from the PDMS stamp onto the device
substrates.24 We measured the BP layer’s thickness with an atomic force microscope (AFM,
Bruker Dimension Icon). Figure 2a shows an AFM line measurement of a 33 nm thick BP flake.
An image of the BP sample is provided in the inset of Figure 2a, and the dashed red line depicts
the direction and location of the AFM line scan.

6
Figure 2. Characterization of a black phosphorus sample. (a) Atomic force microscope line scan
measurement of a BP sample. An image of this BP sample is shown in the inset. The direction of the line
scan is represented with the dashed red line in the inset, as well as BP’s orthogonal axes. (b) Measured
absorption at 3.4 µm of the sample in (a) with a fit line. The BP rests on an Al2O3-on-Au substrate.
Absorption was approximated with A(α) = 1 - R - T (T = 0). α is the incident light’s linear polarization
angle measured from the AC axis, which is depicted in the inset of (a). (c) Polarization-resolved Raman
spectra of the BP sample shown in (a). 532 nm incident light was polarized along the AC or ZZ direction.
After measuring the thickness of BP samples, we measured their anisotropic axes. BP exhibits
two distinct axes: The AC axis exhibits high optical absorption and carrier mobility; The ZZ axis
exhibits low optical absorption and carrier mobility.6,13 Figure 2b plots the polarization-resolved
absorption at 3.4 µm of the BP flake in Figure 2b. The BP flake was on an Al2O3-on-Au
substrate, and absorption was measured with a Fourier-transform infrared spectrometer (FTIR,
Bruker). Each measurement point is the estimated absorption A(α) = 1 - R - T (T = 0) at 3.4 µm
taken at one linear polarization angle α. α is depicted in the inset of Figure 2a, and the AC axis is
aligned with α = 0°. The AC and ZZ axes are clearly distinguishable in Figure 2b because
absorption is maximized at α = 0° and minimized at α = 90°. The result in Figure 2b is fitted to
the equation A(α) = AAC cos2
(α) + AZZ sin2
(α), where AAC/ZZ is the maximum/minimum
absorption at 3.4 µm.
We also measured the AC and ZZ axes with Raman spectroscopy (Renishaw, 532 nm excitation).
The polarization-resolved Raman spectra of the same flake in Figure 2a is shown in Figure 2c.

7
The characteristic Ag
1
, B2g, and Ag
2
peaks are evident. Measuring both the FTIR and Raman
spectra are necessary because there are reported inconsistencies in the literature as to if the Ag
2
peak is maximized along the AC or ZZ axis.23

8
Chapter 4: Integrated Metasurfaces for Black Phosphorus Photodetectors
One challenge of using thin-film BP is the limited absorption (~ 3% absorption for a 15 nm thick
film). In order to enable thin-film, high-absorption BP photodetectors, a variety of detection
mechanisms and nanostructures have been used to increase the amount of collected signal. For
instance, photogating and gate voltage tuning effects can dramatically increase the performance
of BP photodetectors.7,25 Additionally, integrated nanostructures such as dielectric waveguides,
plasmonic metamaterials, and optical cavities have been shown to improve the absorption and
quantum efficiency in BP devices.26-34 Generally, these methods do not affect the intrinsic
absorption of BP (i.e., the imaginary part of its refractive index). Rather, they increase the
amount of incident radiation that is collected by a BP absorber layer, which in turn boosts the
device’s signal-to-noise ratio (SNR).
We identified two nanostructured metasurfaces that increase photodetection in the MWIR and
are easily integrated with BP: a resonant metallic grating and monolithic dielectric metalens. The
resonant metallic grating is discussed in Chapters 5 and 6, in which we show that it increased the
room temperature performance of MWIR BP photoconductors and photodiodes at selected
wavelengths or across large wavelength ranges. Chapter 7 summarizes our experiment of
integrating a monolithic dielectric metalens with a BP photodiode. In Chapter 8, we discuss how
these metasurfaces could be scaled to BP FPAs.

9
Chapter 5: Resonant Grating-Enhanced Black Phosphorus Photoconductors
The first metasurface that we studied is a metal-insulator-metal (MIM) metasurface grating.35,36
An MIM structure consists of a metallic periodic grating and back reflector that encapsulate a
non-metallic absorber layer.37 The MIM structure supports a lateral resonant mode within the
absorber layer, and the spectral properties of this mode are dictated by the its structural
parameters (e.g. thickness, periodicity, depth).38,39 Impinging light couples to the mode, which
recirculates within the absorber layer. Because the round-trip propagation path increases, the
amount of light absorbed within the absorber layer also increases.38 In a previous study, MIM
structures were numerically predicted to increase the amount of light absorbed in thin-film BP.35
We experimentally showed that an adapted MIM structure significantly increases absorption in
thin-film BP photodetectors.36 The MIM structure is shown in Figure 5-1a. MIMs were
fabricated on a silicon substrates and consist of a 60 nm thick Au back reflector, Al2O3 dielectric
layer, BP absorber layer, and 50 nm thick Au/Ti gratings.40 The BP layer was mechanically
exfoliated from a bulk crystal and characterized with the methods described in Chapter 3. The
back reflector’s thickness is sufficient such that MWIR radiation is not transmitted. Also, the
back reflector could also serve as a back voltage gate that controls the carrier density of the BP
channel.18 But modulating the carrier density of the BP layer inside this MIM structure could
spectrally shift the location of its resonant mode and may modify carrier collection.19 The
dielectric layer blocks a short circuit path through the top metallic contacts, BP, and back
reflector.41 We selected Al2O3 because of its transparency and relatively dispersion-less refractive
index in the MWIR. The dielectric and metallic layers were deposited with e-beam evaporation

10
(Angstrom, Kurt J. Lesker), and the gratings were patterned with e-beam lithography at 100 kV
(Raith).
The MIM’s periodic unit cell is defined by the structural parameters shown in Figure 5-1a:
dielectric thickness TD, BP thickness TB, periodicity A, and grating length L. Only three unit cells
are displayed in Figure 5-1a, but the grating was designed by assuming infinite unit cells. To
characterize the MIM’s resonant properties, we designed and fabricated four MIM structures
with a varying L and fixed A = 1000 nm, TD = 80 nm, and TB = 33 nm. The MIMs were
fabricated with 30 unit cells each, and an scanning electron microscope (SEM) image of these
four devices is shown in Figure 5-1b.
We measured the four MIM structures that are shown in Figure 5-1b with an FTIR. Their lengths
are L = 520 nm, L = 540 nm, L = 580 nm, and L = 650 nm. Figure 5-1c plots the absorption
spectra (A = 1 - R - T, T = 0) at linear polarization angles α = 0 and 90° of the L = 520 nm
grating. α is the angle between the incident linear polarization and the AC axis, which is shown
in Figure 5-1b (also see Figure 2a, Chapter 3). Additionally, Figure 5-1c includes the absorption
spectrum at α = 0° of the BP film on the Al2O3 and Au layers but with no MIM. When compared
to the no-MIM case, the MIM’s resonance clearly increased absorption around a resonant
wavelength of λ = 3.58 µm. Also, the absorption of the MIM is maximized when α = 0°, and the
MIM is nearly completely reflective when α = 90°. Because the grating itself acts similarly to a
wire grid polarizer, light is reflected when the incident electric field is parallel to the grating
lines.42 The inset of Figure 5-1c tracks absorption at the wavelength λMax = 3.58 µm as a function
of α. Figure 5-1d shows the spectra of all four gratings L = 520, 540, 580, and 650 nm at α = 0°.

11
Figure 5-1d reveals that as L increases, λMax shifts to higher wavelengths, which is consistent
with Reference 35.
Figure 5-1. Metal-insulator-metal gratings with black phosphorus. (a) Metal-insulator-metal (MIM)
grating structure with a black phosphorus (BP) absorber layer and its structural parameters. A is the
grating period, L is the grating length, TB is the thickness of the BP, and TD is the thickness of the Al2O3.
(b) Scanning electron microscope image of four distinct MIM gratings each with A = 1000 nm on a TB =
33 nm and TD = 80 nm sample. The inset is a zoomed-in image of the grating with L = 650 nm. (c)
Measured absorption spectra of the L = 520 nm MIM grating at linear polarization angles α = 0° and α =
90° and the same sample without a grating at α = 0°. The inset is the maximum absorption value plotted
as a function of α. (d) Measured absorption spectra (α = 0°) of the four gratings in (b). (e) Measured and
simulated absorption at α = 0° of the L = 520 nm MIM and the same sample with no MIM. (f) Measured
and simulated wavelength λMax at which absorption is maximized for the gratings in (d). The dashed line
is a linear function fitted with simulated data points.
Figures 5-1e and 5-1f compare the measured results with finite-difference time-domain (FDTD)
electromagnetic simulations. We calculated the MIM structures’ absorption spectra with periodic
simulations containing one unit cell.35 The optical constants of BP are borrowed from Reference
13. The MIM’s resonant mode was illuminated by a plane wave source, and absorption was

12
extracted with power flux monitors. Figure 5-1e compares the FDTD-simulated and FTIRmeasured spectra for the L = 520 nm device. A similar resonant peak is observable in both the
simulated and measured spectra. There is a noticeable reduction in absorption from the
simulation to the measurement, which could be attributed to loss from our evaporated gold, the
bulk BP optical constants from Reference 13, or the simulated structure assuming infinite unit
cells but the fabricated structure consisting of finite number of unit cells. Figure 5-1f plots λMax
of the four gratings. The simulated and measured trends as L is varied are mostly linear. The
plotted dashed line was fitted to the simulated values of λMax. There is a blueshift between the
measured and simulated λMax, which is likely due to using bulk material data from Reference 13
at a thin BP film thickness. The inset of Figure 5-1f confirms that the resonant mode at λMax
strongly confines the H field within the BP layer.
To demonstrate that MIMs boost photodetector performance, we fabricated two photoconductor
detectors onto one TBP = 29 nm BP film. One photoconductor was enhanced with an MIM
device, and the other photoconductor had no integrated grating. Figure 5-2a is an SEM image of
both of the photoconductors. To protect the photoconductors from oxidation, a thin photoresist
layer (~60 nm) was applied on top of the devices. A horizontal voltage bias VDS was applied
across identical photodetection areas of approximately 30 × 30 µm2
. The MIMs were oriented so
that the grating lines are perpendicular to the BP film’s AC axis, which maximizes absorption
when light is along the direction of VDS.
35 Electron-hole pairs are generated when incident
radiation is absorbed by the photoconductor and are then swept across the AC axis as
photocurrent.8,25

13
We used the results in Figure 5-1 to design the photoconductor in Figure 5-2a, which has an
MIM centered around a testing wavelength of λ = 3.37 µm. By using the data in Figure 5-1f, we
found that a grating length of L = 490 nm resulted in λMax = 3.37 µm. The λMax = 3.37 µm testing
laser emitted Plaser = 34 µW within an approximately 40 × 40 µm2
spot. To extract the
photocurrent generated in both detectors shown in Figure 5-2a, we focused the λ = 3.37 µm laser
beam to the center of each detector’s Adect = 30 × 30 µm2
active region. Therefore, the incident
power that impinged on each detector is Pinc = Plaser (Adect / Aspot) = ~19 µW. The incident power
was controlled with neutral density filters (Thorlabs) and confirmed with an optical power meter
(Thorlabs). The signal’s frequency was modulated at 200 Hz with an optical chopper (Thorlabs),
photocurrent was extracted with a lock-in amplifier (Stanford Research Systems), and the bias
VDS was applied with a source measure unit (Keithley). The photoconductors were measured at
room temperature.
Figure 5-2. Metasurface grating-integrated black phosphorus photoconductor. (a) Scanning electron
microscope image of two BP photoconductors. One photoconductor has an integrated metasurface
grating, and the other photoconductor has no grating. (b) Current-voltage measurement of the integratedgrating photoconductor: IOn is the current when the device is illuminated, IDark is the dark current, and IPH
= IOn - IDark. (c) IPH and responsivity at three different device biases as a function of incident power.
The measured photocurrent in the MIM-integrated photoconductor is shown in Figure 5-2b as a
function of VDS. IOn is the current when the device is illuminated, IDark is the dark current, and IPH

14
is the photocurrent calculated as IPH = IOn - IDark. VDS and IPH exhibit a linear relationship.8
Figure
5-2c shows the MIM photoconductor’s generated IPH and responsivity as a function of the
incident light’s power. Responsivity is defined as the ratio of photocurrent to incident power. We
found that the MIM photoconductor’s responsivity was roughly linear in this range of incident
power, which is likely due to the photogating effect not being the dominant photodetection
mechanism in our device architecture.25 In the range of voltage biases and incident powers in
Figure 5-2c, the photoconductor’s maximum responsivity was 106 mA W-1
.
Finally, we compared the MIM photoconductor’s responsivity to the bare photoconductor’s
responsivity. Figure 5-3 shows the responsivity of both photoconductors at different values of α.
From the polar plot, the MIM structure enhanced responsivity from 12 to 77 mA W-1 (~7 times)
at VDS = 1 V. The polarization-resolved responsivity follows the same trend with α from the inset
of Figure 5-1c. The enhanced responsivity is due to MIM’s resonance, which is shown by the
spectra in Figure 5-1. Figure 5-3 also includes the fit line R[α] = RMax cos2
(α - φ) + RMin sin2
(α -
φ). Due to a slight misalignment of the measurement setup, φ = 3° yielded the best fit.

15
Figure 5-3. Room-temperature responsivity enhancement in a grating-enhanced black phosphorus
photoconductor. Polarization-resolved photodetector responsivity for a photoconductor with its grating’s
resonance centered around the testing wavelength λ = 3.37 µm and a photoconductor with no resonant
grating. α is angle of the incident light’s linear polarization, and φ is angle between α = 0° and the fitted
line.
Both detectors were measured with the same technique to obtain the data in Figure 5-2 and at
VDS = 1 V. Responsivity was measured at a lower bias of VDS = 1 V to prevent overheating of the
photoconductors due to the time that was required for this measurement. Over the duration of the
measurement, we did not observe any significant degradation in absorption or responsivity
because of the thin photoresist passivation layer that was used to protect the detectors from
oxidation. The incident radiation’s polarization was controlled with a half wave plate (Thorlabs)
and a linear polarizer (Bruker).

16
Chapter 6: Black Phosphorus Molybdenum Disulfide Photodiodes with
Broadband Metasurface Gratings
In Chapter 5, we established that the performance of a BP photoconductor can be enhanced by
integrating an MIM structure with the device.31 But the MIM-integrated photoconductor
increased performance only around a select resonant wavelength, and the device exhibited
moderate performance at room temperature. Thin-film MoS2, which is a transition metal
dichalcogenide and 2D material, can be integrated with a BP layer to create a more robust
heterostructure photodetector. BP-MoS2 interface is a heterojunction: The BP is a p-type MWIR
absorber, and the MoS2 is an n-type hole barrier. Also, the MoS2 has the advantage of acting as
both a MWIR-transparent optical window and a passivation layer that protects the BP absorber
from water vapor and oxygen.9,22 Previous work has shown that BP-MoS2 heterojunction
photodiodes are capable of room temperature photodetection in the MWIR.8,43-49 However,
performance in the MWIR is still limited by the low absorption of thin BP films. In this Chapter,
we show that integrated MIMs, with broadband absorption spectra, can increase the performance
of BP-MoS2 photodiodes across a large range of wavelengths in the MWIR.
We designed and fabricated BP-MoS2 photodiodes. Figure 6-1a is a diagram of a BP-MoS2
photodiode. The device’s layer stack includes an Au back reflector, BP layer with thickness TBP,
MoS2 layer with thickness TMo, and 50 nm thick Ti/Au contacts. The thickness of the metallic
layers is 50 nm. The Au back reflector and Ti/Au serve as contacts to apply a voltage bias VDS.
Figure 6-1b is an SEM image of a fabricated BP-MoS2 photodiode. Photodiodes were fabricated
onto a thermal oxide on silicon substrate. The metallic layers were deposited with e-beam

17
evaporation (CHA Industries). The BP and MoS2 layers were created with the mechanical
exfoliation method described in Chapter 3. BP samples with thicknesses around 80 nm and MoS2
samples with thicknesses around 11 nm were used for our photodiodes. The heterostructure layer
thicknesses in Figure 6-1b, TBP = 83.1 and TMo = 11.7 nm, were measured with an AFM. Figure
6-1c plots the absorption spectra, measured with a linearly polarized FTIR, of the device in
Figure 6-1b, as well as a diagram of its AC and ZZ axes. The angle of linear polarization θ is
shown in Figure 6-1b, and absorption was estimated with A = 1 - R - T (T = 0 because no light
transmits through the Au back reflector). It is clear from Figure 6-1c that absorption is higher
along the AC axis. The Raman spectra of the device is included as the inset of Figure 6-1c. As
indicated by Figure 6-1c, we found that the Ag
2
peak is maximized along the AC axis for this
device.23

18
Figure 6-1. Black phosphorus photodiode with an integrated metasurface grating. (a) Device schematic of
a black phosphorus (BP) molybdenum disulfide photodiode structure, in which TBP and TMo are the
thicknesses of the BP and MoS2 layers respectively. (b) Scanning electron microscope (SEM) image of a
BP-MoS2 photodiode. The angle between BP’s orthogonal axes is denoted with θ. (c) Absorption spectra
(A = 1 - R - T, T = 0) of the device in (b) measured with linearly polarized light along θ = 0° (AC axis)
and θ = 90° (ZZ axis). The inset is the Raman spectra with linearly polarized 532 nm light along θ = 0°
and θ = 90°. (d) Device schematic of a BP-MoS2 photodiode after integrating the metasurface grating
with period A and width W. (e) SEM image of the same device in (b) after integrating the metasurface.
The inset shows a zoomed-in view of the grating. (f) Simulated and measured absorption spectra of the
device in (b) at θ = 0° and θ = 90° after integrating the metasurface. The inset is the simulated intensity
profile of |H|2
at λ = 3 µm.
We designed and integrated MIM structures with BP-MoS2 photodiodes. Figure 6-1d is the
device architecture of a BP-MoS2 photodiode after integrating an MIM grating. Similar to the
photoconductor from Chapter 5, the MIM supports a lateral resonant mode by enclosing the BP
and MoS2 layers between a metallic back reflector and periodic grating with period A and width
W.
37,38 We fabricated MIMs onto photodiodes with e-beam lithography (Raith) patterning and
then successive deposition of 5 nm Ti and 50 nm Au with e-beam evaporation (CHA Industries).

19
An SEM image of the same device in Figure 6-1b after integrating an MIM is shown in Figure 6-
1e. The inset of Figure 6-1e shows the periodic metallic grating. Figure 6-1f plots the FTIRmeasured absorption spectra of the MIM-integrated photodiode at θ = 0° and θ = 90°. Integrating
the MIM (A = 450 and W = 235 nm) creates a clear absorption peak at θ = 0°. At θ = 90°,
absorption is minimized.31,42 Additionally, Figure 6-1f shows the FDTD-simulated absorption
spectrum of the MIM-integrated photodiode. The FDTD spectrum matches well with the FTIR
spectrum. The refractive indices of BP and MoS2 were respectively borrowed from References
13 and 50. The inset of Figure 6-1f is the simulated field profile of |H|2
at λ = 3 µm, which
reveals that the field is strongly confined to the heterostructure layers.
The absorption spectrum of the MIM in Figure 6-1 can be expanded by including more
resonators in the structure’s unit cell. An MIM structure with two distinct grating widths within
one unit cell is shown in Figure 6-2a. The unit cell’s period is 2A and contains two distinct
gratings widths W1 and W2. Furthermore, Figure 6-2b shows an MIM with three gratings W1, W2,
and W3. The added resonators / grating widths W2 and W3 support additional resonant modes that
broaden the absorption spectrum.51-55 Figure 6-2c shows the FTIR-measured and FDTDsimulated absorption spectra (θ = 0°) of the MIM in Figure 6-2a. The two distinct grating widths
W1 and W2 create the two distinct resonant peaks. The device in Figure 6-2c has the structural
parameters TBP = 75.4, TMo = 11.7, A = 450, W1 = 200, and W2 = 300 nm. Figure 6-2d is the
FTIR-measured and FDTD-simulated absorption spectra of the MIM in Figure 6-2b and has the
structural parameters TBP = 83.2, TMo = 12.4, A = 600, W1 = 230, W2 = 320 nm, and W3 = 410
nm. Because of the grating with W3, a third resonance is observable. When compared to the

20
single-resonator system in Figure 6-1, the multi-resonator systems exhibit absorption spectra
with larger widths.
Figure 6-2. Multi-resonator metasurface gratings. (a) Metasurface-integrated BP-MoS2 photodiode with a
unit cell that has two gratings: one grating has a width W1 and the other has a width W2. (b) BP-MoS2
photodiode with a triple-resonator grating. Like the double resonator system, one unit cell includes three
gratings with widths W1, W2, and W3. (c) Measured and simulated absorption spectra (A = 1 - R - T, T = 0)
of a double-resonator metasurface grating. (d) Measured and simulated absorption of a triple-resonator
metasurface grating.
The spectral locations and widths of the single, double, and triple MIM’s resonant peaks can be
determined by changing their geometries. Respectively, Figure 6-3a and 6-3b show the FTIRmeasured and FDTD-simulated absorption spectra of three single-resonator MIMs. The MIMs in
Figure 6-3a and 6-3b have period of A = 450 nm and different grating widths W1. As W1 is
increased, the resonance shifts to higher wavelengths. A list that includes all of the structural
parameters for each device in Figure 6-3 can be found in Table 1. Figure 6-3c and 6-3d show the
FTIR-measured and FDTD-simulated absorption spectra of three double-resonator MIMs. The
two absorption peaks’ spectral locations can be independently controlled by varying either W1 or

21
W2. An increasing W1 or W2 for a fixed period A results in the corresponding resonance shifting
to a higher wavelength. Figure 6-3e and 6-3f are the FTIR-measured and FDTD-simulated
absorption spectra of two triple-resonator MIMs. By increasing W1, W2, and W3 by the same
amount, the absorption spectrum shifts to higher wavelengths while retaining the spectral
distance between the three distinct resonant peaks.
Figure 6-3. Absorption spectra of metasurface gratings with varying geometries. (a) Measured and (b)
simulated absorption spectra (A = 1 - R - T, T = 0) of three single-resonator devices (MIM 1-3). The
structural parameters for all devices in this Figure are included in the text as Table 1. (c) Measured and (d)
simulated absorption spectra of three double-resonator devices (MIM 4-6). (e) Measured and (f) simulated
absorption spectra of two triple-resonator devices (MIM 7-8).

22
Table 1. Structural parameters of each device in Figure 6-3.
MIM TBP (nm) TMo (nm) A (nm) W1 (nm) W2 (nm) W3 (nm)
1 83.1 11.7 450 235 - -
2 78.3 12.6 450 280 - -
3 77.9 11.3 450 310 - -
4 75.4 11.7 450 200 300 -
5 79.6 12.7 450 230 320 -
6 76.3 12.2 450 245 330 -
7 83.2 12.4 600 230 320 410
8 82.0 10.3 600 280 370 460
To show that broadband MIM structures increase photodetector performance, we compared the
room temperature photodetection capabilities of BP-MoS2 photodiodes before and after
metasurface integration. We denote a single-resonator photodiode as Device 1 and a tripleresonator photodiode as Device 2, which have the structural parameters of MIM 1 and MIM 7 in
Table 1 respectively. Figure 6-4 includes both Device’s absorption spectrum, dark current, and
photocurrent. Photocurrent was measured at room temperature with testing laser wavelengths λ1
= 3.39 and λ2 = 3.88 µm. Each photodiode’s active area is approximately 50 × 50 µm2
.
Absorption was measured with an FTIR, and dark current was measured with a source measure
unit (Keithley). To extract photocurrent, the emission from both lasers (Thorlabs) was modulated
with a mechanical chopper (Thorlabs) at 200 Hz, and photocurrent was extracted with a lock-in
amplifier (Stanford Research Systems). Each laser was controlled to emit at 1.2 W cm-2, which
corresponds to approximately 30 µW of incident light on each photodiode. The incident light’s
polarization was controlled with a half wave plate and polarizer (Thorlabs).

23
Figure 6-4. Room temperature performance of two black phosphorus photodiodes. Device 1 has an
integrated metasurface with a single resonance, and Device 2 has an integrated metasurface with three
resonances. (a) Measured absorption spectra (A = 1 - R - T, T = 0) of Device 1. Device 1 has same the
structural parameters as MIM 1 in Table 1. The testing wavelengths λ1 = 3.39 and λ2 = 3.88 µm are shown
with vertical lines. (b) Dark current as a function of applied voltage bias VDS in Device 1. (c) Measured
current in Device 1 with linearly polarized λ1 = 3.39 µm illumination (IOn) both before and after
integration of the metasurface, and dark current (IOff). The inset is the polarization-resolved responsivity
at λ1 = 3.39 µm and VDS = -0.5 V after integrating the metasurface (θ is defined in Figure 6-1). (d) IOn in
Device 1 measured with 30 µW of linearly polarized λ2 = 3.88 µm illumination before and after the
integration of the metasurface, and IOff. The inset is the polarization-resolved responsivity at λ2 = 3.88 µm
and VDS = -0.5 V after integrating the metasurface. (e) Measured absorption spectra of Device 2. Device 2
has the same structural parameters as MIM 7 in Table 1. (f) Dark current as a function of applied voltage
bias VDS in Device 2. (g) IOff and IOn in Device 2 with 30 µW of linearly polarized λ1 = 3.39 µm light
before and after integrating the metasurface. (h) IOff and IOn in Device 2 with 30 µW of linearly polarized
λ2 = 3.88 µm light before and after integrating the metasurface. The insets of (g) and (h) are the
polarization-resolved responsivity after integrating the metasurface at λ1 = 3.39 and λ2 = 3.88 µm,
respectively, and VDS = -0.5 V.

24
Device 1 is a BP-MoS2 photodiode with a single-resonator MIM. The absorption spectrum of
Device 1 and lines that represent λ1 and λ2 are shown in Figure 6-4a, and dark current as a
function of VDS is shown in Figure 6-4b. Figure 6-4c is the photoresponse at λ1 = 3.39 µm of
Device 1 before and after integrating the MIM grating. IOn is the total generated current when the
device is illuminated with approximately 30 µW of linearly polarized light, and IOff is the current
with no incident light (dark current). Linearly polarized incident radiation was aligned with the
AC axis of the device such that photocurrent generation was maximized. After the integrating the
MIM, photocurrent increased by 6.7 times at λ = 3.39 µm and VDS = -0.5 V. Photocurrent under
reverse bias was higher than photocurrent under forward bias.8,9,33 The inset of Figure 6-4c is the
polarization-resolved responsivity R at a bias of VDS = -0.5 V as a function of θ (see Figure 6-1b).
R maximizes at 737 mA W-1 when θ = 0° and minimizes when θ = 90°. Figure 6-4d plots IOn and
IOff at λ2 = 3.88 µm (θ = 0°) before and after the inclusion of the MIM grating. After the inclusion
of the MIM, the photocurrent increased by 3.9 times at λ = 3.88 µm and VDS = -0.5 V. The inset
of Figure 6-4d is the polarization-resolved responsivity as a function of θ at a bias of VDS = -0.5
V. It exhibits a maximized responsivity of 191 mA W-1 in addition to the same anisotropic
behavior as the inset in Figure 6-4c. The photocurrent and responsivity increased more at λ1 than
at λ2, which is due to the resonant peak in Figure 6-4a being further from wavelength λ2 than λ1.
Device 2 is a BP-MoS2 photodiode with a triple-resonator MIM. The absorption spectrum is
shown in Figure 6-4e, and the dark current is shown in Figure 6-4f. Figure 6-4g plots IOff and IOn
at λ1 = 3.39 µm measured before and after MIM integration. Photocurrent increased by 7.5 times
at λ1 = 3.39 µm and VDS = -0.5 V. The polarization-resolved responsivity at VDS = -0.5 V is
shown as the inset of Figure 6-4g, which has a maximum responsivity of 818 mA W-1. Similarly,

25
Figure 6-4h shows IOff and IOn measured at λ2 = 3.88 µm with and without the MIM, and the inset
of Figure 6-4h tracks polarization-resolved responsivity at VDS = -0.5 V. At λ2, the MIM
increased photocurrent by a factor of 12.8 times to a maximum responsivity of 614 mA W-1
.
When compared to Device 1, Device 2 exhibits enhanced photocurrent at both λ1 and λ2, which is
because of the multi-resonator MIM’s broad absorption spectra. The performance enhancement
from the integrated MIMs is summarized in Table 2, which lists R and specific detectivity D* for
both Devices. D* is calculated with the equation D* = R A1/2 (2eIOff)
-1/2: R is responsivity, A is
active area, e is elementary charge, and IOff is dark current.
Table 2. Responsivity R and specific detectivity D* for Device 1 and Device 2 with and without the
metasurface grating.
λ1 λ2
R (mA W-1
) D* (cm Hz1/2
W-1)
R (mA W-1
) D* (cm Hz1/2
W-1)
Device 1 No Grating 110 6.37 × 108 49 2.84 × 108
With Grating 737 4.26 × 109 191 1.11 × 109
Device 2 No Grating 109 8.83 × 108 48 3.89 × 108
With Grating 818 6.62 × 109 614 4.98 × 109

26
Chapter 7: Metalens-Integrated Black Phosphorus Photodetector
In addition to the MIM gratings, we integrated dielectric metalenses with BP-MoS2 photodiodes
to enhance photodetection in the MWIR. Metamaterial lenses, or metalenses, are planar arrays of
subwavelength nanostructures that control the phase, transmission, and polarization of light.56,57
When monolithically integrated into the substrate of a photodetector, a metalenses can
concentrate light onto the absorber layer’s active region.58-60 Figure 7-1 illustrates this concept:
the metalens, with area Alens, concentrates light through the substrate into the device’s active area
Adect. The focusing of light from a larger Alens to a smaller Adect increases the incident power
density on the detector and thereby boosts the SNR.60 We integrated a metalens with a BP-MoS2
photodiode’s silicon substrate and demonstrated its capability to enhance MWIR detector
performance.

27
Figure 7-1. Concept a metalens-integrated black phosphorus photodiode. Incident light is focused by the
metalens with area Alens into a photodiode with a smaller area Adect, thereby increasing the impinging
power density on the detector.
Our metalens design is an array of cylindrical silicon nanorod unit cells. Silicon was selected as
the substrate and nanorod material because of its high refractive index and transparency in the
MWIR, Cylindrical nanorods were selected as the subwavelength nanostructure because of their
rotational symmetry that creates polarization insensitivity to incident light.61,62 The metalens
reconstructs the phase profile given by the following equation:62
(, ) = 2 −
ଶగ௡
ఒ
(ඥ
ଶ +
ଶ +
ଶ − ) (1)
In Equation 1, x and y are the position on the metalens array, n is the refractive index of the
substrate that light transmits through, and f is the focal length at a given wavelength λ.
62 We used
f = 300 µm, which is the thickness of the silicon substrate that has a relatively dispersion-less

28
index of n = 3.43. The needed phase at any x,y on the metalens area is found with Equation 1.
Then, a specific nanorod that meets the required phase is placed at that position x,y. To produce
any phase in the range of 0 to 2π, we created a library of nanorods with varying unit cell
geometry.
The nanorod unit cell’s geometry and is shown in Figure 7-2a. Its structural parameters are
height h, diameter d, and gap spacing g. h and g are fixed at 2.5 and 0.3 µm respectively, and d
varies to create relative phase shifts in the range of 0 to 2π. Figure 7-2b is the FDTD-simulated
phase shift and transmittance of the nanorod unit cell at both λ = 3 and 4 µm calculated over the
range of diameters d = 0.8 to 1.7 µm. Our diameter range produces a phase coverage from 0 to
2π at both wavelength λ = 3 and 4 µm, which allows our unit cell library to be used for metalens
designs at both wavelengths. Details on nanorod unit cell FDTD simulations can be found at the
end of this Chapter.
We fixed the distance between the edges of each nanostructure, which is in contrast to
conventional periodic designs in which the centers between each nanostructure are fixed.63 We
imposed a constant spacing g between the edges of the nanorod and unit cell. This constraint
allows for a greater number of unit cells to occupy the same area than in the case if d + 2g
summed to a fixed period (i.e., g is not fixed).59,63 Also, a constant spacing g guarantees that two
adjacent nanorods are never touching each other, which aids in fabrication processes and
minimizes unwanted coupling effects.59 We designed our silicon metalens with this fixed gap
spacing constraint.

29
Figure 7-2. Design of a silicon metalens for focusing mid-wave infrared light. (a) Nanorod unit cell with
height h, diameter d, and gap spacing g. (b) Simulated phase shift (normalized to 2π) and transmittance at
3 and 4 µm for a nanorod unit cell with variable d and fixed h = 2.5 µm and g = 0.3 µm. (c) Metalens
design map for focusing mid-wave infrared light through a f = 300 µm thick substrate. Only the center
part of the full metalens design is shown. Each marker represents one nanorod, and the size and color of a
marker represents the corresponding the nanorod’s diameter d.
We designed a metalens with a diameter of 275 µm and that focuses λ = 3 µm light inside a
silicon substrate with focal length of f = 300 µm. The center section of the silicon metalens
design is shown in Figure 7-2c. Each circular marker represents one individual nanorod. To
begin the design process, a nanorod is placed at x,y = 0 µm to impart the phase shift calculated
with Equation 1 (note that the phase at x,y = 0 µm is 2π). The second nanorod is then placed by
calculating the required phase at the next position. The position of this second nanorod is found

30
by moving in the +x-direction by g, according to the fixed gap spacing constraint. Nanorods with
the same diameter as the second nanorod are then evenly distributed along a ring that is defined
by its position.63 This process is iterated until the metalens reaches the desired diameter. In
Figure 7-2c, as the nanorod diameters decrease as their positions move further from the origin.
The abrupt increase in diameter is due to ensuring a continuous phase profile while selecting
nanorods within the range of diameters in Figure 7-2b. We fabricated and characterized the
design in Figure 7-2.
Figure 7-3a is an SEM image of a fabricated metalens design. The silicon substrate is undoped,
double-side polished, and 300 µm thick. The inset of Figure 7-3a shows clearly distinguishable
nanorods with vertical sidewalls. All fabrication details are discussed at the end of this Chapter.
We measured the metalens’ focal spot with the apparatus in Figure 7-3b. A filtered 1200 K
blackbody source with a 1.5 cm aperture illuminates the metalens. We assumed that the distance
between the source and sample (~30 cm) was sufficient to approximate collimated plane waves
with normal incidence. Then, a MWIR camera (640 × 512 pixels, Jet Propulsion Laboratory),
attached with a 36× objective lens, records xy-plane images that are inside the silicon substrate at
discrete positions along the z-direction. z = 0 µm is the vertical position of the metalens, and z =
300 µm is the opposite plane of the silicon substrate. It is important to note that the camera’s
movement in z is multiplied by the refractive index of silicon.58 To see the focal spot, an xyz
image cube is reconstructed from the xy-plane images. Then, xz-plane images extracted extracted
from the xyz cube reveal the location of the focal spot.

31
Figure 7-3c is an image the focal spot at λ = 3050 nm, which was obtained by placing a
narrowband filter in front of the blackbody source. There focal spot is clearly located at the
silicon-air interface at z = 300 µm. Additionally, the metalens concentrates light in an area Alens
into a nominal area Adect. Figure 7-3d is the measured and simulated (FDTD) normalized
intensity profile of focal spot at z = 300 µm.
Figure 7-3. Metalens characterization. (a) Scanning electron microscope image of a silicon metalens. The
inset shows the constituent nanorods. (b) Setup for characterizing the focal spot of a metalens. (c)
Measured xz-plane image of the metalens’ focal spot at λ = 3050 nm. (d) Measured and simulated
normalized intensity profile of the focal spot at z = 300 µm and λ = 3050 nm.
We measured and simulated our metalens’ focal spot at multiple wavelengths. Figure 7-4 shows
measured and simulated images the focal spot at five wavelengths λ = 3050, 3390, 3900, 4220,
and 4665 nm. The spatial location of the focal spots in both the measurements and simulations
are generally in good agreement. The location of the focal spot decreases in z as λ increases, and
this trend is consistent in both the measurements and simulations. In our design, we did not
attempt to address this chromatic aberration. But recent work shows that optimization algorithms
can be used to create metalenses with reduced chromatic aberrations.56,57 At all five testing
wavelengths, the metalens concentrates MWIR light into a clear spot on the opposite edge of the
substrate.

32
Figure 7-4. Measured and simulated focal spots. The measured and simulated position and chromatic
trend of the focal spots are in good agreement at five testing wavelengths (3050, 3390, 3900, 4220, and
4665 nm). The dashed lines indicate the silicon-air interface.
After imaging the metalens’ focal spot, we evaluated its capability to enhance MWIR
photodetection at room temperature. To quantify the enhancement in photodetection, we
compared the performance of two photodiodes: one photodiode’s substrate has an integrated
metalens, and the other photodiode’s substrate has no integrated metalens. Figure 7-5a shows a
BP-MoS2 photodiode and integrated metalens fabricated onto the opposite sides of the substrate.
Figure 7-5b shows a BP-MoS2 photodiode with no integrated metalens. By comparing the
measured photocurrent in the metalens-integrated photodiode to that of the device with no
metalens, we were able to quantify the performance enhancement due to the metalens.

33
Figure 7-5. Black phosphorus photodiodes with integrated metalenses. (a) Photodiode with an integrated
metalens. (b) Photodiode with no integrated metalens. (c) Measured absorption spectrum (A = 1 - R - T)
of a black phosphorus molybdenum disulfide photodiode along its orthogonal axes. The inset is a
scanning electron microscopy image of the metalens-integrated photodiode.
We fabricated the two BP-MoS2 photodiodes for comparison with the same processes, layer
thicknesses, and active areas. Each BP-MoS2 heterojunction was exfoliated from bulk crystals
(see Chapter 2) with BP/MoS2 layer thicknesses of 36/9 nm, which were measured with an
atomic force microscope. The approximate active area of both photodiodes is 30 × 30 µm2
. One
photodiode was aligned to the center of the metalens (i.e., the location of the focal spot), and the
other photodiode was fabricated onto a blank Si substrate. Fabrication details can be found at the
end of this Chapter. After fabricating the detectors, we identified their AC and ZZ axes. Figure 7-
5c shows the measured absorption spectra, estimated by A = 1 - T - R, of the metalens-integrated
BP-MoS2 photodiode along its AC and ZZ axes. The inset of Figure 7-5c is an SEM image of the
photodetector after depositing Ti/Au contacts. Even though the BP-MoS2 stack rests on undoped
float-zone silicon, we suspended the contacts with an Al2O3 spacer layer that ensures electrical
isolation between the substrate and the metallic contacts. After fabricating the metalens-enhanced
and unenhanced photodiodes, we measured room temperature photocurrent generation in both
devices.

34
Figure 7-6. Metalens-enhanced mid-wave infrared photodetection. (a) Measured current as a function of
voltage bias for the photodiode with no integrated metalens: IOff is the dark current and IOn is the current
under λ = 3.39 or 3.88 µm illumination. (b) IOff and IOn for a photodetector with an integrated metalens:
IOn exceeds that of the photodetector with no integrated metalens. (c) Extracted photocurrent IPH = IOn -
IOff and responsivity at λ = 3.39 µm for both the metalens-enhanced and unenhanced photodetectors. (d)
Extracted photocurrent IPH and responsivity at λ = 3.88 µm for both the metalens-integrated and
unintegrated photodetectors.
We measured room temperature photocurrent generation in the two photodiodes shown in Figure
7-5a and 7-5b at two laser wavelengths λ = 3.39 and 3.88 µm. Linearly polarized emission from
both lasers was fixed to ~0.84 W cm-2 (~7.5 µW of incident power), modulated at 200 Hz with an
optical chopper (Thorlabs), and aligned to the AC axis. A vertical voltage bias was applied across

35
the photodiode with a source measure unit (Keithley), and the generated photocurrent was
extracted with a lock-in amplifier (Stanford Research Systems). Figure 7-6a shows the dark
current (IOff) and total generated current with λ = 3.39 and 3.88 µm illumination (IOn) in the
photodiode with no integrated metalens. Similarly, Figure 7-6b plots IOff and IOn at both
wavelengths for the photodiode with the integrated metalens. IOn is noticeably greater in the
photodiode with the integrated metalens. Also, IOff in both devices is comparable. Figure 7-6c
and 7-6d respectively show the extracted photocurrent and responsivity of both photodiodes at λ
= 3.39 and 3.88 µm. As in previous Chapters, photocurrent is defined as IPH = IOn - IOff, and
responsivity is defined as the ratio of generated photocurrent to incident power. Photocurrent and
responsivity at λ = 3.39 µm were about 6.65 times higher than in the case with no metalens.
Additionally, photocurrent and responsivity in the presence of the metalens increased by about
6.31 times at λ = 3.88 µm. At both of the testing wavelengths, the integrated metalens enhanced
detector performance in the MWIR. To evaluate how the integrated metalens increased SNR, we
calculated specific detectivity D* for both photodiodes with the responsivity and dark current
data in Figure 7-6. The detector with no metalens exhibited D* = 3.03 × 108
and 1.56 × 108
cm
Hz1/2 W-1 at λ = 3.39 and 3.88 µm, respectively. And the detector with the integrated metalens
exhibited D* = 2.02 × 109
and 9.81 × 108
cm Hz1/2 W-1 at λ = 3.39 and 3.88 µm, respectively. At
both λ = 3.39 and 3.88 µm, the metalens enhanced photodetector SNR.
The remaining paragraphs of this Chapter detail the simulation and fabrication processes for the
metalens-integrated photodetectors. All simulations in this Chapter were performed with the
FDTD method. Nanorod unit cell data was obtained with periodic boundary conditions in the xand y-directions and a normally incident plane wave source in the z-direction. The transmission

36
and phase shift for a given unit cell was extracted with frequency-domain monitors placed in
front of and behind the structure. Focal spots were simulated with a FDTD region of 120 × 120
μm2
at the center of the metalens. The FDTD region was defined with perfectly matched layers at
each boundary. To reduce the simulation time, near field data was extracted at single wavelengths
to calculate far-field focal spot images, and anti-symmetric and symmetric conditions were
respectively imposed in the x- and y-directions.
The photodiodes discussed in this Chapter were fabricated on a double side polished, 300 μm
thick, and undoped float-zone silicon substrate. Alignment markers for the front side of the
substrates were patterned onto a photoresist layer with a direct laser writer (Heidelberg). Then, 5
nm of Ti and 50 nm of Au were successively deposited with e-beam deposition (CHA Industries).
Identical alignment markers (in position and size) were then created on the back side of the
substrates using the same patterning and deposition process. The direct laser writer is capable of
aligning patterns to markers on either side of a substrate, which enables the exact positioning of
alignment markers on the back side of the substrates to identical markers on the front side. After
both sides of the substrate had matching sets of alignment markers, we fabricated metalenses
onto the front side and photodetectors onto the back side. Metalenses were etched into the front
side of the substrates. The metalens design was pattered by using e-beam lithography (Raith).
Then, metalenses were cryoetched into the substrate using SF6 and O2 at -120°C (Oxford). The
patterned resist acted as an etch mask. Photodetectors were fabricated onto the back side of the
substrates. BP/MoS2 flakes with layer thicknesses around 35/10 nm were selected and transferred
onto the substrates. Then, 50 nm Al2O3 dielectric insulators were patterned with e-beam
lithography (Raith) and deposited with e-beam deposition (Angstrom). Finally, metallic contacts

37
were deposited on top of the dielectric insulators by first patterning with e-beam lithography
(Raith) and then successively depositing 5 nm of Ti and 60 nm of Au with e-beam deposition
(CHA Industries). Photodiodes with no integrated metalens were fabricated onto blank
substrates.

38
Chapter 8: Conclusion, Outlook, and Future Research Directions
We demonstrated two integrated metasurfaces for boosting the performance of BP photodetectors
in the MWIR. First, we showed that resonant MIMs enhance the responsivity of BP
photoconductors MWIR. We fabricated and characterized MIMs that increased absorption in thin
film BP. The spectral location of an MIM’s resonance could be tuned by changing its structural
parameters, and the MIM boosted responsivity by up to ~7 times in a thin-film BP
photoconductor. Then, we showed that the resonant behavior of MIM metasurface gratings can
be expanded to large spectral ranges.
We fabricated BP-MoS2 photodiodes with integrated single-resonator MIM structures that
increased absorption around a selected wavelength. Then, we characterized double- and tripleresonator MIMs with broadband absorption spectra that can be controlled by tuning their
geometry. The broadband absorption spectra resulted in an enhanced room temperature
responsivity and specific detectivity at multiple wavelengths.
We also integrated a monolithic dielectric metalens with a BP-MoS2 photodiode to enhance SNR
in the MWIR. We characterized our metalens by imaging focal spots at five testing wavelengths
and then showed that it increased the incident power density on the substrate-detector interface.
Then, we integrated a BP-MoS2 photodiode with the metalens. The integrated metalens increased
room temperature responsivity at multiple testing wavelengths in the MWIR. In summary, we
showed that integrated MIMs and metalenses enhance BP photoconductors and photodiodes at
selected wavelengths or over broad spectral ranges.

39
In Chapter 4, we stated that the MIM grating and dielectric metalens were investigated because
of their scalability to BP FPAs. The MIM gratings are fabricated in a single lithographic step,
which is scalable to large arrays of BP photodetectors that could be enabled by large area growth
methods.64-68 Each pixel in such an array could have different integrated MIMs that are tailored
to enhance performance at specific wavelengths or around large wavelength ranges. Additionally,
integrated metalenses are scalable to FPAs. Each pixel of a metalens-integrated FPA would have
a corresponding metalens on the opposite side of the substrate. Moreover, the concentrated
incident light also allows for smaller device pitches. Devices with smaller areas are desirable for
FPAs because of the reduction of parasitic effects within the absorber material. However, small
active areas decrease the optical collection area, which results in a reduced SNR that the
integrated metalenses can counteract. In conclusion, we designed, fabricated, and characterized
two metasurfaces that enhance BP photodetector performance in the MWIR and are scalable to
FPAs.

40
References
1 Green, R. O. et al. Imaging Spectroscopy and the Airborne Visible/Infrared Imaging Spectrometer
(AVIRIS). Remote Sensing of Environment 65, 227-248 (1998).
https://doi.org:https://doi.org/10.1016/S0034-4257(98)00064-9
2 Rieke, M., Kelly, D. & Horner, S. in Optics and Photonics. (SPIE).
3 Chapman, J. W. et al. Spectral and Radiometric Calibration of the Next Generation Airborne
Visible Infrared Spectrometer (AVIRIS-NG). Remote Sensing 11 (2019).
https://doi.org:10.3390/rs11182129
4 Holly, A. B. et al. Snow and Water Imaging Spectrometer (SWIS): optomechanical and system
design for a CubeSat-compatible instrument. Proc.SPIE 9611, 961103 (2015).
https://doi.org:10.1117/12.2190013
5 Gu, R. et al. MBE growth of HgCdTe on GaSb substrates for application in next generation
infrared detectors. Journal of Crystal Growth 468, 216-219 (2017).
https://doi.org:https://doi.org/10.1016/j.jcrysgro.2016.12.034
6 Xia, F., Wang, H. & Jia, Y. Rediscovering black phosphorus as an anisotropic layered material for
optoelectronics and electronics. Nature Communications 5, 4458 (2014).
https://doi.org:10.1038/ncomms5458
7 Chen, X. et al. Widely tunable black phosphorus mid-infrared photodetector. Nature
Communications 8, 1672 (2017). https://doi.org:10.1038/s41467-017-01978-3
8 Long, M. et al. Room temperature high-detectivity mid-infrared photodetectors based on black
arsenic phosphorus. Science Advances 3, e1700589 (2017).
https://doi.org:10.1126/sciadv.1700589
9 Bullock, J. et al. Polarization-resolved black phosphorus/molybdenum disulfide mid-wave
infrared photodiodes with high detectivity at room temperature. Nature Photonics 12, 601-607
(2018). https://doi.org:10.1038/s41566-018-0239-8
10 Duong, D. L., Yun, S. J. & Lee, Y. H. van der Waals Layered Materials: Opportunities and
Challenges. ACS Nano 11, 11803-11830 (2017). https://doi.org:10.1021/acsnano.7b07436
11 Wu, J., Wang, N., Yan, X. & Wang, H. Emerging low-dimensional materials for mid-infrared
detection. Nano Research 14, 1863-1877 (2021). https://doi.org:10.1007/s12274-020-3128-7
12 Deng, L. Y. et al. Extremely high extinction ratio terahertz broadband polarizer using bilayer
subwavelength metal wire-grid structure. Applied Physics Letters 101, 011101 (2012).
https://doi.org:10.1063/1.4729826
13 Morita, A. Semiconducting black phosphorus. Applied Physics A 39, 227-242 (1986).
https://doi.org:10.1007/BF00617267

41
14 Amani, M., Regan, E., Bullock, J., Ahn, G. H. & Javey, A. Mid-Wave Infrared Photoconductors
Based on Black Phosphorus-Arsenic Alloys. ACS Nano 11, 11724-11731 (2017).
https://doi.org:10.1021/acsnano.7b07028
15 Liu, B. et al. Black Arsenic–Phosphorus: Layered Anisotropic Infrared Semiconductors with
Highly Tunable Compositions and Properties. Advanced Materials 27, 4423-4429 (2015).
https://doi.org:https://doi.org/10.1002/adma.201501758
16 Deng, B. et al. Efficient electrical control of thin-film black phosphorus bandgap. Nature
Communications 8, 14474 (2017). https://doi.org:10.1038/ncomms14474
17 Deng, W. et al. Electrically tunable two-dimensional heterojunctions for miniaturized nearinfrared spectrometers. Nature Communications 13, 4627 (2022). https://doi.org:10.1038/s41467-
022-32306-z
18 Biswas, S. et al. Tunable intraband optical conductivity and polarization-dependent epsilon-nearzero behavior in black phosphorus. Science Advances 7, eabd4623
https://doi.org:10.1126/sciadv.abd4623
19 Jun, Y. C. et al. Active tuning of mid-infrared metamaterials by electrical control of carrier
densities. Opt. Express 20, 1903-1911 (2012). https://doi.org:10.1364/OE.20.001903
20 Cohen, B. A. et al. Lunar Flashlight: Illuminating the Lunar South Pole. IEEE Aerospace and
Electronic Systems Magazine 35, 46-52 (2020). https://doi.org:10.1109/MAES.2019.2950746
21 McNutt, L., Johnson, L., Kahn, P., Castillo-Rogez, J. & Frick, A. in AIAA SPACE 2014
Conference and Exposition AIAA SPACE Forum (American Institute of Aeronautics and
Astronautics, 2014).
22 Abate, Y. et al. Recent Progress on Stability and Passivation of Black Phosphorus. Advanced
Materials 30, 1704749 (2018). https://doi.org:https://doi.org/10.1002/adma.201704749
23 Ling, X. et al. Anisotropic Electron-Photon and Electron-Phonon Interactions in Black
Phosphorus. Nano Letters 16, 2260-2267 (2016). https://doi.org:10.1021/acs.nanolett.5b04540
24 Hou, C. et al. Multilayer Black Phosphorus Near-Infrared Photodetectors. Sensors 18 (2018).
https://doi.org:10.3390/s18061668
25 Guo, Q. et al. Black Phosphorus Mid-Infrared Photodetectors with High Gain. Nano Letters 16,
4648-4655 (2016). https://doi.org:10.1021/acs.nanolett.6b01977
26 Wang, S. et al. Integration of Black Phosphorus Photoconductors with Lithium Niobate on
Insulator Photonics. Advanced Optical Materials 11, 2201688 (2023).
https://doi.org:https://doi.org/10.1002/adom.202201688
27 Huang, L. et al. Waveguide-Integrated Black Phosphorus Photodetector for Mid-Infrared
Applications. ACS Nano 13, 913-921 (2019). https://doi.org:10.1021/acsnano.8b08758

42
28 Youngblood, N., Chen, C., Koester, S. J. & Li, M. Waveguide-integrated black phosphorus
photodetector with high responsivity and low dark current. Nature Photonics 9, 247-252 (2015).
https://doi.org:10.1038/nphoton.2015.23
29 Chen, C. et al. Three-Dimensional Integration of Black Phosphorus Photodetector with Silicon
Photonics and Nanoplasmonics. Nano Letters 17, 985-991 (2017).
https://doi.org:10.1021/acs.nanolett.6b04332
30 Venuthurumilli, P. K., Ye, P. D. & Xu, X. Plasmonic Resonance Enhanced Polarization-Sensitive
Photodetection by Black Phosphorus in Near Infrared. ACS Nano 12, 4861-4867 (2018).
https://doi.org:10.1021/acsnano.8b01660
31 Lien, M. R. et al. Resonant Grating-Enhanced Black Phosphorus Mid-Wave Infrared
Photodetector. Nano Letters 22, 8704-8710 (2022). https://doi.org:10.1021/acs.nanolett.2c03469
32 Tian, R. et al. Black Phosphorus Photodetector Enhanced by a Planar Photonic Crystal Cavity.
ACS Photonics 8, 3104-3110 (2021). https://doi.org:10.1021/acsphotonics.1c01168
33 Yan, W. et al. Spectrally Selective Mid-Wave Infrared Detection Using Fabry-Pérot Cavity
Enhanced Black Phosphorus 2D Photodiodes. ACS Nano 14, 13645-13651 (2020).
https://doi.org:10.1021/acsnano.0c05751
34 Yadav, S. N. S., Chen, P.-L., Liu, C. H. & Yen, T.-J. Plasmonic Metasurface Integrated Black
Phosphorus-Based Mid-Infrared Photodetector with High Responsivity and Speed. Advanced
Materials Interfaces 10, 2202403 (2023). https://doi.org:https://doi.org/10.1002/admi.202202403
35 Audhkhasi, R. & Povinelli, M. L. Gold-black phosphorus nanostructured absorbers for efficient
light trapping in the mid-infrared. Opt. Express 28, 19562-19570 (2020).
https://doi.org:10.1364/OE.398641
36 Lien, M. R. et al. Resonant Grating-Enhanced Black Phosphorus Mid-Wave Infrared
Photodetector. Nano Letters (2022). https://doi.org:10.1021/acs.nanolett.2c03469
37 Liu, X., Starr, T., Starr, A. F. & Padilla, W. J. Infrared Spatial and Frequency Selective
Metamaterial with Near-Unity Absorbance. Physical Review Letters 104, 207403 (2010).
https://doi.org:10.1103/PhysRevLett.104.207403
38 Todorov, Y. et al. Optical properties of metal-dielectric-metal microcavities in the THz frequency
range. Opt. Express 18, 13886-13907 (2010). https://doi.org:10.1364/OE.18.013886
39 Wu, J. et al. TE polarization selective absorber based on metal-dielectric grating structure for
infrared frequencies. Optics Communications 329, 38-43 (2014).
https://doi.org:https://doi.org/10.1016/j.optcom.2014.05.012
40 Telesio, F. et al. Ohmic contact engineering in few–layer black phosphorus: approaching the
quantum limit. Nanotechnology 31, 334002 (2020). https://doi.org:10.1088/1361-6528/ab8cf4
41 Kischkat, J. et al. Mid-infrared optical properties of thin films of aluminum oxide, titanium
dioxide, silicon dioxide, aluminum nitride, and silicon nitride. Appl. Opt. 51, 6789-6798 (2012).
https://doi.org:10.1364/AO.51.006789

43
42 Yamada, I. et al. Mid-infrared wire-grid polarizer with silicides. Opt. Lett. 33, 258-260 (2008).
https://doi.org:10.1364/OL.33.000258
43 Hong, T. et al. Anisotropic photocurrent response at black phosphorus–MoS2 p–n
heterojunctions. Nanoscale 7, 18537-18541 (2015). https://doi.org:10.1039/C5NR03400K
44 Deng, Y. et al. Black Phosphorus–Monolayer MoS2 van der Waals Heterojunction p–n Diode.
ACS Nano 8, 8292-8299 (2014). https://doi.org:10.1021/nn5027388
45 Ye, L., Li, H., Chen, Z. & Xu, J. Near-Infrared Photodetector Based on MoS2/Black Phosphorus
Heterojunction. ACS Photonics 3, 692-699 (2016). https://doi.org:10.1021/acsphotonics.6b00079
46 Wu, F. et al. High efficiency and fast van der Waals hetero-photodiodes with a unilateral
depletion region. Nature Communications 10, 4663 (2019). https://doi.org:10.1038/s41467-019-
12707-3
47 Wang, L. et al. Perovskite/Black Phosphorus/MoS2 Photogate Reversed Photodiodes with
Ultrahigh Light On/Off Ratio and Fast Response. ACS Nano 13, 4804-4813 (2019).
https://doi.org:10.1021/acsnano.9b01713
48 Maimon, S. & Wicks, G. W. nBn detector, an infrared detector with reduced dark current and
higher operating temperature. Applied Physics Letters 89, 151109 (2006).
https://doi.org:10.1063/1.2360235
49 Yuan, H. et al. Polarization-sensitive broadband photodetector using a black phosphorus vertical
p–n junction. Nature Nanotechnology 10, 707-713 (2015).
https://doi.org:10.1038/nnano.2015.112
50 Islam, K. M. et al. In-Plane and Out-of-Plane Optical Properties of Monolayer, Few-Layer, and
Thin-Film MoS2 from 190 to 1700 nm and Their Application in Photonic Device Design.
Advanced Photonics Research 2, 2000180 (2021).
https://doi.org:https://doi.org/10.1002/adpr.202000180
51 Audhkhasi, R., Lien, M. R. & Povinelli, M. L. Experimental Implementation of Metasurfaces for
Secure Multi-Channel Image Encryption in the Infrared. Advanced Optical Materials 11,
2203155 (2023). https://doi.org:https://doi.org/10.1002/adom.202203155
52 Ogawa, S. & Kimata, M. Metal-Insulator-Metal-Based Plasmonic Metamaterial Absorbers at
Visible and Infrared Wavelengths: A Review. Materials 11 (2018).
53 Cheng, C.-W. et al. Wide-angle polarization independent infrared broadband absorbers based on
metallic multi-sized disk arrays. Opt. Express 20, 10376-10381 (2012).
https://doi.org:10.1364/OE.20.010376
54 Jiang, Z. H., Yun, S., Toor, F., Werner, D. H. & Mayer, T. S. Conformal Dual-Band Near-Perfectly
Absorbing Mid-Infrared Metamaterial Coating. ACS Nano 5, 4641-4647 (2011).
https://doi.org:10.1021/nn2004603

44
55 Butun, S. & Aydin, K. Structurally tunable resonant absorption bands in ultrathin broadband
plasmonic absorbers. Opt. Express 22, 19457-19468 (2014).
https://doi.org:10.1364/OE.22.019457
56 Chen, W. T., Zhu, A. Y. & Capasso, F. Flat optics with dispersion-engineered metasurfaces.
Nature Reviews Materials 5, 604-620 (2020). https://doi.org:10.1038/s41578-020-0203-3
57 Wang, S. et al. Broadband achromatic optical metasurface devices. Nature Communications 8,
187 (2017). https://doi.org:10.1038/s41467-017-00166-7
58 Zhang, S. et al. Solid-immersion metalenses for infrared focal plane arrays. Applied Physics
Letters 113, 111104 (2018). https://doi.org:10.1063/1.5040395
59 Wenger, T., Muller, R., Wilson, D., Gunapala, S. D. & Soibel, A. Large metasurface-based optical
concentrators for infrared photodetectors. AIP Advances 11, 085221 (2021).
https://doi.org:10.1063/5.0054328
60 Wenger, T. et al. Infrared nBn detectors monolithically integrated with metasurface-based optical
concentrators. Applied Physics Letters 121, 181109 (2022). https://doi.org:10.1063/5.0121643
61 Fedeli, J. M. & Nicoletti, S. Mid-Infrared (Mid-IR) Silicon-Based Photonics. Proceedings of the
IEEE 106, 2302-2312 (2018). https://doi.org:10.1109/JPROC.2018.2844565
62 Khorasaninejad, M. et al. Polarization-Insensitive Metalenses at Visible Wavelengths. Nano
Letters 16, 7229-7234 (2016). https://doi.org:10.1021/acs.nanolett.6b03626
63 Zhou, Y. et al. A Performance Study of Dielectric Metalens with Process-Induced Defects. IEEE
Photonics Journal 12, 1-14 (2020). https://doi.org:10.1109/JPHOT.2020.2986263
64 Smith, J. B., Hagaman, D. & Ji, H.-F. Growth of 2D black phosphorus film from chemical vapor
deposition. Nanotechnology 27, 215602 (2016). https://doi.org:10.1088/0957-4484/27/21/215602
65 Mao, J. et al. Langmuir–Blodgett fabrication of large-area black phosphorus-C60 thin films and
heterojunction photodetectors. Nanoscale 12, 19814-19823 (2020).
https://doi.org:10.1039/D0NR04537C
66 Zhao, M. et al. Growth Mechanism and Enhanced Yield of Black Phosphorus Microribbons.
Crystal Growth & Design 16, 1096-1103 (2016). https://doi.org:10.1021/acs.cgd.5b01709
67 Xu, Y. et al. Epitaxial nucleation and lateral growth of high-crystalline black phosphorus films on
silicon. Nature Communications 11, 1330 (2020). https://doi.org:10.1038/s41467-020-14902-z
68 Mannix, A. J. et al. Robotic four-dimensional pixel assembly of van der Waals solids. Nature
Nanotechnology 17, 361-366 (2022). https://doi.org:10.1038/s41565-021-01061-5

45
Appendix A: Fellowship Acknowledgement
This work was supported by a NASA Space Technology Graduate Research Opportunity
(80NSSC20K1161).

Abstract (if available)

Abstract Thin-film black phosphorus has emerged as a promising material for room temperature mid-wave infrared photodetection. Among its direct bandgap in the mid-wave infrared, black phosphorus exhibits high carrier mobility, substrate compatibility, and bandgap tunability. However, many of these attractive optoelectronic properties are only observable at smaller film thicknesses, which ultimately inhibits photodetector absorption. To address this challenge, we propose to integrate black phosphorus photodetectors with metasurfaces that boost absorption and performance in the mid-wave infrared. This dissertation begins by providing a background on black phosphorus and integrated metasurface gratings and lenses. Then, we discuss our experimental results from integrating metasurface gratings and lenses with black phosphorus photodetectors. We show that these integrated metasurfaces enhance the room temperature performance of black phosphorus photoconductors and photodiodes in the mid-wave infrared. Finally, we summarize our results and discuss future research directions.

Linked assets

University of Southern California Dissertations and Theses

Conceptually similar

PDF

Low-dimensional asymmetric crystals: fundamental properties and applications

PDF

Light-matter interactions in engineered microstructures: hybrid opto-thermal devices and infrared thermal emission control

PDF

Optimized nanophotonic designs for thermal emission control

PDF

Microphotonics for tailored infrared emission and secure data encryption

PDF

2D layered materials: fundamental properties and device applications

PDF

Designable nonlinear power shaping photonic surfaces

PDF

Resonant light-matter interactions in nanophotonic structures: for manipulating optical forces and thermal emission

PDF

Integrated large-scale monolithic electro-optical systems in standard SOI CMOS process

PDF

Photodetector: devices for optical data communication

PDF

Novel nanomaterials for electronics, optoelectronics and sensing applications

PDF

Modeling and engineering noise in superconducting qubits

PDF

Integrating material growth and device physics: building blocks for cost effective emerging electronics and photonics devices

PDF

Bogoliubov quasiparticles in Andreev bound states of aluminum nanobridge Josephson junctions

PDF

Silicon photonics integrated circuits for analog and digital optical signal processing

$Metasurfaces in 3D applications: multiscale stereolithography and inverse design of diffractive optical elements for structured light$

PDF

Metasurfaces in 3D applications: multiscale stereolithography and inverse design of diffractive optical elements for structured light

PDF

Nanophotonic light management in thin film silicon photovoltaics

PDF

GaN power devices with innovative structures and great performance

PDF

Integrated silicon waveguides and specialty optical fibers for optical communications system applications

PDF

Van der Waals material electronic devices for memory and computing

PDF

Building blocks for 3D integrated circuits: single crystal compound semiconductor growth and device fabrication on amorphous substrates

Asset Metadata

Creator Lien, Max Ray (author)

Core Title Metasurface-integrated black phosphorus photodetectors

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Electrical Engineering

Degree Conferral Date 2023-12

Publication Date 10/06/2023

Defense Date 10/05/2023

Publisher Los Angeles, California (original), University of Southern California (original), University of Southern California. Libraries (digital)

Tag black phosphorus,infrared,layered 2D materials,light trapping,metamaterials,OAI-PMH Harvest,photodetectors,photonics

Format theses (aat)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Povinelli, Michelle (committee chair), Levenson-Falk, Eli (committee member), Wang, Han (committee member)

Creator Email max.ray.lien@gmail.com,mrlien@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-oUC113758817

Unique identifier UC113758817

Identifier etd-LienMaxRay-12416.pdf (filename)

Legacy Identifier etd-LienMaxRay-12416

Document Type Dissertation

Format theses (aat)

Rights Lien, Max Ray

Internet Media Type application/pdf

Type texts

Source 20231013-usctheses-batch-1101 (batch), University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the author, as the original true and official version of the work, but does not grant the reader permission to use the work if the desired use is covered by copyright. It is the author, as rights holder, who must provide use permission if such use is covered by copyright.

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA

Repository Email cisadmin@lib.usc.edu