Solid-state imaging devices, commonly found in cameras and scientific instruments, rely on silicon (Si) based charge-coupled device (CCD) or CMOS sensor technology. However, Si based CCD or CMOS sensor has constrained spectral range (< 1.0 mu m) due to absorption characteristics of silicon (Si), and inadequate quantum efficiency (QE) over broad spectral ranges. To overcome these limitations, we propose a photodetector structure that enable broad spectral range detection, covering visible to shortwave infrared (VIS-SWIR) wavelength. This breakthrough detector achieves high QE (>90%) over wide spectral ranges and offers bandwidth e >30 GHz. To address the challenge in photo responses beyond the visible region, we utilize III-V compound semiconductor material to fabricate the detector utilizing matured InP substrates. The objective is to develop a monolithic photodiode and array that covers wavelengths from 0.4 mu m to 1.75 mu m, with high frequency response (>8 GHz) and high QE. The use of matured substrates and III-V semiconductor materials simplifies manufacturing, improves spectral coverage, and enhances performance significantly compared to existing Si technologies. The photodetector s can be either top or bottom illuminated and feature a single set of absorption layers designed to achieve desired QE and speed. This photodetector can be used as a single element or in an array configuration which operated independently with a metal line connection scheme enabling rapid and random addressing of individual pixels. Additionally, these uncooled photodetectors offer the advantage of performing reliably under various temperature variations. This detector holds great potential benefits for mul tispectral imaging, advanced communication systems, and sensing applications. We present fabrication and characterization results of the fabricated test structures.

We present an innovative solution to improve the efficiency of thermophotovoltaic (TPV) devices by tackling the problem of sub-bandgap photon losses. We propose an optimized design for thin-film mirrors using inverse electromagnetic design principles, thereby enhancing the average reflectivity and photon re-use. Our method surpasses the traditional Bragg mirror by employing a gradient-descent based optimization over Bragg mirror geometrical parameters, leveraging the transfer matrix method for derivative calculations. The optimized structure, based on continuously chirped distributed Bragg reflectors proposed herein demonstrates a remarkable increase in reflectivity beyond 98%, over an almost three-octaves bandwidth (0.1eV-0.74eV). We show that the incident power loss in InGaAs TPV cells at an emitter temperature of 1200°C is significantly reduced. While our work shows considerable promise, further exploration is needed to ascertain the practicability and robustness of these designs under various operational conditions. This study thus provides a major step forward in TPV technology, highlighting a new route towards more effective energy conversion systems.

Multilayer optical coatings are used to control the reflection, transmission, and absorption of light in a variety of applications. However, the design of these coatings is a challenging and computationally expensive task. In this article, we present an inverse-design method that provides an efficient and automated way to design one-dimensional optical coatings with superior optical properties. Our approach involves starting with a desired optical response and working backward to determine the optimal coating structure that can achieve that response. To accomplish this, we define a figure of merit (FOM) that maximizes the reflection or transmission within a desired spectral range and polarization. We identify the design variables, calculate the derivatives of the FOM, and iteratively update the thickness of each layer until the desired FOM is achieved. Our method applies to a broad variety of one-dimensional optical coatings, and we demonstrate its effectiveness by designing broadband mirrors and anti-reflection coatings. Our results show that the optimized structures outperform the initial fixed-thickness structures, highlighting the potential of inverse design to simplify the design and fabrication of complex multilayer optical coatings with greater precision and efficiency.

Significant efforts have been made on realizing deep ultraviolet (UV) – (200 – 280 nm) detectors; however, target detection elements remain inefficient, bulky, and low sensitivity. An ideal detector, particularly avalanche photodetectors (APDs) for this region must have high gain, high efficiency, and low noise. Ultrawide bandgap (UWB) AlxGa1-xN material system has shown promise to enable the design of high quantum efficient (QE), radiation-hard detectors capable of operating at high temperatures in this region. However, achieving high quality material becomes more difficult at higher Al compositions due to challenges associated with nonuniformity and doping efficiency. Current APDs are unable to provide high QE, low dark current, high multiplication gain, and solar blindness without coatings. We are developing III-Nitrite based APDs for single photon detection to operate filter-free in the deep-UV band. Toward this objective, we present results on development of high quality, high Al composition AlxGa1-xN with high conductivity, and demonstrate first in the world fabricated Separate Absorption and Multiplication (SAM) APD devices with 65% Al composition. Physics-based TCAD simulations utilizing the material system, and device measurement results are also presented. The proposed APD and its array, with single photon detection capability using III-N material systems will have earth science, space, defense, and commercial applications including UV spectroscopy, non-line of sight communications, portable chemical and biological identification systems.

In this work, we design an InGaAs avalanche photodiode (APD) with a higher photocurrent to dark current ratio than a conventional APD. The improvement is based on optical confinement in an optical resonant metastructure, which increases the optical intensity in the detector volume and thus its quantum efficiency. With optical confinement, the absorbing layer can be 20 times thinner than the conventional APD, reducing the intrinsic generation-recombination dark current by the same amount and still be able to produce 90.9% of the photocurrent. The photocurrent to intrinsic dark current ratio can thus be increased theoretically by 18.2 times in concomitant with a lower operating voltage. Higher photocurrent can also be obtained using a thicker absorber. Besides APDs, the optical resonator structure can also be applied to p-i-n photodiodes for similar improvement. To further increase the detection sensitivity, we design an efficient InGaAsP light emitting diode (LED) having a similar resonator geometry. The resonator geometry enables more collimated emission and increases the optical power theoretically by more than 36 times compared to a conventional LED. Integrating the meta- APD and LED together will yield a powerful SWIR transceiver for various low light, LIDAR, and 3-dimensional imaging applications.

In this work, we design an InGaAs avalanche photodiode (APD) with a higher photocurrent to dark current ratio than a conventional APD. The improvement is based on optical confinement in an optical resonator, which increases the optical intensity in the detector volume and thus its quantum efficiency. With optical confinement, the absorbing layer can be 20 times thinner than the conventional APD, reducing the intrinsic generation-recombination dark current by the same amount and still be able to produce 90.9% of the photocurrent. The photocurrent to intrinsic dark current ratio can thus be increased theoretically by 18.2 times in concomitant with a lower operating voltage. Higher photocurrent can also be obtained using a thicker absorber. Besides APDs, the optical resonator structure can also be applied to p-i-n photodiodes for similar improvement. To further increase the detection sensitivity, we design an efficient InGaAsP light emitting diode (LED) having a similar resonator geometry. The resonator geometry enables more collimated emission and increases the optical power theoretically by more than 36 times compared to a conventional LED. Integrating the resonant APD and LED together will yield a powerful SWIR transceiver for various low light, LIDAR, and 3-dimensional imaging applications.

In this work, we designed a wavelength-selective thermal meta-emitter consisting of a periodic array of rings. These rings generate localized surface plasmons to provide large emissivity, narrow band, and highly directional thermal radiation. The large emissivity enables large power throughput. The narrow spectrum matches the GaSb PV cell absorption spectrum and thus reduces the power wasted in the cell and increase the power conversion efficiency (PCE). The collimated emission enables large distance between emitter and cell distance to reduce convectional heating of PV cells. The emitter consists of a thin dielectric layer with an etched ring surface, and this surface is covered with a metal. Thermal emission emerges from the planar side of the dielectric surface when the metal cover is heated by a thermal source. We model different dielectric and metal materials to determine the optimum choice of materials for the meta-emitter. The considered materials are SiC and AlN for dielectric and gold, tungsten, rhodium, tantalum, molybdenum, niobium, chromium, and platinum for metal. We found that while AlN provides a larger power selectivity, SiC yields a better overall PCE because of the better matched emission spectrum with the GaSb PV cell. For the metal cover, we found that tantalum has the second largest power selectivity after gold. Since gold has a low melting point unsuitable for high temperature TPV operation, tantalum becomes the most suitable material for the meta-emitter.

The solar spectrum reaching Earth spans from ultraviolet to infrared wavelengths with a cutoff at around 2.5 μm. About 30% of the irradiance is absorbed by the atmosphere, and the rest is absorbed by the Earth's surface. The radiance available for generating renewable energy ranges from 400 nm to 2.0 μm. Silicon and III-V materials are used for photovoltaic (PV) cells. The PV cells reflect 30% of incoming radiation and to reduce this reflection, antireflection coating (ARC) is being used. Conventional techniques such as single or stacked multi-layer ARC or micro-nanostructures ARC are used. However, they lack in providing broadband and omnidirectional transmission, which limits its conversion efficiency in today’s PV cells. We present broadband ARC achieved with an inverse transfer design, a prospect towards significantly high conversion efficiency PV cells. The ARC exhibit significantly increased transmission more than 96% over the solar spectrum ranges up to 2.5 μm, even with wide angles of incidence from ~ 0° to >70°. This transmission over the same angles of incidence, is significantly higher than that of ARC based on quarter-wavelength optical thickness (QWOT), a state-of-art ARC. The results also rival the transmission performance of state-of-the-art nanostructure-based ARC even at large angles of incidence, but are significantly easier to fabricate using standard e-beam evaporation and/or sputtering. The results show over broadband spectrum ranges, radiation back due to reflection makes to zero, leading to significantly increased conversion efficiency of the PV cells.

The detection of Nd: YAG laser emission at 1.064 microns is important for a number of applications such as active infrared imaging and space LIDAR. We propose a silicon avalanche photodiode (APD) based on micro-scaled photonic metastructures. In one approach, the metastructure contains a hexagonal hole array filled with SiO2 to yield optical diffraction and trapping. With 3D electromagnetic modeling, we optimize the array dimensions and yield 26.0% theoretical absorption at 1.064 microns. It is 59 times larger than that of the planar structure with an additional 18% dark current reduction. In another approach, the metastructure contains a ring array fabricated on the top contact layer to excite localized surface plasmons. The theoretical QE is 52 times larger and the dark current is 8.6 times lower. In addition to active imaging and LIDAR, the present meta-APD designs are also useful for broadband passive imaging, in which Ip/Id can be improved by 3.9 and 8.7 times, respectively. The ring metastructure can also be integrated with an InGaAsP light emitting diode to produce coherent, directional emission at 1.064 micron to further improve SWaP in active applications.

A thermophotovoltaic (TPV) power generation system based on resonator-pixel (RP) emitters and photovoltaic (PV) cells is proposed. The RP structure enables wavelength-selective emission and absorption, which increases the system power throughout and efficiency. An RP structure contains a thin semiconductor material etched into an array of micron-sized rings and covered with a metal layer. The emissivity of the RP emitter is determined by equating it to absorptivity and is modeled by three-dimensional electromagnetic modeling. The result shows that the emitter is able to increase the wavelength selectivity by eight times compared to a bulk emitter. Integrated with an RP PV cell having enhanced absorption at the band edge, the power conversion efficiency can be maintained constant in a wide range of temperatures, while the power throughput increases by 1.7 times. In addition, the RP emission is highly directional. Its far-field beamwidth is less than 10°, with which the emitter and PV cell can be placed at a larger distance to avoid convectional heating on the PV cell. In this work, we also confirm a near-field TPV effect that raises the electrical power by a factor of 2 when the emitter and cell are placed less than 2 μ m apart.

We propose a wavelength-selective thermal emitter based on the resonator-pixel (RP) structure. This passive emitter is designed for thermophotovoltaic (TPV) power generation. It contains a thin SiC material etched into an array of micron-sized rings, and the etched surface is covered with a metal such as tantalum. The emissivity of the emitter is determined by equating it to the absorptivity, which is modeled by 3-dimensional electromagnetic modeling. The result shows that the RP emitter is able to increase the wavelength selectivity by 8 times relative to a bulk SiC emitter. Integrated with a resonant GaSb PV cell with enhanced absorption at the band edge, the power conversion efficiency (PCE) can be maintained constant at 80% between 600 and 1800°C while the power throughput increases by 1.72 times. We further modeled the far-field beam profile of the emitter and found the beamwidth to be less than 10° when the emitter area is larger than 10 × 10 μm2. This highly collimated emission allows larger distance between the emitter and PV cell without reducing its PCE. The thermal management of the TPV system can thus be greatly improved. By modeling the emitter and PV cell together, we also found a near-field TPV effect, which increases the electrical power throughput by a factor of 2.

In this work, we designed an InGaAs APD with a higher photocurrent to dark current ratio than a regular APD. The improvement is based on optical confinement in an optical resonator, which is able to increase the optical intensity in the detector volume and thus its quantum efficiency. With optical confinement, the absorbing layer can be 20 times thinner than the regular APD, reducing the dark current by the same amount and still be able to produce 90.9% of the photocurrent. The I-p/I-d ratio can thus be increased by 18.2 times in concomitant with a lower operating voltage. Higher photocurrent can also be obtained using a thicker absorber. To further increase the detection sensitivity, we designed an efficient InGaAsP LED having the similar resonator geometry. The resonator geometry enables more collimated emission and increases the optical power theoretically by more than 36 times compared with a regular LED. Integrating the resonant APD and LED together will yield a powerful SWIR transceiver for various low light, LIDAR, and 3-dimensional imaging applications.

A midwave infrared light emitting device (LED) with a micro-scaled photonic structure coupling to a resonator is proposed. The photonic structure is used to create localized surface plasmons (LSP), with which significant optical confinement can occur near the surface, thereby increasing the internal emission quantum efficiency. The LED volume is further designed into a resonator, with which the LSP resonates with the radiating mode of the resonator, thereby increasing the light extraction efficiency. The similarly designed structure can also be used as a wavelength-selective passive emitter to suppress the thermal radiation beyond a cutoff wavelength. Therefore, the designed emitter structure can be useful in a wide range of applications.

We present a multi-quantum well (MQW)-based photodetectors design method for a 1-3 μm wavelength selectivity range using the finite difference time domain (FDTD) Lumerical platform. We demonstrate absorption coefficient and power absorption profile modulation in an III-V-based type-II MQW stack embedded with photon-trapping (PT) surface structures. We present an MQW-based photodetectors design space by varying the MQW stacking period, and the well and the barrier dimensions from 100-200 and 5-10 nm respectively. We show that the power absorption in the MQW increases for a fixed wavelength sensitivity range. However, the well and the barrier dimension variation facilitate the wavelength sensitivity range modulation. The upper bound of 3 μm on the wavelength-selectivity is achieved by tuning the well/barrier widths. We further proposed a modified device structure to cap the lower wavelength optical signal and cap them at 1 μm. We also show a tremendous increase in power absorption by introducing photon-trapping holes into the MQW structure. Finally, we extract the effective absorption coefficient of the MQW using the power absorption profile generated in the FDTD framework to show the desired wavelength selectivity. Finally, we utilize the extracted absorption coefficient to perform a COMSOL-based simulation to show a 31% enhancement in quantum efficiency of the MQW detector with the introduction of photon-trapping holes.

Broadband and omnidirectional anti-reflection coatings, Distributed Bragg Reflectors (DBR), and optical bandpass filter coatings in the 3-5 μm and 1-12 μm IR ranges are of crucial importance in the design of high-efficiency MWIR / LWIR detectors, Thermophotovoltaic devices, and IR imaging systems. We discuss a novel inverse design approach to multilayer optical coatings by a Transfer Matrix Method simulation and optimization algorithm. This algorithm iteratively simulates a multilayer structure's reflection and transmission characteristics by the Transfer Matrix Method and updates layer thicknesses to optimize a performance metric – whether reflection, transmission, or filter performance – over a given band and range of angles of incidence. The optimization procedure provides a systematic, computational approach to designing optical coatings with desired reflection, transmission, and absorption characteristics over wavelength bands and angles of incidence of interest in an application. We show that Distributed Bragg Reflectors and Anti-reflection coatings generated by the algorithm outperform conventional quarter-wavelength layer coatings over broadband MWIR and LWIR ranges and angles of incidence as high as 70 degrees, while reducing total component thickness significantly – a major benefit when fabricating optical coatings on semiconducting substrates directly. We also discuss how the approach can be extended to sharp-cutoff broadband bandpass filter design.

Broadband and omnidirectional antireflection coatings in the 3-5 μm and 3-12 μm infrared ranges are of crucial importance in maximizing quantum efficiency in MWIR and LWIR photodetectors, as well as in improving the efficiency of LWIR thermophotovoltaic devices. Conventional approaches to AR coatings on semiconductors, such as quarter-wavelength optical thickness layers, fail to provide consistently high transmission over mid-IR bandwidths and acute angles of incidence. Unconventional approaches such as metasurface and nanostructure AR coatings are difficult and expensive to fabricate on a large scale. We discuss ultra-broadband antireflection coating designed through an inverse design procedure driven by a differential evolution optimization algorithm. Our approach iteratively simulates the transmission characteristics of a multilayer structure using the Transfer-Matrix Method and optimizes layer parameters to maximize the average transmission within a given wavelength band and range of angles of incidence. We present AR coatings which exhibit 98% and 96% average transmission over the 3 μm - 5 μm and 3 μm - 12 μm ranges respectively, over angles of incidence between 0° and 70°. Compared to quarter optical wavelength antireflection coatings over the same ranges, our structures exhibit significantly higher broadband transmission – quarter-wavelength structures saturate near 80% transmission over the 3-5 μm or 3-12 μm ranges, with transmission significantly lowered at large angles. As such, our results rival the transmission performance of state-of-the-art nanostructure AR coatings even at large angles of incidence but are significantly easier to fabricate using standard e-beam evaporation and/or sputtering.

High performance broadband Distributed Bragg Reflectors (DBRs) have become instrumental in the design of high efficiency Thermophotovoltaic devices, improving efficiency by redirecting parasitic photon loss during conversion towards the device for regenerating power. Broadband Bragg Reflectors designed to reflect in the 3-5 micron, 3-12 micron, and 1-12 micron infrared ranges are of crucial importance in such applications to maximize regeneration efficiency in TPV devices, as most parasitic photons fall in these ranges under typical operating temperatures. Additionally, omnidirectional capabilities further improve regeneration efficiency by absorbing parasitic photons at large angles of incidence. In this work, we present a novel inverse design transfer matrix optimization algorithm for designing variable layer thickness broadband DBRs for arbitrary wavelength ranges and angles of incidence, and we showcase the resulting ultra-high reflection DBRs designed with our algorithm, specifically for the 1-12μm IR range. We demonstrate that our DBRs provide very significant increases in average reflection over the IR ranges of interest as compared to state-of-the-art reflective coatings, even at large angles of incidence, and are also optimized to be considerably thinner than quarter wavelength and other state of the art DBRs with comparable reflection spectra. As such, the DBRs designed with our inverse design algorithm have significant applications in the design of very high efficiency TPV systems, while also being thin enough to embed onto devices or windows, and are also easy to fabricate due to their simple multilayer 1-D structure.

State-of-art detectors having response over 2.0+ μm wavelengths require cryogenic coolers to improve SNR and achieve high sensitivity. Next generation detectors are required to operate at room temperature (300 K) to fit small form factor sensor modules for SWaP-efficient space instruments and defense applications. III-V materials-based p-i-n detectors (e.g. InGaAs) over 2.0+ μm wavelengths suffer from lattice mismatch with substrate, resulting in high dark currents, low sensitivity (low SNR), and needing cooling. An avalanche photodiode (APD) with high gain will significantly increase sensitivity above p-i-n detector solutions; however, dark current also multiplies under high gain, leading to increased noise. An APD’s material system dictates noise associated with gain process and depends on ratio of ionization coefficient (hereinafter mentioned as k or k-factor) which dictate the APD excess noise. In APDs, dark current and excess noise limit useful gain and hence the SNR of detection systems. Therefore, it is essential to reduce k-factor and dark current of an APD. We present a first in the world low-noise, high-gain, and high bandwidth 2.0+ μm uncooled APD having k factor of <0.01 enabling low excess noise at a given gain. The APD uses a novel structure and a bandstructure-engineered multiplication region based on III-V material system to reduce k-factor to near zero. We present the APD SACM device design, their simulations, and experimental results of fabricated APD devices. The uncooled 2.0+μm APD matches advantages offered by HgCdTe APDs having k~ zero, but with low dark currents at near room-temperature operation, and also mitigates both HgCdTe APDs and standard III-V p-i-n deficiencies to become an ideal solution benefiting to extend short-wave IR band to 2.0 +μm for imaging applications.

For mid-wave infrared detectors, we designed a meta-surface to enhance quantum efficiency (QE) across 2 to 6 microns. The relative enhancement depends on the intrinsic absorption coefficient α of the material. If α is large at 2×10 4 /cm, a 0.1 micron-thick meta-detector can yield a peak QE of 90%, which is 2.6 times higher than the conventional detector. The improvement is larger with a smaller α. When it is 2000/cm, the improvement is about 10 times with a peak QE of 49%. For energy harvesting devices, we designed several nanostructures etched on top of silicon solar cells to enhance their absorption. Employing an array of nano-columns increases absorption from 55% to 97% at 0.7 microns and from 5.0% to 37% at 1.0 microns. An array of nano-cones further increases the average absorption to 95% between 0.4 to 0.8 microns. The overall integrated absorption is increased by 74% for nano-columns and 92% for nano-cones. For GaAs solar cells, a metasurface can improve photocurrent by 26% from a planar solar cell with a 100 nm-thick absorber.