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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 $\alpha $ of the material. If $\alpha $ is large at $2 \times 10^{4}$ /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 $\alpha $ . 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.

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.

The demonstration of a newly developed compact thermal imager (CTI) on the International Space Station (ISS) has provided not only a technology advancement but a rich high-resolution dataset on global clouds, atmospheric and land emissions. This study showed that the free-running CTI instrument could be calibrated to produce scientifically useful radiance imagery of the atmosphere, clouds, and surfaces with a vertical resolution of ~460 m at limb and a horizontal resolution of ~80 m at nadir. The new detector demonstrated an excellent sensitivity to detect the weak limb radiance perturbations modulated by small-scale atmospheric gravity waves. The CTI’s high-resolution imaging was used to infer vertical cloud temperature profiles from a side-viewing geometry. For nadir imaging, the combined high-resolution and high-sensitivity capabilities allowed the CTI to better separate cloud and surface emissions, including those in the planetary boundary layer (PBL) that had small contrast against the background surface. Finally, based on the ISS’s orbit, the stable detector performance and robust calibration algorithm produced valuable diurnal observations of cloud and surface emissions with respect to solar local time during May–October 2019, when the CTI had nearly continuous operation.

For MWIR IR detectors, we designed a meta-surface to enhance QE across 2 to 6 microns. The relative enhancement depends on the intrinsic absorption coefficient, α, of the material. If α is large at 2x104/cm, a 0.1 micron-thick metadetector can have a peak QE of 90%, which is 2.6 times higher than the conventional detector. The improvement is larger at 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 93% at 0.7 microns and from 5.0% to 20% 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 71% for nano-columns and 91% 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.

In an infrared photodetector, noise current (dark current) is generated throughout the volume of the detector. Reducing the volume will reduce dark current, but the corresponding smaller area will also reduce the received signal. By using a separate antenna to receive light, one can reduce the detector area without reducing the signal, thereby increasing its signal-to-noise ratio (SNR). Here, we present a dielectric resonator antenna (DRA)-coupled infrared photodetector. Its performance is compared to a conventional resonant cavity-enhanced slab detector. The noise equivalent power (NEP) is used as a figure of merit for the comparison. Formulas for the NEP are derived for both cases. A pBp photodiode detector is assumed in the comparison. The active region of the photodiode is an InAs/GaSb Type II Superlattice (T2SL). A Genetic Algorithm is used to optimize the dimensions of the detector and the DRA to achieve the smallest NEP. The result is a photodetector that converts over 85% of the incident light into carriers with a volume reduced by 95%. This optimal geometry leads to an NEP reduced by 6.02 dB over that of the conventional resonant cavity-enhanced slab detector.

A Dielectric Resonator Antenna Coupled Infrared Antimonide Photodetector (DRACAD) is presented. The large capture area of the DRA feeds incident radiation to the subwavelength dimension detector. The reduced dimension of the detector reduces the Noise Equivalent Power (NEP). The large capture area of the DRA increases the captured signal. The combination leads to high Signal-to-Noise Ratios (SNR). The DRACAD is designed using a Genetic Algorithm Optimization to obtain dimensions which maximize the absorbed infrared radiation with the smallest possible detector. The plasmonic metals are modeled as effective lossy sheet impedances. Absorption spectrum simulations between $8-12\mu m$ wavelengths show the DRACAD absorbs nearly 73% of the incident light at $9\mu m$ after all metal losses are accounted for.

With the increasing use of resonant structures in optical devices, broadband optical characterization of the refractive index and extinction coefficient is necessary for accurate simulation and device design. For resonance-enhanced photodetectors, the complex refractive index is necessary to impedance match not only the resonator to air, minimizing the reflection, but also the resonator to the detector element, ensuring absorption occurs in the photodiode. To work towards better resonator-detector coupling, we present the complex refractive index for GaSb and an InAs/GaSb strained layer superlattice designed to be the absorber layer for a long-wave infrared photodetector. The optical properties were extracted using spectroscopic ellipsometry. Several modeling methods will be discussed for both the superlattice and the single-side polished bulk GaSb. Comparison to transmission and reflection values as well as absorption coefficients from literature provide additional confidence in the extraction process. Future work will incorporate these values into a resonance-enhanced photodetector.

III-V GaSb based strained layer superlattice (SLS) can provide broadband detection from 3 to 15 microns and beyond. However, the longer wavelength material has smaller external quantum efficiency (EQE) because of the smaller absorption coefficient and carrier diffusion length. The usual approach of using thicker material for higher EQE is not viable because of the limited carrier collection. In this work, we adopt a metasurface on top of a thin detector material to increase its absorption without increasing the thickness, thereby increasing EQE. The metasurface consists of an array of rings. These rings excite different optical responses in different wavelength regimes, thus yielding a broad detection spectrum. In the MWIR regime, they diffract light collectively, and in the LWIR and VLWIR regimes, they focus light as Fresnel lenses and excite localized surface plasmon polaritons (SPPs). Through these different optical interactions, the incident radiation is redirected and trapped in the detector volume. When the trapped light interferes constructively, the optical intensity builds up to a large value, leading to a large absorption in a thin material. A properly designed metastructure is thus able to achieve a large EQE in a broad spectrum. The thin absorbing layer also allows several of them stacking together to form a multi-color detector. Narrowband detection can be obtained in each color using circular posts instead of rings. By exploiting different optical field profiles under different optical responses, a voltage-switchable detection scheme can be implemented to detect four discrete wavelengths ranging from MWIR to VLWIR. The broad and narrow band designs thus show the flexibility of metastructures. To compare the present metastructure approach with the more conventional approaches using microlenses and Fresnel lenses that can also reduce dark current while maintaining a sizable QE, we optimize the respective structures and compare the merits under different approaches.

In 1988 DARPA provided funding to NASA’s Goddard Space Flight Center to support the development of GaAs Quantum Well Infrared Photodetectors (QWIP). The goal was to make a single element photodetector that might be expandable to a two-dimensional array format. Ultimately, this led to the development of a 128 x 128 element array in collaboration with ATT 2) broad spectral response and; 3) higher sensitivity. We have collaborated extensively with QmagiQ, LLC and Northwestern University to further pursue and advance the SLS technology ever since we started back in 2012. In December of 2018 we launched the first SLS-based IR camera system to the International Space Station on board the Robotic Refueling Mission #3 (RRM3). This paper will describe the evolution of QWIP technology leading to the current development of SLS-based imaging systems at the Goddard Space Flight Center over the past 30 years.

We are developing small pixel 12-μm pitch resonator-QWIPs for infrared imaging at 8 μm and 10 μm. The materials and geometries are designed to yield 44% quantum efficiency (QE) at 1.0 × 1018 cm−3 doping and 56% at 1.2 × 1018 cm−3 for the 8 μm detectors and 47% at 1.0 × 1018 cm−3 for the 10 μm detector. Because of production variability and excess detector tunneling current at large bias, the maximum QE for the 8 μm detectors is measured to be 24.6% and 56.4%, respectively, while that for the 10 μm detector is 44%. Focal plane arrays of 1280 × 1024 resolution were produced from the lower doping, 8 μm material. Their performance is the same as the test detectors up to the maximum internal bias of the readout circuit. From the test detector I-V characteristics, the performance can be further projected to higher bias, at which the FPAs can offer 23 mK sensitivity at 3.7 ms integration time from the lower doping material and 21 mK at 3.2 ms from the higher doping material. For the 10 μm FPAs, the sensitivity is projected to be 30 mK at 3.8 ms. Therefore, we have demonstrated high sensitivity and fast R-QWIP FPAs down to 12-μm pitch. To expand the application envelope of the resonator-pixel technology, we have deigned resonant geometries for other infrared materials. Experimental efforts are underway.

In this presentation, we will discuss the advantages of using an in-pixel digital read out integrated circuit and type-II strained layer superlattice detector array technology to elevate the operating temperature of the focal plane array for Earth remote sensing instruments.

A systematic calibration approach is presented to correlate the digital output of an infrared camera and the scene temperature. Aided by the optoelectronic properties of the camera, as few as two experimental data points are needed to establish this correlation. This approach can readily include the effects of atmospheric transmission, scene emissivity, and different background subtractions. Hence, the temperature conversion in flight can be reliably obtained from laboratory calibration. The conversion function can also be used to identify the camera’s thermal sensitivity and temperature resolution, which are important information in different space missions. In applying this calibration procedure to a laboratory camera and the compact thermal imager onboard the International Space Station, its validity is confirmed.

We recently proposed a resonator structure to increase the quantum efficiency (QE) of a quantum well infrared photodetector. In this paper, we performed a systematic study on its QE as a function of detector size and doping density. We found that the experimental result generally agrees with the prediction from electromagnetic modeling, and the resonant coupling scheme is effective with the pixel pitch as small as $6~\mu \text{m}$ . In addition, we modeled the potential detector performance at even smaller pixel pitches and found that the maximum QE occurs at 3- $\mu \text{m}$ pixel pitch for 9- $\mu \text{m}$ detection, where QE is predicted to be above 80%.

Long-wavelength infrared (LWIR) focal plane arrays (FPAs) needed for Earth Science imaging, spectral imaging, and sounding applications have always been among the most challenging in infrared photodetector technology due to the rigorous material growth, device design and fabrication demands. Future small satellite missions will present even more challenges for infrared FPAs, as operating temperature must be increased so that cooler (and radiator) volume, mass, and power can be reduced. To address this critical need, we are working on following three technologies. 1) Type-II superlattice (T2SL) nBn type barrier infrared detector, which combines the high operability, spatial uniformity, temporal stability, scalability, producibility, and affordability advantages of III-V detectors. 2) The resonator pixel technology, which uses metasurface light trapping techniques to achieve strong absorption in a small detector absorber volume, thereby enabling enhanced QE and/or reduced dark current. 3) High dynamic range 3D Readout IC (3D-ROIC), which integrates a digital reset counter with a conventional analog ROIC to provide a much higher effective well capacity than previously achievable. The resulting longer integration times are especially beneficial for high flux/dark current LWIR applications as they can improve signal-to-noise ratio and/or increase the operating temperature. By combining these three technologies, this project seeks to demonstrate a cost-effective, high performance LWIR FPA technology with significantly higher operating temperature and sensitivity than previously attainable, and with the flexibility to meet a variety of Earth Science TIR measurement needs, particularly the special requirements of small satellite missions.

Long-wavelength infrared (LWIR) focal plane arrays (FPAs) needed for Earth Science imaging, spectral imaging, and sounding applications have always been among the most challenging in infrared photodetector technology due to the rigorous material growth, device design and fabrication demands. Future small satellite missions will present even more challenges for LWIR FPAs, as operating temperature must be increased so that cooler (and radiator) volume, mass, and power can be reduced. To address this critical need, we are working on following three technologies. 1) Type-II superlattice (T2SL) barrier infrared detector (BIRD), which combines the high operability, spatial uniformity, temporal stability, scalability, producibility, and affordability advantages of the quantum well infrared photodetector (QWIP) FPA with the better quantum efficiency and dark current characteristics. A mid-wavelength infrared (MWIR) T2SL BIRD FPA is a key demonstration technology in the (6U) CubeSat Infrared Atmospheric Sounder (CIRAS) funded under the ESTO InVEST Program. A LWIR T2SL BIRD FPA is also being developed under the ESTO SLI-T Program for future thermal infrared (TIR) land imaging needs. 2) The resonator pixel technology, which uses nanophotonics light trapping techniques to achieve strong absorption in a small detector absorber volume, thereby enabling enhanced QE and/or reduced dark current. 3) High dynamic range 3D Readout IC (3DROIC), which integrates a digital reset counter with a conventional analog ROIC to provide a much higher effective well capacity than previously achievable. The resulting longer integration times are especially beneficial for high flux/dark current LWIR applications as they can improve signal-to-noise ratio and/or increase the operating temperature. By combining the aforementioned technologies, this project seeks to demonstrate a cost-effective, high-performance LWIR FPA technology with significantly higher operating temperature and sensitivity than previously attainable, and with the flexibility to meet a variety of Earth Science TIR measurement needs, particularly the special requirements of small satellite missions.

In this presentation, we will discuss the advantages of using an in-pixel digital read out integrated circuit and resonator pixel technology with the type-II strained layer superlattice detector array to elevate the operating temperature of the focal plane array for Earth remote sensing applications.

In this presentation, we will discuss the advantages of using an in-pixel digital read out integrated circuit and metasurface resonator pixel technology with the type-II strained layer superlattice detector array to elevate the operating temperature of the focal plane array for Earth remote sensing applications.

In this paper, we report our recent efforts in achieving high performance in Antimonides type-II superlattice (T2SL) based infrared photodetectors using the barrier infrared detector (BIRD) architecture, resonator pixel light coupling mechanism, and digital read out integrated circuits (DROICs).

Long-wavelength infrared (LWIR) focal plane arrays (FPAs) needed for Earth Science imaging, spectral imaging, and sounding applications have always been among the most challenging in infrared photodetector technology due to the rigorous material growth, device design and fabrication demands. Future small satellite missions will present even more challenges for LWIR FPAs, as operating temperature must be increased so that cooler (and radiator) volume, mass, and power can be reduced. To address this critical need, we are working on following three technologies. 1) Type-II superlattice (T2SL) barrier infrared detector (BIRD), which combines the high operability, spatial uniformity, temporal stability, scalability, producibility, and affordability advantages of the quantum well infrared photodetector (QWIP) FPA with the better quantum efficiency and dark current characteristics. A mid-wavelength infrared (MWIR) T2SL BIRD FPA is a key demonstration technology in the (6U) CubeSat Infrared Atmospheric Sounder (CIRAS) funded under the ESTO InVEST Program. A LWIR T2SL BIRD FPA is also being developed under the ESTO SLI-T Program for future thermal infrared (TIR) land imaging needs. 2) The resonator pixel technology, which uses nanophotonics light trapping techniques to achieve strong absorption in a small detector absorber volume, thereby enabling enhanced QE and/or reduced dark current. 3) High dynamic range 3D Readout IC (3DROIC), which integrates a digital reset counter with a conventional analog ROIC to provide a much higher effective well capacity than previously achievable. The resulting longer integration times are especially beneficial for high flux/dark current LWIR applications as they can improve signal-to-noise ratio and/or increase the operating temperature. By combining the aforementioned technologies, this project seeks to demonstrate a cost-effective, high-performance LWIR FPA technology with significantly higher operating temperature and sensitivity than previously attainable, and with the flexibility to meet a variety of Earth Science TIR measurement needs, particularly the special requirements of small satellite missions.

In this presentation, we will report our recent efforts in achieving high performance in Antimonides type-II superlattice (T2SL) based infrared photodetectors using the barrier infrared detector (BIRD) architecture. The recent emergence of barrier infrared detectors such as the nBn [1] and the XBn [2] have resulted in mid-wave infrared (MWIR) and long-wave infrared (LWIR) detectors with substantially higher operating temperatures than previously available in III-V semiconductor based MWIR and LWIR detectors. The initial nBn devices used either InAs absorber grown on InAs substrate, or lattice-matched InAsSb alloy grown on GaSb substrate, with cutoff wavelengths of $\sim 3.2 \ \mu \mathrm{m}$ and $\sim 4\ \mu \mathrm{m}$ , respectively. While these detectors could operate at much higher temperatures than existing MWIR detectors based on InSb, their spectral responses do not cover the full ( $3 - 5.5 \mu \mathrm{m})$ Mwir atmospheric transmission window. There also have been nBn detectors based on the InAs/GaSb T2SL absorber [3], [4].

We will discuss a development of digital infrared sensor using type-II superlattices. Our goal is to elevate the operating temperature of the sensor to reduce the size, weight, and power of the remote sensing instruments.

In this work, the performance of a 640 × 512 long-wavelength resonant quantum well infrared photodetector (R-QWIP) focal plane array (FPA) was evaluated as a function of operating temperature, bias, and photon flux using an F/2.2 optic. From these FPA measurements an assessment of the dark current, noise, conversion efficiency and noise-equivalent temperature difference is provided herein. Histogram results are used to support a statistical interpretation of operability and non-uniformity across the R-QWIP FPA. In addition, single pixel devices fabricated from the same wafer lot enabled supplemental noise gain and spectral response measurements. The spectral response of this R-QWIP structure was confirmed to peak around 8.3 microns with a spectral bandwidth or approximately 1 micron (full-width half maximum) and the noise gain measurements were used to provide an estimation of the expected external quantum efficiency (conversion efficiency = quantum efficiency ∗ gain).

Resonator-quantum well infrared photodetectors (R-QWIPs) are the next generation of QWIP detectors that use resonances to increase the quantum efficiency (QE). To achieve the expected performance, the detector geometry must be produced in precise specification. In particular, the height of the diffractive elements (DE) and the thickness of the active resonator must be uniformly and accurately realized to within 0.05 lm accuracy and the substrates of the detectors have to be removed totally. To achieve these specifications, two optimized inductively coupled plasma (ICP) etching processes are developed. Using these etching techniques, we have fabricated a number of R-QWIP test detectors and FPAs with the required dimensions and completely removed the substrates of the test detectors and FPAs. Their QE spectra were tested to be in close agreement with the theoretical predictions. The operability and spectral non-uniformity of the FPA is about 99.57% and 3% respectively.

Resonator-Quantum Well Infrared Photo detectors (R-QWIPs) are the next generation of QWIP detectors that use resonances to increase the quantum efficiency (QE). Recently, we are exploring R-QWIPs for broadband long wavelength applications. To achieve the expected performance, two optimized inductively coupled plasma (ICP) etching processes (selective and non-selective) are developed. Our selective ICP etching process has a nearly infinite selectivity of etching GaAs over Ga1-xAlxAs. By using the etching processes, two format (1Kx1K and 40x40) detectors with 25 μm pixel pitch were fabricated successfully. In despite of a moderate doping of 0.5 × 1018 cm-3 and a thin active layer thickness of 0.6 or 1.3 μm, we achieved a quantum efficiency 35% and 37% for 8 quantum wells and 19 quantum wells respectively. The temperature at which photocurrent equals dark current is about 66 K under F/2 optics for a cutoff wavelength up to 11 μm. The NEΔT of the FPAs is estimated to be 22 mK at 2 ms integration time and 60 K operating temperature. This good result thus exemplifies the advantages of R-QWIP.

There are potential applications for high operating temperature (HOT) Hg1-xCdxTe (MCT) infrared (IR) devices for both defense and commercial applications. MCT absorption can be raised either by increasing the absorber thickness or by lowering the operating temperature. However, if we can increase MCT absorption by using plasmonic materials, it will help in both HOT operation and increasing signal to noise ratio. We studied three Gold (Au) plasmonic structures for the absorption enhancement of MCT materials in the wavelength band of 3-7 μm. By using RF module in COMSOL multiphysics software, we found a large increase (1.85X) of MCT absorption when the optimized Au plasmonic structure is used. The increase in absorption at 4.5-6 μm is higher than that at 3-4 μm. The increased absorption due to plasmonic structure allows absorber thickness to decrease which leads to decrease in dark current and HOT devices. We report here record 90% increase in absorption of MCT material at room temperature at peak wavelength around 3.4 μm by using optimized plasmonic structure.

Rigorous electromagnetic field modeling is applied to calculate the quantum efficiency of various quantum well infrared photodetector (QWIP) geometries. We found quantitative agreement between theory and experiment for corrugated-QWIPs, grating-coupled QWIPs, and enhanced-QWIPs, and the model explains adequately the spectral lineshapes of the quantum grid infrared photodetectors. After establishing our theoretical approach, we used the model to optimize the detector structures for 12-micron pixel pitch focal plane arrays.

We have been developing corrugated quantum well infrared photodetector (C-QWIP) technology for long wavelength applications. A number of large format 1024 × 1024 C-QWIP focal plane arrays (FPAs) have been demonstrated. In this paper, we will provide a detailed analysis on the FPA performance in terms of quantum efficiency η and compare it with a detector model. We found excellent agreement between theory and experiment when both the material parameters and the pixel geometry were taken into account. By changing the number of quantum wells, doping density, spectral bandwidth and pixel size, a range of η from 13% to 37% was obtained. This range of η, combined with the wide spectral width, enables C-QWIPs to be operated at a high speed. For example, model analysis shows that a C-QWIP FPA with 10.7 μm cutoff and 25 μm pitch will have a thermal sensitivity of 16 mK at 2 ms integration time with f/2 optics in the presence of 900 readout noise electrons.

Corrugated quantum-well infrared photodetectors (C-QWIPs) have been proposed for long-wavelength infrared detection. In this work, we optimize the detector structure and produce a number of large format focal plane arrays (FPAs). Specifically, we adopt one-corrugation-per-pixel geometry to increase the active detector volume and incorporate a composite cover layer to preserve the large sidewall reflectivity, which results in a large detector quantum efficiency. We also optimize the detector material structure such as the final state energy, the doping density, and the number of quantum well periods to improve the FPA operation under the existing readout electronics. As a result, high FPA sensitivity has been achieved, and their characteristics are in agreement with the detector model. Based on this model, we perform a systematic analysis on the FPA performance with a wide range of detector and system parameters. We find that C-QWIP FPAs are capable of high-speed imaging especially for those with longer cutoff wavelengths.