Effects of gamma and proton irradiation, and of forward bias minority carrier injection, on minority carrier diffusion and photoresponse were investigated for long-wave (LW) and mid-wave (MW) infrared detectors with engineered majoritycarrier barriers. The LWIR detector was a type-II GaSb/InAs strained-layer superlattice pBiBn structure. The MWIR detector was a InAsSb/AlAsSb nBp structure without superlattices. Room temperature gamma irradiations degraded the minority carrier diffusion length of the LWIR structure, and minority carrier injections caused dramatic improvements, though there was little effect from either treatment on photoresponse. For the MWIR detector, effects of room temperature gamma irradiation and injection on minority carrier diffusion and photoresponse were negligible. Subsequently, both types of detectors were subjected to gamma irradiation at 77 K. In-situ photoresponse was unchanged for the LWIR detectors, while that for the MWIR ones decreased 19% after cumulative dose of ~500 krad(Si). Minority carrier injection had no effect on photoresponse for either. The LWIR detector was then subjected to 4 Mrad(Si) of 30 MeV proton irradiation at 77 K, and showed a 35% decrease in photoresponse, but again no effect from forward bias injection. These results suggest that photoresponse of the LWIR detectors is not limited by minority carrier diffusion.

Airborne meteorological instruments are key to climate science, of which understanding cloud formation is an important component. Cloud formation involves the entrainment of air with dissimilar convective, thermal, and humidity properties. Improved understanding of these meteorological parameters can improve forecasting. The National Oceanic and Atmospheric Administration (NOAA) currently relies on outboard sensors with 25 Hz sampling, giving 10 m spatial resolution during nominal fixed wing aircraft flights. Higher-resolution humidity and temperature data are needed. We describe a novel laser absorption-based instrument that can make high sensitivity airborne measurements with 25 cm spatial resolution. The use of mid-infrared (MIR) quantum cascade lasers (QCLs) enables high sensitivity humidity measurements based on strong fundamental vibrations of water vapor. Sampling at 1 MHz (averaged data output exceeding 1 kHz) enables the high spatial resolution from a jet platform that accesses near the ground to 45,000 ft. This technology could be extended using complementary lasers to identify any Chemical, Biological, Radiological, and Nuclear (CBRN) threats, or hazardous material incidents, based on their MIR absorption features. Additional applications could also include highly accurate information for combating wildfires and high-speed identification of anomalous gas-phase species in semiconductor processing.

Surface dust blown by a lunar lander can spoof sensors and damage lander and other surface and orbital assets. Since many countries seek to use and leverage the Moon in the coming decades, this is potentially a defense issue. Empirical data on Plume Surface Interactions (PSI) from lander-mounted instruments are needed to determine particle size distributions. We report a feasibility study of laser light-scattering for particle sizing. Calculations suggest that distributions of particle sizes in the range 0.1 to 10 microns can be accurately determined from laser-propagation decay using 4 to 8 wavelengths between 0.4 to 2 microns. Lab standards have been created based on calibrated showers of silica spheres and known concentrations and sizes of SiC grit in resin rods. Experiments were performed using lasers from 0.4 to 10 micron wavelength. For visible wavelengths, a point Si detector or images taken with a Si CCD camera were used to record scattered intensity vs propagation distance. At long-wave infrared, a pyroelectric detector or bolometer array were used. Characteristic decay lengths were determined by an algebraic sliding aperture method suitable for rapid and automated analysis. The experiments confirm theoretical expectations for Mie scattering by simple distributions of spherical particles. These results inform future experiments for testing the inverse problem of extracting more complicated size distributions from decay lengths measured using multiple wavelengths.

Airborne meteorological instruments are key to climate science, of which understanding cloud formation is an important component. Cloud formation involves the entrainment of air with dissimilar convective, thermal, and humidity properties. Improved understanding of these meteorological parameters can improve forecasting. The National Oceanic and Atmospheric Administration (NOAA) currently relies on outboard sensors with 25 Hz sampling, giving 10 m spatial resolution during nominal fixed wing aircraft flights. Higher-resolution humidity and temperature data are needed. We describe a novel laser absorption-based instrument that can make high sensitivity airborne measurements with 25 cm spatial resolution. The use of mid-infrared (MIR) quantum cascade lasers (QCLs) enables high sensitivity humidity measurements based on strong fundamental vibrations of water vapor. Sampling at 1 MHz (averaged data output exceeding 1 kHz) enables the high spatial resolution from a jet platform that accesses near the ground to 45,000 ft. This technology could be extended using complementary lasers to identify any Chemical, Biological, Radiological, and Nuclear (CBRN) threats, or hazardous material incidents, based on their MIR absorption features. Additional applications could also include highly accurate information for combating wildfires and high-speed identification of anomalous gas-phase species in semiconductor processing.

The Planetary Atmospheres Minor Species Sensor (PAMSS) is an intracavity laser absorption spectrometer that uses a mid-infrared quantum cascade laser in an open external cavity for sensing ultra-trace gases with parts-per-billion sensitivity. PAMSS was flown on a balloon by Near Space Corporation from Madras OR to 30 km on 17 July 2014. Based on lessons learned, it was modified and was flown a second time to 32 km by World View Enterprises from Pinal AirPark AZ on 8 March 2015. Successes included continuous operation and survival of software, electronics, optics, and optical alignment during extreme conditions and a rough landing. Operation of PAMSS in the relevant environment of near space has significantly elevated its Technical Readiness Level for trace-gas sensing with potential for planetary and atmospheric science in harsh environments.

High pixel temperatures for IR scene projector arrays face materials challenges of oxidation, diffusion, and recrystallization. For cost effective development of new high-temperature materials, we have designed and fabricated simplified pixels for testing. These consist of resistive elements, traces, and bond pads sandwiched between dielectric layers on Si wafers. Processing involves a pad exposure etch, a pixel outline etch, and an undercut etch to thermally isolate the resistive element from the substrate. Test pixels were successfully fabricated by electron-beam lithography using a combination of wet and dry etching.

Streaming Process for Electrode-less Electrochemical Deposition (SPEED) method is used to create complex thin-film structures, such as KBNNO, in a single step, in contrast to hydrothermal approaches with separate nanoparticle growth and deposition processes. This new ferroelearic oxide [KNbO3](1-x)[BaNi1/2Nb1/2O3-delta](x) or "KBNNO" has an alloy-tunable band gap as low as 1.1 eV, so that its absorption can be tailored to match the solar spectrum. At the same time, it has a reasonably large polarization allowing for charge separation across the bulk, sizeable photocurrents, and open-circuit voltages V-oc that exceed the band gap, potentially leading to efficiencies that exceed those possible for standard pn-junction cells. Physical characterization of KBNNO films demonstrate the microstructure and stoichiometry of SPEED-produced thin-films, ratio of elements needed to achieve an ideal band gap of similar to 1.39 eV, the effect on film chemistry, microstructure, and band gap of annealing, the practical separation of excited carriers at room temperature, the maximum achievable polarization and its temperature dependence, and the conditions for ideal poling. Photovoltaic characterization of KBNNO cells will determine the efficiency, the relative strengths of dark and photo currents, the open circuit voltage, the short circuit current, and cell fill factor (FF).

We have invented a novel photodetector by mating a surface plasmon resonance coupler with a graphene field effect transistor. The device enables wavelength selectivity for spectral sensing applications. Surface plasmon polaritons (SPPs) are generated in a 50 nm thick Ag film on the surface of a prism in the Kretschmann configuration positioned 500 nm from a graphene FET. Incident photons of a given wavelength excite SPPs at a specific incidence angle. These SPP fields excite a transient current whose amplitude follows the angular resonance spectrum of the SPP absorption feature. Though demonstrated first at visible wavelengths, the approach can be extended far into the infrared. We also demonstrate that the resonant current is strongly modulated by gate bias applied to the FET, providing a clear path towards large-scale spectral imagers with locally addressable pixels.

A mid-infrared intracavity laser absorption spectrometer for trace gas sensing is demonstrated. An external-cavity multi-mode quantum cascade laser with central wavelength 8.0 μm was combined with a scanning Fabry-Perot interferometer, which analyzed the change of the laser emission spectrum caused by introducing an analyte inside the cavity. The detection mechanism is based on monitoring the laser spectrum dynamics at adiabatically changing laser conditions in long pulse operation mode. Fast acquisition and vapor exchange allow nearly real-time analyte detection. Sensitivity at the level of 1 × 10−5 cm−1 was demonstrated based on a weak water vapor absorption line.

A polarized photoresponse to mm-wave radiation over the frequency range of 40 to 108 GHz is demonstrated in a grating-gated high electron mobility transistor (HEMT) formed by an InGaAs/InP heterostructure. The photoresponse is observed within the plasmon resonance absorption band of the HEMT, whose gate consists of a 9 μm period grating that couples incident radiation to plasmons in the 2D electron gas. Gate-bias changes the channel carrier concentration, causing a corresponding change in photoresponse in agreement with theoretical expectations for the shift in the plasmon resonance band. The noise equivalent power is estimated to be 235 pW/Hz1/2.

A mid-infrared intracavity laser absorption spectrometer based on an external cavity multi-mode quantum cascade laser is combined with a scanning Fabry-Perot interferometer is used as tunable narrow band transmission filter to analyze the laser emission spectrum. Sensitivity as a trace gas detector at 8.1 micron wavelengths has been demonstrated based on a weak water vapor line at an absorption coefficient of 1 x 10-5 cm-1. For molecules of reasonably strong absorption cross section (10-17 cm2), this corresponds to a detection limit of 40 ppb.

Tunable resonant absorption by plasmons in the two-dimensional electron gas (2DEG) of grating-gated InP- and Graphene-based HEMTs are investigated. Fourier-spectrometer-obtained transmission resonances are observed over a wide spectral band from mm wavelengths to THz frequencies. These results are found to be consistent with grating period and 2DEG sheet charge density dependent theoretical calculations. The temperature dependence of these transmission resonances as a function of temperature is also reported for both devices. Such devices have potential as a chip-scale frequency-agile THz imaging spectrometers for man-portable or space-based spectral-sensing applications.

Plasmon excitation in the two dimensional electron gas (2DEG) of grating-gated high electron mobility transistors (HEMTs) gives rise to terahertz absorption lines, which may be observed via transmission spectroscopy. Such absorption resonances may alter the channel conductance, giving a means for tunable terahertz detection. The transmission spectrum may be calculated analytically by making simplifying assumptions regarding the electron distribution. Such assumptions can limit the usefulness of such analytical theories for device optimization. Indeed, significant differences between experimentally observed resonances and theory have been noted and explained qualitatively as due to additional, unanticipated, sheets of charge in the device. Here, we explore finite element method (FEM) simulations, used to obtain realistic carrier profiles. Simulated plasmon spectra do not support previous explanations of red-shifting due to interactions with additional neighboring charge distributions. Simulations do show unexpected plasmon resonances associated with the unanticipated sheet charge, named virtual-gate, as well as the expected resonances associated with the 2DEG. Plasmonic modes determined from these investigations are able to account for the measured absorption lines which were previously thought to be red-shifted 2DEG plasmons. Additionally, the same simulation approach was applied to proposed graphene-based devices to investigate their plasmon resonance spectra.

Small metal particles are investigated as scattering centers to increase the effective optical thickness of thin-film solar cells. The particular type of particles used is known as "metal-black", well known as an IR absorber for bolometric infrared detectors. Gold-black was deposited on commercial thin-film solar cells using a thermal evaporator in nitrogen ambient at pressures of ~1 Torr. A broad range of length scales, as revealed by scanning electron microscope images gives rise to effective scattering over a range of wavelengths across the solar spectrum. The solar cell efficiency was determined both as a function of wavelength and for a solar spectrum produced by a Xe lamp and appropriate filters. Up to 20% increase in short-circuit photo-current, and a 5% increase in efficiency at the maximum power point, were observed.

Efficiency enhancement of thin film solar cells was obtained using deposited gold-black particles as scattering centers. Gold black is shown to give a broad particle size distribution from nm to several microns. This results in plasma resonance-enhanced scattering and intensified local electric field over a broad spectrum. Initial experiments on thin film solar cells with gold black showed an increase in shortcircuit photocurrent 20% in the 350 to 1000 nm spectral range and an increase in the efficiency of 7%. Photoemission electron microscopy (PEEM) reveals the spectral and spatial distribution of plasmon resonances for gold black using UV, visible, and near IR wavelength sources. Plasma resonances occur out to at least 800 nm wavelength, as evidenced by localized regions of high photoemission.

Tunable resonant absorption by plasmons in the two-dimensional electron gas (2DEG) of grating-gated HEMTs is known for a variety of semiconductor systems, giving promise of chip-scale frequency - agile THz imaging spectrometers. In this work, we present our approach to measurement of electrical response to millimeter waves from backward-wave oscillators (BWO) in the range 40-110 GHz for InP-based HEMTs. Frequency-modulation of the BWO with lock-in amplification of the source-drain current gives an output proportional to the change in absorption with frequency without contribution from non-resonant response. This is a first step in optimizing such devices for man-portable or space-based spectral-sensing applications.

Intracavity Laser Absorption Spectroscopy (ICLAS) at IR wavelengths offers an opportunity for spectral sensing with sufficient sensitivity to detect vapors of low vapor pressure compounds such as explosives. Reported here are key enabling technologies for this approach, including multi-mode external-cavity quantum cascade lasers and a scanning Fabry-Perot spectrometer to analyze the laser mode spectrum in the presence of a molecular intracavity absorber. Reported also is the design of a compact integrated data acquisition and control system. Applications include military and commercial sensing for threat compounds, chemical gases, biological aerosols, drugs, and banned or invasive plants or animals, bio-medical breath analysis, and terrestrial or planetary atmosphere science.

Mid-IR spectrometers with adequate resolution for chemical sensing and identification are typically large, heavy, and require sophisticated non-stationary optical components. Such spectrometers are limited to laboratory settings. We propose an alternative based on semiconductor micro-fabrication techniques. The device consists of several enabling parts: a compact broad-band IR source, photonic waveguides, a photon-to-surface-plasmon transformer, a surface-plasmon sample-interaction region, and an array of silicon ring-resonators and detectors to analyze the spectrum. Design considerations and lessons learned from initial experiments are presented.