We present a conceptual design for an in-situ methane gas sensor that could be deployed rapidly to suspected sites of spurious methane emission. Based on the remote detection of suspected methane leaks now possible with a class of satellites currently in orbit or soon to be launched, in-situ sensors would be deployed to the location of the detection, and accurate measurements of methane leak rates would be reported. Our design is very high level at this time, but incorporates a spectral capability that allows switching on and off the wavelengths of peak methane emission to facilitate detection. Unlike the case for the satellite-based sensing of the gas, our design will lead to the quantification of smallest levels of methane leaks. Two approaches will be considered: ground sensors detecting methane in emission against a cooler sky background, and aerial sensors detecting the gas in absorption against the ground scene as a background source. Sensitivity plays a key role, with the infrared detector working in the 2.4-micron region and operating with near-theoretical sensitivity, with the limiting noise sources set by the background levels for the ground sensor. Consideration of levels of detector dark current, based on the background signal level, and required detector operation temperatures will be derived. The paper reports on the conceptual design, with details on the electronics approach needed to realize the needed levels of sensitivity. Performance quantification will be accomplished through simulation using accurate noise models.

A diversified infrared technology base has been developed over the recent decades for various civilian and military sensing needs. The technology has been optimized to balance performance and affordability constraints for a variety of end-use goals. Simplistically, these goals might involve the detection and measurement of nearby, bright sources that fill even the largest angular fields-of- view of pixels in simple, low-magnification systems for which abundant signal makes possible infrared detection and measurement with less-sensitive, uncooled sensor arrays. At another extreme are ultra-cryogenically-cooled systems operating below thermoelectric cooler capabilities and which enable the detection and measurement of much fainter sources that underfill even the tiny angular pixel fields of view set by the diffraction limit of large, high magnification optical systems. Our emphasis is closer to the latter for the applications described here. As one example of the environmental monitoring capabilities made possible in the infrared, gas leak detection in transmission pipelines is vitally important for safe operation and for protecting the environment by accounting for and assessing the impact of leaks that adversely affect climate change. Gas leak detection in the infrared spectrum is facilitated by the distinctive spectral fingerprints of fundamental molecular vibrational modes which can be exploited for the detection of the gas. Sensitivity becomes paramount for many applications requiring faint signal detection, and large sensor array formats facilitate surveillance coverage. Many climate change assessments are expected to involve wide-area coverage of Earth scenes with revisit times sufficiently short to capture important transitory events. Shorter term monitoring of containment compliance requires detecting sufficiently small gas leak flows over broad expanses of the Earth's surface with high detection sensitivities. In this paper we described supporting technologies in the areas of sensor arrays and optical sub-systems, with an emphasis on dispersive spectrometers. There are a plethora of applications involving the stewardship of a range of biological assets, both in the ocean and on land environments, as well as large-scale sensing of atmospheric properties, including concentrations of greenhouse gases.

Large-scale molecular dynamics (MD) simulations, along with bond-order interatomic potentials, have been employed to study defect production, clustering and their evolution within high energy displacement cascades in semiconductors. Based on the MD results, the damage density within a cascade core is evaluated, and used to describe a new energy partition function. In addition, we have further developed a model to determine the non-ionizing energy loss (NIEL) for semiconductors, which can be used to predict the displacement damage degradation induced by space radiation on electronic components. The atomic-level based NIEL model has been applied to GaAs and GaN. At low energies, the most surviving defects are single interstitials and vacancies, and only 20% of the interstitial population is contained in clusters in GaAs, but a direct-impact amorphization in GaAs occurs with a high degree of probability during the cascade lifetime for Ga PKAs (primary knock-on atoms) with energies higher than 2 keV. However, a large number of atoms will be displaced during the collisional phase with a compacted cascade volume in GaN, and consequently, a great number of displaced atoms recombine signicantly with vacancies at the same time, i.e., a pseudo-metallic behavior (PMB). This leads to the result that the majority of surviving defects are just single interstitials or vacancies for all recoil energies considered with only a small number of defects forming clusters. The total number of defects simulated in GaN can be very well predicted by the simplied Norgett, Robison and Torrens (NRT) formula due to the PMB, in contrast to GaAs where the defect number becomes much larger than the NRT value. The calculated NIEL in GaN is often found smaller than that predicted by a model based on the simple Kinchin-Pease formula. The comparisons of defect creation, density and effective NIEL in GaN to those of GaAs suggest that GaN may be much more resistant to displacement damage than GaAs, and therefore, very suitable for use in high-power space-energy systems and space-probe applications.

We describe our approach for the modeling of space objects on the basis of 3-D designs supplemented with surface material assignments and their associated electro-optical properties. Challenges with this approach include incorporating Bi-directional Reflectance Distribution Function (BRDF) models that are both representative and robust over a range of wavelengths. In view of the small angles subtended by most space objects observed at a distance, any limitations of the applicability of BRDFs measured with much larger laboratory fields of view need to be assessed. In addition, any observational effects associated with partial coherence under conditions addressed by the van Cittert-Zernike theorem require investigation. More comparisons of predictions and observations are needed to guide these developments.

Large-scale molecular dynamics (MD) simulations, along with bond-order interatomic potentials, have been applied to study the defect production for lattice atom recoil energies from 500 eV to 20 keV in gallium arsenide (GaAs). At low energies, the most surviving defects are single interstitials and vacancies, and only 20% of the interstitial population is contained in clusters. However, a direct-impact amorphization in GaAs occurs with a high degree of probability during the cascade lifetime for Ga PKAs (primary knock-on atoms) with energies larger than 2 keV. The results reveal a non-linear defect production that increases with the PKA energy. The damage density within a cascade core is evaluated, and used to develop a model that describes a new energy partition function. Based on the MD results, we have developed a model to determine the non-ionizing energy loss (NIEL) in GaAs, which can be used to predict the displacement damage degradation induced by space radiation on electronic components. The calculated NIEL predictions are compared with the available data, thus validating the NIEL model developed in this study.

Classical molecular dynamics (MD), along with a bond-order potential for GaAs, has been used to study threshold displacement energies (Ed) of Ga and As recoils. Considering the crystallographic symmetry of GaAs, recoil events are confined in four unit stereographic triangles. To investigate the displacement energy’s dependence on crystallographic orientation, more than 3600 recoil events were simulated to uniformly sample values of Ed. Various defect configurations produced at these low energy recoils and the separation distances of Frenkel pairs were quantified and outlined. For both Ga and As, the minimum, $E_{\rm{d}}^{{\rm{min}}}$ , is found to be 8 eV, but the maxima, $E_{\rm{d}}^{{\rm{max}}}$ , are 22 and 28 eV for Ga and As, respectively. The distribution of Ed within unit stereographic triangles indicates that Ed shows a weak dependence on the recoil directions, in contrast to other semiconductors. The average threshold displacement energy is 13 ± 1 eV, which is in excellent agreement with available experiments.

A previous paper described LWIR pupil imaging, and an improved understanding of the behavior of this type of sensor for which the high-sensitivity focal plane array (FPA) operated at higher flux levels includes a reversal in signal integration polarity. We have since considered a candidate methodology for efficient, long-term calibration stability that exploits the following two properties of pupil imaging: (1) a fixed pupil position on the FPA, and (2) signal levels from the scene imposed on significant but fixed LWIR background levels. These two properties serve to keep each pixel operating over a limited dynamic range that corresponds to its location in the pupil and to the signal levels generated at this location by the lower and upper calibration flux levels. Exploiting this property for which each pixel of the Pupil Imager operates over its limited dynamic range, the signal polarity reversal between low and high flux pixels, which occurs for a circular region of pixels near the upper edges of the pupil illumination profile, can be rectified to unipolar integration with a two-level non-uniformity correction (NUC). Images corrected real-time with standard non-uniformity correction (NUC) techniques, are still subject to longer-term drifts in pixel offsets between recalibrations. Long-term calibration stability might then be achieved using either a scene-based non-uniformity correction approach, or with periodic repointing for off-source background estimation and subtraction. Either approach requires dithering of the field of view, by sub-pixel amounts for the first method, or by large off-source motions outside the 0.38 milliradian FOV for the latter method. We report on the results of investigations along both these lines.

A previous paper described LWIR Pupil Imaging with a sensitive, low-flux focal plane array, and behavior of this type of system for higher flux operations as understood at the time. We continue this investigation, and report on a more detailed characterization of the system over a broad range of pixel fluxes. This characterization is then shown to enable non-uniformity correction over the flux range, using a standard approach. Since many commercial tracking platforms include a “guider port” that accepts pulse width modulation (PWM) error signals, we have also investigated a variation on the use of this port to “dither” the tracking platform in synchronization with the continuous collection of infrared images. The resulting capability has a broad range of applications that extend from generating scene motion in the laboratory for quantifying performance of “realtime, scene-based non-uniformity correction” approaches, to effectuating subtraction of bright backgrounds by alternating viewing aspect between a point source and adjacent, source-free backgrounds.

We describe an integrated sensor assembly serving as both a component technology demonstration and a potential means of detecting distant point sources of infrared radiation. The objective of the demonstration was to show that usefully long integration times could be achieved with a low-background and well capacity, LWIR focal plane array optimized for use with cooled optics in space. The system controls extraneous background radiation with a small (150 μm) cooled pinhole that nevertheless transmits all the radiation of a point source collected by the fore-optic. Broad waveband response (~3 to 12 μm) results from optimization of the fore-optic for both MW and LWIR, as well as from a broadband anti-reflection coating on the field lens that is used at the pinhole to reimage the entrance aperture and its surrounding cold stop. Integration times in excess of 10 msec have been achieved for room temperature backgrounds with the FPA cold stage operated at 50 Kelvin, and noise performance has been bracketed with single frames of data collected over several integration times and over several minutes duration. However, anomalous signal behavior has been observed as the temperature of a remote blackbody increases. Although operation to date has been with a lower operability, engineering grade FPA, plans are to eventually upgrade to a higher quality device.

Various approaches now exist for obtaining spectral imagery over a broad range of infrared wavelengths. One involves use of a single grating element in two grating orders with dualband focal plane array (FPA) technology - an approach offering high efficiency over both the MWIR & LWIR, and obviating the need for separate focal plane arrays, dispersing elements, and optical beamsplitters. Another approach achieves similar results by exploiting an FPA having broad wavelength response with an innovative grating having useable efficiency extending beyond the single octave limits of traditional gratings. Significant advantages result in either case for space-based hyperspectral imagers, for which a reduction in cryo-cooled mass translates into prodigious savings in overall payload mass, cryo-cooling requirements, and waste heat removal. By contrast, longer term approaches might realize infrared "hyperspectral pixels" in 2-D imaging focal plane arrays. In this case, each pixel would detect different wavelengths of radiation at different depths, and the resulting "spectral photocurrents" would be transported to read-out circuitry through a vertical grid of electrical contacts. Although not yet realized in practice, the conceptual basis for accomplishing this with the widely-available HgCdTe detector material has been described. With regard to employment, space-based thermal hyperspectral imaging (HSI) is characterized by coarser ground resolution as a result of aperture diameter limitations and diffraction considerations at the longer infrared wavelengths. The resulting sub-pixel detections based on spectral signature are often complementary with higher resolution, shorter wavelength, panchromatic imagery. Overlapping fields-of-view between the two sensor types on the dayside of the earth enable simultaneous correlation of infrared spectral signatures with spatially-resolved scene features; data collects on the night-side are limited to the thermal hyperspectral images and would await correlations with high resolution visible imagery at the next daytime opportunity.

We report current-voltage data for back-illuminated mesa photodiode test structures fabricated by arsenic-diffusion into n-type LPE HgCdTe films. Arsenic diffusion was carried out in a sealed quartz ampoule containing a source of both Hg and As. The arsenic-diffused p-on-n photodiodes were characterized at 70 K and 80 K. The cutoff wavelength was about 11 μm at 80 K. The data for 400 μm diameter photodiodes fabricated by the arsenic diffusion process are very similar to those from a conventional two-layer LPE P-on-n process for material with approximately the same cutoff wavelength. We outline process and doping level changes that should improve detector performance.

Over the last several years the development of type-II Strained Layer Superlattice (SLS) infrared photodetectors has yielded devices that may offer plausible alternative technology to conventional mercury cadmium telluride (MCT)-based photodetectors. Prevailing theory predicts that SLS-based detector technologies will have several potential advantages over MCT technologies, including lower dark currents and higher operating temperatures. However, experimentally it has been found that conventional p-on-n and n-on-p SLS detectors have high dark current and thus, do not reach theoretically predicted performance benchmarks. The two prevailing contributors to this high dark current are the generation-recombination (GR) current and surface leakage currents, the latter resulting from the mesa sidewall exposure. A recently emerging technology that utilizes a uni-polar barrier design nBn has been shown to reduce dark current, while keeping the inherent advantages of SLS. Specific advantages of SLS over MCT include wavelength tunability, improved uniformity, and operability potentially at a reduced manufacturing cost. This report presents some recent experimental findings for the electrical and optical response of an nBn detector composed of an InAs/GaSb SLS absorber (n) and contacts (n) with an AlGaSb barrier (B). Results include the intrinsic determination of the diffusion current, and the GR current for the nBn device. Also presented is the optical response of the InAs/GaSb nBn detector at 77K over a broad range of operating biases. Dark current measurements over the 10K-300K temperature range were undertaken to extract the activation energies in the heterostructure.

This paper reports the first demonstration of the megapixel-simultaneously-readable and pixel-co-registered dual-band quantum well infrared photodetector (QWIP) focal plane array (FPA). The dual-band QWIP device was developed by stacking two multi-quantum-well stacks tuned to absorb two different infrared wavelengths. The full width at half maximum (FWHM) of the mid-wave infrared (MWIR) band extends from 4.4 to 5.1μm and the FWHM of a long-wave infrared (LWIR) band extends from 7.8 to 8.8μm. Dual-band QWIP detector arrays were hybridized with custom fabricated direct injection read out integrated circuits (ROICs) using the indium bump hybridization technique. The initial dual-band megapixel QWIP FPAs were cooled to 70K operating temperature. The preliminary data taken from the first megapixel QWIP FPA has shown system NEΔT of 27 and 40mK for MWIR and LWIR bands, respectively.

An overview of the properties of the absorption coefficient of mercury cadmium telluride that may make this material useful for intrinsic hyperspectral detection is presented. A review of recent work on modeling the absorption coefficient is provided, and new directions for achieving an analytical representation with higher fidelity are suggested.