NASA’s New Observing Strategies (NOS) thrust provides a framework for identifying technology advances needed to exploit newly available observational capabilities, including high-quality instruments on constellations of SmallSats and CubeSats, that enable measurement of phenomena that could not be studied using previously available techniques. Satellite developers and operators require software tools to simulate new technologies and validate new mission concepts that can incorporate a dynamic set of observing assets with various instruments located at different vantage points. These new mission concepts include many more design variables than traditional missions, requiring tools to facilitate trade analysis and concept validation in an iterative fashion, similar to an Observing System Simulation Experiment (OSSE) framework. Several recent projects address design and operational trades by designing software packages such as the Trade-space Analysis Tools for Constellations (TAT-C) co-developed by Stevens Institute of Technology, the Simulation Toolset for Adaptive Remote Sensing (STARS) developed by The Ohio State University, and the Virtual Constellation Engine (VCE) developed by the University of Southern California. Each tool has different but complementary capabilities and can be run independently. However, linking capabilities using modern web-based service application programming interfaces (APIs) contributes to a powerful modeling ecosystem for the Earth Science community with well-defined interfaces that facilitate interoperability with existing mission planning tools. This presentation will describe the capabilities of each individual software tool as well as recent efforts to integrate their capabilities to evaluate and mature constellation mission concepts as part of the NOS thrust.
Due to the demand for increasingly large format focal plane arrays, smaller and smaller pixels are required for high resolution imaging. A promising technique for backside illuminated devices is self-aligned etching of the mesas, or using the metal contact pad as the etch mask. In this work, we report on the self-aligned etching of two Type-II superlattice materials and some of their constituent material components to create pixels with subwavelength dimensions in a longwave infrared detector. Palladium was used as the primary mask material to prevent the exposure of the gold contacts to the etch plasma. The inductively coupled plasma conditions were varied, including varying the etch gas composition through different ratios of BCl3 and Cl2, and the etch rate and sidewall angle were measured. Using a mixture of BCl3 and Cl2 produced higher etch rates at room temperature than previously reported results at high temperatures with similar sidewall angles, thus reducing undesired diffusion of the device stack layers.
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.
KEYWORDS: Sensors, Antennas, Long wavelength infrared, Absorption, Dielectrics, Resonators, Infrared sensors, Chemical elements, Signal to noise ratio, Etching
Antenna coupled detectors break the intrinsic tradeoff between signal and noise by “collecting over a large area” and “detecting over a small area”. Most antenna coupled detectors in the infrared rely on a metal resonator structure. However, there are losses associated with metallic structures. We have demonstrated a novel long-wave infrared (LWIR) detector that combines a dielectric resonator antenna with an antimonide-based absorber. The detector consists of a 3D, subwavelength InAsSb absorber embedded in a resonant, cylindrical dielectric resonator antenna made of amorphous silicon. This architecture enables the antimonide detection element to shrink to deep subwavelength dimensions, thereby reducing its thermal noise. It is important to note that this concept only applies when (a) the detector noise is limited by bulk noise mechanisms with negligible surface leakage currents and (b) the dominant source of current in the device is due to dark current (such as diffusion) that scales with the volume of the detector. The dielectric resonator enhances the collection of photons with its resonant structure that couples incident radiation to the detector. We will present results on the absorption in structures with and without the dielectric resonator antenna. The signal to noise enhancement in the LWIR photodiodes integrated with the dielectric resonator antenna using radiometric characterization will be discussed.
Terahertz (THz) spectral signatures have been measured for a variety of explosive materials and precursors. These
signatures were measured by THz Time Domain Spectroscopy, using ultrashort pulsed lasers coupled with electro-optic
materials to generate and detect THz radiation. Transmission and reflection spectra were measured across a frequency
range from 0.2 to 2.5 THz for solid and liquid materials. These spectra are reported in terms of index of refraction and
absorption coefficient, both of which can be calculated from transmission or reflection data. The value of THz spectral
signatures for the development of future explosives sensing systems is discussed.
We fabricated and tested a low temperature cell which is mounted directly on the second stage of a CTI-Cryogenics Model 22C CRYODYNE CRYOCOOLER. The vacuum system consists of a room temperature vacuum shroud, a radiation shield maintained at 77K and the cell which is mounted directly to the second stage of the cryocooler. The ultimate cell temperature is 12.4 Kelvin, and the low temperature limit increases at a rate of 5.6 Kelvin/Watt. We achieve a cell temperature of 22 Kelvin under typical experimental conditions of approximately 29 milli Torr helium, slow flowing gas, and a heated injector. The absorption path length of the cell is 3.35 cm, and the window clear aperture is 1.27 cm. We preformed a series of experiments in which we determined the translational temperatures of vibration- rotation transitions in the band of CO for different cell temperatures. The results of our tests are discussed in this paper.
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