Pyroelectric lithium tantalate (LiTaO3 or LT) wafers were integrated with subwavelength resonant absorbers to create spectrally selective room-temperature detectors at submillimeter wavelengths and terahertz (THz) frequencies. Photoresponse resonances were measured using a tunable backward wave oscillator (BWO) in the range of 0.3-1.0 THz. The resulting sensor characteristics agree well with design predictions from electrodynamic simulations, which reveal the dependence of resonant absorption on design parameters. Maximum responsivity and detectivity without electrical amplification were similar to 340 V/W and 6x10(6) Jones [cm center dot root Hz) / W], respectively. The selectivity and responsivity can be understood in terms of inductor-capacitor (LC) resonances that heat the pyroelectric near the surface, followed by rapid thermal equilibration through the bulk. The response voltage that appears between surfaces is in reasonable agreement with the estimated temperature rise and the known LT pyroelectric coefficient. An array of such detectors may function as a compact THz spectrometer for standoff detection of chemical compounds including (e.g.,) opioids.

Dust plumes lofted by a lunar lander threaten operations and assets. To inform prediction and protection, we are developing a lander-mounted laser-based instrument to obtain empirical particle-size distributions in ejecta plumes. The particle size distribution is determined from laser propagation decay at multiple wavelengths. We present laboratory experiments on static particle suspensions of independently known particle-size distributions in liquid, and of dropped particle showers in an evacuable chamber, to confirm theoretical expectations for laser propagation decay constants. We predict the laser-propagation decay-constant spectrum for plumes of lunar simulant generated by tethered rocket tests. We describe progress on a prototype system for tethered rocket tests and some of the calibration issues.

Surface dust blown by a lunar lander is a threat to operations and assets. Multi-national lunar exploitation makes this a potential defense problem. To aid prediction and protection, we are developing a lander-mounted laser-based instrument to obtain empirical particle-size distributions in ejecta plumes. The method is based on analysis of laser propagation decay at multiple wavelengths. System design depends on expected laser propagation decay lengths in the cloud of lunar particles lofted by the lander rockets. We present laboratory experiments to confirm theoretical expectations for laser propagation decay constants for independently known particle size distributions. The method of extracting particle size distributions from measured decay constants at multiple wavelengths is demonstrated. Predictions are made for decay constants in lunar plumes with representative regolith size distributions and minerology.

Highly sensitive, broadly tunable detectors are needed for future sensing applications and quantum information systems. A promising material for these challenges comprises stacked graphene sheets having a “magic” twist angle between their in-plane symmetry axes. This material displays superconductivity with a 2 K transition temperature. We investigate a proposed design for a fast and sensitive detector of THz and mm-waves based on antenna-coupled magic-angle-twist-graphene Josephson junctions. The considered non-bolometric detection mechanism depends on the decrease in the maximum zero-voltage DC current when AC current is driven through the junction. Finite element electrodynamic simulations favor the bowtie over log-periodic, square spiral, and Archimedean spiral antenna designs. Responsivity, noise-equivalent-power, and the prospects for single-photon detection are estimated. Graphene sheets stacked with “magic” twist angle display superconductivity. Josephson junctions can be created by selective gating. We investigate a non-bolometric detection mechanism that promises simultaneous high speed and sensitivity, with potential application to quantum cryptography in the 5G band. A noise-equivalent-photon flux of 1 photon every 6 ns is the projected sensitivity for THz and mm-waves.

Metal-insulator-metal (MIM) resonant absorbers comprise a conducting ground plane, a thin dielectric, and thin separated metal top-surface structures. The dielectric SiO 2 strongly absorbs near 9 μm wavelength and has correspondingly strong long-wave-infrared (LWIR) dispersion for the refractive index. This dispersion results in multiple absorption resonances spanning the LWIR, which can enhance broad-band sensitivity for LWIR bolometers. Similar considerations apply to silicon nitride Si 3 N 4 . TiO 2 and AlN have comparatively low dispersion and give simple single LWIR resonances. These dispersion-dependent features for infrared MIM devices are demonstrated by experiment, electrodynamic simulation, and an analytic model based on standing waves.

The minority carrier diffusion length was directly measured by the variable-temperature Electron Beam-Induced Current technique in InAs/GaSb type-II strain-layer-superlattice infrared-detector structures. The Molecular Beam Epitaxy-grown midwave infrared superlattices comprised 10 monolayers of InAs and 10 monolayers of GaSb to give a total absorber thickness of 4 μm. The diffusion length of minority electrons in the p-type absorber region of the p-type/barrier/n-type structure was found to increase from 1.08 to 2.24 μm with a thermal activation energy of 13.1 meV for temperatures ranging from 77 to 273 K. These lengths significantly exceed the individual 10-monolayer thicknesses of the InAs and GaSb, possibly indicating a low impact of interface scattering on the minority carrier diffusion length. The corresponding minority electron mobility varied from 48 to 65 cm2/V s. An absorbed gamma irradiation dose of 500 Gy halved the minority carrier diffusion length and increased the thermal activation energy to 18.6 meV, due to creation of radiation-induced defect recombination centers.

Ternary lead chalcogenides, such as PbS_xSe_1-x, offer the possibility of room-temperature infrared detection with engineered cut-off wavelengths within the important 3-5 micron mid-wave infrared (MWIR) wavelength range. We present growth and characterization of aqueous spray-deposited thin films of PbSSe. Complexing agents in the aqueous medium suppress unwanted homogeneous reactions so that growth occurs only by the heterogeneous reaction on the hydrophilic substrate. The strongly-adherent films are smooth with a mirror-like finish. The films comprise densely packed grains with tens of nm dimensions and a total film thickness of ∼400-500 nm. Measured optical constants reveal absorption out to at least 4.5 μm wavelength and a ∼0.3 eV bandgap intermediate between those of PbS and PbSe. The semiconducting films are p-type with resistivity ∼1 and 85 Ohm-cm at 300 and 80 K, respectively. Sharp x-ray diffraction peaks identify the films as Clausthalite-Galena solid-state solution with a lattice constant that indicates an even mixture of PbS and PbSe. The photoconductive response is observed at both nitrogen and room temperature up to at least 2 kHz chopping frequency.

Addition of wavelength selective absorbers on microbolometers tends to increase their thermal mass and slow their infrared response times. Making the bolometric material an integral part of the absorber and minimizing layer thicknesses is one possible way to maintain high detector speeds. Here, we study experimentally the effect on permittivity of adding a layer of semiconducting VO_x between two layers of SiO_2. Additionally, we investigate theoretically the effect on resonance wavelength of thinning the metal in metal-insulator-metal plasmonic resonant absorbers.

The Planetary Atmospheres Minor Species Sensor (PAMSS) is an ultra-trace gas sensor. This paper reports its transition from a Technical Readiness Level of 4 (TRL4) to TRL 5 and an established path forward to TRL6. This report describes tests of PAMSS in chambers that simulate a balloon flight to 30 km. Lessons learned inform a number of improvements, which are being implemented for a balloon flight planned for June 2014.

A quantum cascade laser at IR wavelengths with an open external cavity presents an opportunity for spectral sensing of molecular compounds that have low vapor pressure. The sensitivity of such a system is potentially very high due to extraordinarily long effective optical paths that can be achieved in an active cavity. We demonstrate here an external cavity mid-IR QCL molecular absorption sensor using a fixed Fabry-Perot etalon as the spectrum analyzer. The system is sensitive to the water vapor present in the laboratory air with an absorption coefficient of just 9.6 x 10-8 cm-1. The system is sensitive enough to detect the absorption coefficient of TNT vapor at room temperature.

Infrared absorbance is investigated in gold-black, a porous nano-structured conducting film. A two level full factorial optimization study with evaporation-chamber pressure, boat current, substrate temperature, and degree of polymer infusion (for hardening) was performed. Polymer infusion was found generally to reduce absorbance in the long wave IR but has little effect at THz wavelengths, although for samples with the highest absorbance there is a slight improvement in the absorbance figure of merit (FOM) in both wavelength regimes. The characteristic length scales of the structured films vary considerably as a function of deposition parameters, but the IR FOM is found to be weakly correlated with these distributions, which are determined by wavelet analysis of scanning electron micrographs.

A scanning Fabry-Perot transmission filter composed of a pair of dielectric mirrors has been demonstrated at millimeter and sub-millimeter wavelengths. The mirrors are formed by alternating quarter-wave optical thicknesses of silicon and air in the usual Bragg configuration. Detailed theoretical considerations are presented for determining the optimum design. Characterization was performed at sub-mm. wavelengths using a gas laser together with a Golay cell detector and at mm-wavelengths using a backward wave oscillator and microwave power meter. High resistivity in the silicon layers was found important for achieving high transmittance and finesse, especially at the longer wavelengths. A finesse value of 411 for a scanning Fabry-Perot cavity composed of three-period Bragg mirrors was experimentally demonstrated. Finesse values of several thousand are considered to be within reach. This suggests the possibility of a compact terahertz Fabry-Perot spectrometer that can operate in low resonance order to realize high free spectral range while simultaneously achieving a high spectral resolution. Such a device is directly suitable for airborne/satellite and man-portable sensing instrumentation.

The temporal dynamics, spectrum, and gain of the far–infrared p-Ge laser for composite cavities consisting of an active crystal and passive transparent elements have been studied with high temporal and spectral resolution. Results are relevant to improving the performance of mode-locked or tunable p-Ge lasers using intracavity modulators or wavelength selectors, respectively. It is shown that an interface between the active p-Ge crystal and a passive intracavity spacer causes partial frequency selection of the laser modes, characterized by a modulation of their relative intensities. Nevertheless, the longitudinal mode frequencies are determined by the entire optical length of the cavity and not by resonance frequencies of intracavity sub-components. Operation of the p-Ge laser with multiple interfaces between Ge, Si, and semi-insulating GaAs elements, or a gap, is demonstrated as a first step toward a p-Ge laser with an external quasioptical cavity and distributed active media.