Thermoelectric terahertz detectors comprising silicon and platinum lines that join at an antenna feed were designed, fabricated, and tested. These room-temperature detectors generate their own electrical signal in response to heating at the feed without external biasing. Hence, these detectors can be cheap, compact, and energy efficient. The full length of the bowtie antenna was chosen to be 6.3 mu m. Numerical simulations revealed a resonant response near 5 THz (60-mu m wavelength). Characterization using a blackbody source gave 207-kV/W responsivity, 0.24-pW/root Hz noise equivalent power (NEP), and 3.3 x 10(9) cmroot Hz/W (Jones) detectivity (D & lowast;) in the 3.8-8-THz band. These figures of merit are comparable with the best values for THz uncooled thermal detectors.

Our antenna-coupled thermoelectric THz detectors use sputtered Sb2Te3 films as one of the key materials. After sputtering at room temperature, films must be activated by annealing, which introduces Sb-O infrared vibrational bands for temperatures exceeding 200 °C. Far-infrared reflectance spectra and X-ray diffraction data suggest that the oxidation occurs mainly at the interface between film and glass substrate and does not affect the bulk of the film. Detector characterization data is presented for antenna-coupled Sb2Te3-Bi2Te3 thermoelectric junction detectors.

We investigate the contact resistance of twisted bilayer graphene devices encapsulated by hexagonal boron nitride (hBN). Encapsulation ensures an atomically flat interface that supports emerging new states in twistronics, but it also necessitates an etch step to expose the graphene for electrical contacts. The dry-etch process by sulfur hexafluoride (SF6) etches hBN but stops on graphene. In our case, graphene only serves as an imperfect etch mask, resulting in a porous structure in graphene after the SF6 etch, possibly due to inadvertent residual oxygen during the etch process. We investigate electrical contacts to graphene surfaces exposed by this process. Etched pores expose edges which participate in the contact, which combined with fluorination of the graphene surface, provide variables for optimizing contact resistance.

Sputter depositions of Sb2Te3 and Bi2Te3 thin films are optimized for applications that require photolithographic patterning, such as antenna-coupled thermoelectric THz/mm-wave detection and radiant heat energy harvesting. Sputtered films of both materials require thermal activation treatment at temperatures exceeding 175 °C, which must be done after patterning. We report the results of a new optimization study with sputtering on unheated substrates followed by annealing. Our two-level full-factorial approach assumes three factors, namely RF power during sputtering, deposition time, and subsequent annealing temperature. Infrared reflectance spectroscopy, profilometry, and resistivity measurements show that annealing thickens the films, increases their refractive indices, and decreases resistivity by increasing the carrier relaxation time. Seebeck coefficients and lift-off patterning are significantly improved over previous results with films deposited on heated substrates. Examples of fabricated mm-wave detector devices, their simulation-guided design, and initial characterization results are presented.

We are developing an instrument to determine particle size distributions in dust clouds with a method based on analysis of light propagation decay in images of scattered laser beams. Laboratory experiments, calculations, and tethered- rocket flight testing are used to validate the method and hardware. An application is to investigate lunarlander plume-surface interactions that create broad particle-size distributions with complex dynamics.

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.

We have measured desorption of atomic hydrogen from graphene devices on hexagonal boron nitride. Compared to graphene devices on silicon oxide, chemisorbed hydrogen desorbs over much narrower range of temperatures, indicating that hydrogen interacts more uniformly with graphene on hexagonal boron nitride. Our results indicate that hexagonal boron nitride will make an ideal substrate for achieving the intrinsic adsorbate-graphene interaction and developing graphene-based sensors, hydrogen storage devices and other applications that can be affected by graphene-adsorbate interactions.

In this work, we present a numerical and experimental study of the photothermal heating of microholes array (MHA) bowtie antennas on gold metal films. The article first shows how MHAs in gold metal films can absorb more heat depending on the geometry or placement of the MHA in each gold metal films. We show numerically and experimentally that the photothermal heating exhibits a maximum when the MHAs are patterned in a bowtie-like geometry. Numerical studies were carried out by using COMSOL multiphysics and experimental results were obtained using thermal imaging microscopy for several different MHAs geometries, respectively. Our results showed a good agreement between experimental data and the numerical simulations, validating that MHAs bowtie antennas on gold metal films heat significantly more and make them ideal for use in photothermal applications.

We propose an annotation technique that may enhance characterization of hexagonal boron nitride flake thickness on 300-nm silicon oxide substrate via machine learning-enhanced machine vision of optical microscopy data. Manually interpreting optical microscopy data is labor intensive, time-consuming, and faulted by human subjectivity due to periodic behavior in light reflectance as thickness increases, resulting in colors that are visually difficult to disambiguate. To address this issue, we propose a labeling technique incorporating thickness-related shadowing around flake borders to differentiate thicknesses when colors are similar. By focusing on annotation labeling, we aim to solve the limitations of conventional human characterization and a tool for machine vision model training. Our technique promises a precise means of flake thickness identification by optical microscopy alone, bypassing labor-intensive scanning probe characterization. Our approach promises accelerated characterization of hBN thickness when implemented with machine vision, with potential applications for enhancing efficiency in 2D materials research and development. Depiction of annotation labeling technique incorporating thickness-related shadowing around flake borders to differentiate thicknesses when colors are indistinguishable for two flakes (110-nm (left) and 225-nm (right)) in a trilayer stack of hBN/SiO2/Si on 300-nm oxide.

We are developing antenna-coupled thermoelectric junctions for infrared detection and energy harvesting. Joule heating at the antenna feed from currents induced by antenna-collected radiation generates thermoelectric voltage and current for detection and energy conversion. COMSOL Multiphysics simulations identify Sb2Te3 and Bi2Te3 thermoelectric materials as ideal for the thermocouple junction at the antenna feed. We optimized RF sputtered telluride films by performing a 2-level full-factorial experiment with three factors–argon pressure, substrate temperature, and RF power—with power factor as the response. Preliminary photolithographically patterned devices designed for mm-waves using less suitable materials gave immeasurably low response to radiation from a backward wave oscillator, which is explained by the simulations that motivate this study. We are developing antenna-coupled thermoelectric junctions for infrared detection and energy harvesting. Joule heating at the antenna feed from currents induced by antenna-collected radiation generates thermoelectric voltage and current for detection and energy conversion. COMSOL Multiphysics simulations identify Sb2Te3 and Bi2Te3 thermoelectric materials as ideal for the thermocouple junction at the antenna feed. We optimized RF sputtered telluride films by performing a 2-level full-factorial experiment with three factors—argon pressure, substrate temperature, and RF power—with power factor as the response. Preliminary photolithographically patterned devices designed for mm-waves using less suitable materials gave immeasurably low response to radiation from a backward wave oscillator, which is explained by the simulations that motivate this study.

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.

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.

Carbon nanotube forests (CNTFs) were grown on a patterned substrate to form square pixelated arrays. Two-level full factorial optimization first determined the best conditions for synthesis by chemical vapor deposition catalyzed by iron (Fe) nanoparticles deposited on oxidized silicon substrates. Varied parameters included growth temperature, growth time, and acetylene-to-hydrogen gas flow rate ratio. Argon was used as a carrier gas. Unpatterned CNTF heights were grown with values from 15.3 to 185.7 microns. Reactive ion etching of the substrate in oxygen plasma dramatically improved forest growth rates. Uniform square 7 × 7 pixel arrays were produced by contact photolithography and lift-off of the deposited Fe. Each pixel was subdivided into square islands separated by gaps with different island and gap dimensions, which ranged from 4 to 50 microns and 1 to 10 microns, respectively. The results demonstrate the fabrication of thermally and electrically isolated vertically aligned CNTF islands, which have applications to batteries, sensors, infrared absorbers, and infrared or electron emitters.

Terahertz imaging systems require small, low cost and low power systems operating at room-temperature. Terahertz Seebeck nanoantennas are room temperature detectors which generate voltage due to incident electromagnetic radiation, they also provide polarization sensitivity, directivity, small footprint, tunability and the possibility of integration into electronic and photonic circuits. In this work a gold bowtie nanoantenna is designed and optimized to detect electromagnetic radiation at 2 THz. The resulting device is a gold bowtie antenna with asymmetric connection lines to optimize the Seebeck voltage. The connection lines are made of Sb2Te3 and Bi2Te3 to increase the generated voltage due to the incident electromagnetic radiation. Simulation results obtained using COMSOL Multiphysics are presented. The fabrication of the resulting optimized device was performed using photolithography and liftoff. The materials were deposited by sputtering. The fabricated device includes an external heater to measure the effective Seebeck coefficient. Experimental results of the effective Seebeck coefficient of the device as well as response measurements are presented and compared to Multiphysics simulations.

We describe a controllable infrared plasmonic laser attenuator with application to infrared detector characterization. The device promises a broad range of effective radiant temperature and fine temperature control/resolution near ambient. The enabling mechanism is controllably-frustrated surface-plasmon-resonance using a Kretschmann prism coupler. The predicted wide dynamic range depends on optical and geometric tolerances for the coupler. To investigate these, a mid-wave-infrared coupler comprising a conducting Ga-doped ZnO film deposited directly on a right-angle sapphire prism was fabricated, tested, and compared with theory. The attenuation resonance was observed by measuring specular reflection of a p-polarized quantum cascade laser beam at 4.45 mu m wavelength as a function of internal incidence angle from the coated prism face. The predicted resonance for comparison was based on ellipsometrically-obtained optical constants and film thickness. Near perfect match between theory and experiment was achieved after adjusting for experimental uncertainties in optical parameters. The results quantify the accuracy and precision with which optical constants and geometrical parameters must be known to achieve the predicted performance.

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.