Joel M. Hensley

High-efficiency thermophotovoltaic energy conversion enabled by a metamaterial selective emitter

Thermophotovoltaics (TPV) is the process by which photons radiated from a thermal emitter are converted into electrical power via a photovoltaic cell. Selective thermal emitters that can survive at temperatures at or above ∼1000°C have the potential to greatly improve the efficiency of TPV energy conversion by restricting the emission of photons with energies below the photovoltaic (PV) cell bandgap energy. In this work, we demonstrated TPV energy conversion using a high-temperature selective emitter, dielectric filter, and 0.6xa0eV In0.68Ga0.32As photovoltaic cell. We fabricated a passivated platinum and alumina frequency-selective surface by conventional stepper lithography. To our knowledge, this is the first demonstration of TPV energy conversion using a metamaterial emitter. The emitter was heated to >1000°C, and converted electrical power was measured. After accounting for geometry, we demonstrated a thermal-to-electrical power conversion efficiency of 24.1±0.9% at 1055°C. We separately modeled our system consisting of a selective emitter, dielectric filter, and PV cell and found agreement with our measured efficiency and power to within 1%. Our results indicate that high-efficiency TPV generators are possible and are candidates for remote power generation, combined heat and power, and heat-scavenging applications.

CO2 laser-based dispersion interferometer utilizing orientation-patterned gallium arsenide for plasma density measurements

A dispersion interferometer based on the second-harmonic generation of a carbon dioxide laser in orientation-patterned gallium arsenide has been developed for measuring electron density in plasmas. The interferometer includes two nonlinear optical crystals placed on opposite sides of the plasma. This instrument has been used to measure electron line densities in a pulsed radio-frequency generated argon plasma. A simple phase-extraction technique based on combining measurements from two successive pulses of the plasma has been used. The noise-equivalent line density was measured to be 1.7 × 10(17) m(-2) in a detection bandwidth of 950 kHz. One of the orientation-patterned crystals produced 13 mW of peak power at the second-harmonic wavelength from a carbon dioxide laser with 13 W of peak power. Two crystals arranged sequentially produced 58 mW of peak power at the second-harmonic wavelength from a carbon dioxide laser with 37 W of peak power.

An External Cavity 4.7 Terahertz Quantum Cascade Laser

An anti-reflection coated 4.7 terahertz quantum cascade laser coupled to an external cavity formed by a single moving mirror frequency tunes up to 4 wavenumbers with mode hops and around 0.4 wavenumbers without mode hops.

Standoff detection from diffusely scattering surfaces using dual quantum cascade laser comb spectroscopy

Using dual optical frequency comb (OFC) spectroscopy in the longwave infrared (LWIR), we demonstrate standoff detection of trace amounts of target compounds on diffusely scattering surfaces. The OFC is based on quantum cascade lasers (QCL) that emit ~1 Watt of optical power under cw operation at room temperature over coherent comb bandwidths approaching 100 cm-1. We overlap two nearly identical 1250 cm-1 QCL OFC sources so that the two interfering optical combs create via heterodyne a single comb in the radio frequency (rf) that represents the entire optical spectrum in a single acquisition. In a laboratory scale demonstration we show detection of two spectrally distinct fluorinated silicone oils, poly(methyl-3,3,3-trifluoropropylsiloxane) and Krytox™, that act as LWIR simulants for security relevant compounds whose room temperature vapor pressure is too low to be detected in the gas phase. These target compounds are applied at mass loadings of 0.3 to 90 μg/cm2 to sanded aluminum surfaces. Only the diffusely scattered light is collected by a primary collection optic and focused onto a high speed (0.5 GHz bandwidth) thermoelectrically cooled mercury cadmium telluride (MCT) detector. At standoff distances of both 0.3 and 1 meter, we demonstrate 3 μg/cm2 and 1 μg/cm2 detection limits against poly(methyl-3,3,3-trifluoropropylsiloxane) and Krytox™, respectively.

Plasma density measurement using a dispersion interferometer based on second-harmonic generation in orientation-patterned GaAs

A dispersion interferometer based on second-harmonic generation in orientation-patterned GaAs has been used to measure the line density of electrons in a radio-frequency plasma with a detection limit of 7 × 10¹⁶ m^-2.

Multi-cavity far infrared 2D plasmonic phenomena with inhomogeneous screening

We have observed coherent interaction of coupled far infrared 2D plasma cavities in a GaAs/AlGaAs FET. These results have been modeled with a transmission line formalism that treats both unscreened and screened 2D plasmons.