H. Saxena

Temperature dependence of plasmonic terahertz absorption in grating-gate gallium-nitride transistor structures

Strong plasmon resonances have been observed in the terahertz transmission spectra (1–5 THz) of large-area slit-grating-gate AlGaN/GaN-based high-electron-mobility transistor (HEMT) structures at temperatures from 10 to 170 K. The resonance frequencies correspond to the excitation of plasmons with wave vectors equal to the reciprocal lattice vectors of the metal grating, which serves both as a gate electrode for the HEMT and a coupler between plasmons and incident terahertz radiation. Wide tunability of the resonances by the applied gate voltage demonstrates potential of these devices for terahertz applications.

Tunable two-dimensional plasmon resonances in an InGaAs/InP high electron mobility transistor

Voltage-tunable plasmon resonances in the two-dimensional electron gas (2DEG) of a high electron mobility transistor (HEMT) fabricated from the InGaAs/InP materials system are reported. The device was fabricated from a commercial HEMT wafer by depositing source and drain contacts using standard photolithography and a semitransparent gate contact that consisted of a 0.5 μm period transmission grating formed by electron-beam lithography. Narrow-band resonant absorption of terahertz radiation was observed in transmission in the frequency range of 10–50 cm−1. The resonance frequency depends on the gate-tuned sheet charge density of the 2DEG. The observed separation of resonance fundamental from its harmonics and their shift with gate bias are compared with theory.

Grating-gate tunable plasmon absorption in InP and GaN based HEMTs

Gate-voltage tunable plasmon resonances in the two dimensional electron gas of high electron mobility transistors (HEMT) fabricated from the InGaAs/InP and AlGaN/GaN materials systems are reported. Gates were in the form of a grating to couple normally incident THz radiation into 2D plasmons. Narrow-band resonant absorption of THz radiation was observed in transmission for both systems in the frequency range 10 - 100 cm-1. The fundamental and harmonic resonances shift toward lower frequencies with negative gate bias. Calculated spectra based on the theory developed for MOSFETs by Schaich, Zheng, and McDonald (1990) agree well with the GaN results, but significant differences for the InGaAs/InP device suggest that modification of the theory may be required for HEMTs in some circumstances.

Intracavity laser absorption spectroscopy using mid-IR quantum cascade laser

Intracavity Laser Absorption Spectroscopy (ICLAS) at IR wavelengths offers an opportunity for spectral sensing with sufficient sensitivity to detect vapors of low vapor pressure compounds such as explosives. Reported here are key enabling technologies for this approach, including multi-mode external-cavity quantum cascade lasers and a scanning Fabry-Perot spectrometer to analyze the laser mode spectrum in the presence of a molecular intracavity absorber. Reported also is the design of a compact integrated data acquisition and control system. Applications include military and commercial sensing for threat compounds, chemical gases, biological aerosols, drugs, and banned or invasive plants or animals, bio-medical breath analysis, and terrestrial or planetary atmosphere science.

Plasmon resonance response to millimeter-waves of grating-gated InGaAs/InP HEMT

Tunable resonant absorption by plasmons in the two-dimensional electron gas (2DEG) of grating-gated HEMTs is known for a variety of semiconductor systems, giving promise of chip-scale frequency- agile THz imaging spectrometers. In this work, we present our approach to measurement of electrical response to millimeter waves from backward-wave oscillators (BWO) in the range 40-110 GHz for InP-based HEMTs. Frequency-modulation of the BWO with lock-in amplification of the source-drain current gives an output proportional to the change in absorption with frequency without contribution from non-resonant response. This is a first step in optimizing such devices for man-portable or space-based spectral-sensing applications.

Infrared Intracavity Laser Absorption Spectrometer

A spectral sensing method with sufficient sensitivity to detect vapors of low vapor-pressure compounds such as explosives would have great promise for defense and security applications. An opportunity is Intracavity Laser Absorption Spectroscopy (ICLAS) at IR wavelengths. Our approach is based on multi-mode external-cavity quantum cascade lasers and a scanning Fabry-Perot spectrometer to analyze the laser mode spectrum in the presence of a narrow band intracavity absorber. This paper presents results of numerical solution of laser rate equations that support feasibility of kilometer effective active-cavity path lengths and sensitivity to concentrations of 10 ppb. This is comparable to the saturated vapor pressure of TNT. System design considerations and first experimental results are presented at 10 and 70 μm wavelengths.

Investigation of plasmonic resonances in the two-dimensional electron gas of an InGaAs/InP high electron mobility transistor

The observation of THz regime transmission resonances in an InGaAs/InP high electron mobility transistor (HEMT) can be attributed to excitation of plasmons in its two-dimensional electron gas (2DEG). Properties of grating-based, gate-voltage tunable resonances are shown to be adequately modeled using commercial finite element method (FEM) software when the HEMT layer structure, gate geometry and sheet charge concentration are taken into account. The FEM results are shown to produce results consistent with standard analytical theories in the 10-100 cm-1 wavenumber range. An original analytic formula presented here describes how the plasmonic resonance may change in the presence of a virtual gate, or region of relatively high free charge carriers that lies in the HEMT between the physical grating gate and the 2DEG. The virtual gate and corresponding analytic formulation are able to account for the red-shifting experimentally observed in plasmonic resonances. The calculation methods demonstrated here have the potential to greatly aid in the design of future detection devices that require specifically tuned plasmonic modes in the 2DEG of a HEMT, as well as giving new insights to aid in the development of more complete analytic theories.

Tunable THz plasmon resonances in InGaAs/InP HEMT

Voltage-tunable plasmon resonances in a InGaAs/InP high electron mobility transistor (HEMT) are reported. The gate contact consisted of a 0.5 micron period metal grating formed by electron-beam lithography. Narrow-band resonant absorption of THz radiation was observed in transmission in the range 10 - 50 cm-1. The resonance frequency red-shifts with increasing negative gate bias as expected. Photo-response to a tunable far-IR laser is reported. The device may have application in high-frame-rate THz array detectors for spectral imaging with real-time chemical analysis.

Tunable far-IR detectors/filters based on plasmons in two-dimensional electron gases in InGaAs/InP heterostructures

Plasmons can be generated with photons in the two dimensional electron gas (2-deg) of high electron mobility transistors (HEMTs). Because the plasmon frequency at a given wavevector depends on sheet charge density, a gate bias can tune the plasmon resonance. This effect allows a properly designed HEMT to be used as a voltage-tunable narrow-band detector or filter. This work reports on both the theory and design of such a device in the InP materials system and discusses its potential uses. By using a sub-micron grating to couple incident radiation to a high sheet charge 2-deg, a minimum detectible wavelength of roughly 26 microns is obtained. Fabrication issues, terahertz response, and tunability are discussed. Because of its small size, this novel device could find use in spaceborne remote sensing application.

Plasmon Mediated, InGaAs/InP, Tunable Far-IR Detector

Plasmon resonances in the two dimensional electron gas (2-deg) of a high electron mobility transistor (HEMT) can affect transport properties. The resonance frequency depends on the gate-tuned sheet charge density of the 2deg and on the characteristic length of the gate metallization by which free space THz radiation couples to the plasmon. Thus, this type of device can be used as a tunable detector. This work presents an experimental investigation of such a device fabricated from the InGaAs/InP material system. E-beam lithography was used to fabricate a gate in the form of a grating with sub-micron period. Sensitivity of the conductance to incident THz fields is reported. Direct absorption of THz, temperature effects, and the effects of source to drain current on system performance are also investigated. It is expected that this class of device will find use in space-borne remote sensing applications.