Michael C. Wanke

Terahertz photoconductivity and plasmon modes in double-quantum-well field-effect transistors

Double-quantum-well field-effect transistors with a grating gate exhibit a sharply resonant, voltage tuned terahertz photoconductivity. The voltage tuned resonance is determined by the plasma oscillations of the composite structure. The resonant photoconductivity requires a double-quantum well but the mechanism whereby plasma oscillations produce changes in device conductance is not understood. The phenomenon is potentially important for fast, tunable terahertz detectors.

Single-quantum-well grating-gated terahertz plasmon detectors

A grating-gated field-effect transistor fabricated from a single-quantum well in a high-mobility GaAs–AlGaAs heterostructure is shown to function as a continuously electrically tunable photodetector of terahertz radiation via excitation of resonant plasmon modes in the well. Different harmonics of the plasmon wave vector are mapped, showing different branches of the dispersion relation. As a function of temperature, the resonant response magnitude peaks at around 30K. Both photovoltaic and photoconductive responses have been observed under different incident power and bias conditions.

Millimeter wave mixing using plasmon and bolometric response in a double-quantum-well field-effect transistor

Heterodyne mixing characteristics of a double-quantum-well field-effect transistor (DQW FET) from 94 to 145GHz are reported. The DQW FET exhibits two physically distinct mixing responses. Near pinch-off, the device behaves as a broadband bolometric mixer with intermediate frequency (IF) bandwidth of 620MHz. Away from pinch-off it shows an electrically tunable resonant plasmon response. Mixing on a plasmon resonance yields an IF with significant harmonic distortion, signaling a complicated nonlinear mechanism, and shows a wide IF bandwidth >2GHz.

Large-area metamaterials on thin membranes for multilayer and curved applications at terahertz and higher frequencies

A possible path for fabricating three-dimensional metamaterials with curved geometries at optical and infrared frequencies is to stack flexible metamaterial layers. We have fabricated highly uniform metamaterials at terahertz frequencies on large-area, low-stress, free-standing 1 μm thick silicon nitride membranes. Their response remains comparable to that of similar structures on thick substrates as measured by the quality factor of the resonances. Transmission measurements with a Fourier transform infrared spectrometer highlight the advantage of fabricating high frequency metamaterials on thin membranes as etalon effects are eliminated. Releasing the membranes enables layering schemes and placement onto curved surfaces in order to create three-dimensional structures.

Transformation of the multimode terahertz quantum cascade laser beam into a Gaussian, using a hollow dielectric waveguide

We demonstrate that a short hollow dielectric tube can act as a dielectric waveguide and transform the multimode, highly diverging terahertz quantum cascade laser beam into the lowest order dielectric waveguide hybrid mode, EH(11), which then couples efficiently to the free-space Gaussian mode, TEM(00). This simple approach should enable terahertz quantum cascade lasers to be employed in applications where a spatially coherent beam is required.

Enhanced responsivity in membrane isolated split-grating-gate plasmonic terahertz detectors

A 50-fold increase in responsivity of plasmon resonant detectors is achieved by thermally isolating the detector on a thin membrane. Terahertz radiation is resonantly absorbed in the grating-gated channel while temperature modulation is sensed by the resistance of a narrow center region biased to pinch off. Thermal isolation enhances the temperature rise on absorption. Detectors with and without the additional thermal isolation demonstrate a linear power dependence.

Far-Infrared Spectrum Analysis Using Plasmon Modes in a Quantum-Well Transistor

Excitation of resonant plasmon modes by far-infrared (FIR) radiation in a quantum-well transistor is used to analyze the spectral content of FIR illumination at frequencies between 0.58 and 0.99 THz. A split grating gate design that allows localized pinch-off of the transistor channel greatly enhances FIR response and allows completely electrical tuning of the plasmon resonance, enabling broadband FIR spectrum analysis without moving parts. A voltage ramp applied to the gate can generate a spectrum at video rate

Nonequilibrium many-body theory of intersubband lasers

We present a theory for intersubband lasers, based on the solution of the Maxwell-semiconductor Bloch equations for the laser field and active medium. The collision contributions are treated within the relaxation rate approximation, where the relaxation rates are determined by microscopic scattering calculations. The theory is suitable for investigating steady-state as well as dynamical laser characteristics. As examples of applications of the theory, we examine the thermal dependence of the laser output versus current density curve and the response to modulation of the injection current, for a three-subband laser. The influence of the nonparabolicity of the conduction band and Hartree-Fock many-body effects are investigated.

Interaction between metamaterial resonators and intersubband transitions in semiconductor quantum wells

We report on the coupling and interaction between the fundamental resonances of planar metamaterials (split ring resonators) and intersubband transitions in GaAs/AlGaAs quantum wells structures in the mid-infrared. An incident field polarized parallel to the sample surface is converted by the metamaterial resonators into a field with a finite component polarized normal to the surface and interacts strongly with the large dipole moment associated with quantum well intersubband transitions.

Heterodyne Mixing of Terahertz Quantum Cascade Lasers Using a Planar Schottky Diode

Terahertz quantum cascade lasers (QCLs) have been used together with a monolithic planar Schottky diode receiver to study the heterodyne mixing between dual internal modes of a QCL and between a single mode of a QCL and a known molecular line from a molecular gas laser. Dual-mode mixing shows that the intrinsic linewidth of a free-running QCL is les30 kHz . Mixing against a molecular laser line gives a high precision measurement of a QCLs absolute frequency and can show transient turn-on behavior in a pulsed QCL.