S.K. Lyo

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.

Real-space and energy representations for the interface-roughness scattering in quantum-well structures

We show that the real space representation of the interface roughness as a fluctuating potential in the coordinate space is equivalent to the usual energy-fluctuation representation for intrasublevel scattering in multi-interface quantum well structures with generally shaped confinement-potential profiles. The coordinate picture is, however, more general and can be used for higher-order effects and multi-sublevel scattering in tunnel-coupled multi-quantum-well structures. The result is employed to study the interface-roughness-limited mobility of tunnel-coupled double quantum wells at low temperatures.

Quantized magneto-thermopower in tunnel-coupled ballistic channels: sign reversal and oscillations

The quantized electron-diffusion thermoelectric power S is studied for a general one-dimensional band structure in ballistic channels with applications to a single-quantum-well channel and tunnel-coupled double-quantum-well channels in a perpendicular magnetic field. We find a field-induced sign reversal of S and oscillations in double-well channels.

High-Q photonic bandgap resonant cavities: From mm-wave to optical regime

The authors have realized a new class of high-Q resonant cavity using two-dimensional photonic bandgap (PBG) structures and showed that its Q-value can be as high as {approximately}23,000 in the mm-wave regime. They further show that its modal properties, such as the resonant frequency, modal linewidth and number of modes, can be tuned by varying the cavity size. In addition, they present a new nano-fabrication technique for constructing PBG resonant cavities in the near infrared and visible spectral regime.

Suppression of impurity and interface-roughness back-scattering in double quantum wires: theory beyond the Born approximation

The effect of higher-order corrections to the Born approximation is studied for the previously obtained giant conductance enhancement in tunnel-coupled double quantum wires in a magnetic field by including both impurity and interface-roughness scattering. The enhancement is caused by an abrupt suppression of back-scattering of electrons which occurs when the chemical potential is in the anticrossing gap of the ground tunnel-split doublet. The calculated conductance enhancement is large, and the relative higher-order correction to the enhancement is found to be significant for long-range scattering potentials. However, this relative higher-order correction will be reduced as the range of scattering potentials becomes small. The correction depends on various effects, such as the magnetic field, electron and impurity densities, impurity positions, symmetric and asymmetric doping profiles, centre barrier thickness, and degree of interface roughness.

Terahertz absorption by resonant plasmon excitations in grating-gated quantum wells

Terahertz detection using excitations of plasmon modes offers a high-speed, high resolution, and frequency-selective alternative to existing technology. Plasmons in high mobility quantum well two-dimensional electron gas (2DEG) systems can couple to radiation when either the channel carrier density, or the incident radiation, is spatially modulated with appropriate periodicity. Grating-gated terahertz detectors having a voltage tunable frequency response have been developed based on this principle. A continuous wave THz photomixer was used to characterize the resonant absorption in such devices. At the fundamental 2DEG plasmon frequency, defined by the grating and the quantum well carrier density, a 20% change in transmission was observed. As the resonance is tuned from the natural plasmon frequency through application of a gate bias, it shifts as expected, but the transmission change drops to only a few percent.

Magnetoconductance of independently tunable tunnel-coupled double quantum wires

The authors report on their recent experimental studies of vertically-coupled quantum point contacts subject to in-plane magnetic fields. Using a novel flip-chip technique, mutually aligned split gates on both sides of a sub micron thick double quantum well heterostructure define a closely-coupled pair of ballistic one-dimensional (1D) constrictions. They observe quantized conductance steps due to each quantum well and demonstrate independent control of each ID constriction width. In addition, a novel magnetoconductance feature at {approximately}6 T is observed when a magnetic field is applied perpendicular to both the current and growth directions. This conductance dip is observed only when 1D subbands are populated in both the top and bottom constrictions. This data is consistent with a counting model whereby the number of subbands crossing the Fermi level changes with field due to the formation of an anticrossing in each pair of 1D subbands.

Suppression of direct-transition phonon side bands in the magnetoluminescence from doped quantum wells

We present a theory for and the observation of LO-phonon side bands for transitions between the electron and hole Landau levels . The side band of the primary line in an n-GaAs/InGaAs/AlGaAs quantum well is absent at low temperatures. The side bands of the secondary lines are relatively strong, growing with in excellent agreement with the theory. The suppression of small- side bands is due to the interference between the electron - phonon and hole - phonon recombination channels and the magnetic quantization in two dimensions.

LDRD final report on Bloch Oscillations in two-dimensional nanostructure arrays for high frequency applications.

We have investigated the physics of Bloch oscillations (BO) of electrons, engineered in high mobility quantum wells patterned into lateral periodic arrays of nanostructures, i.e. two-dimensional (2D) quantum dot superlattices (QDSLs). A BO occurs when an electron moves out of the Brillouin zone (BZ) in response to a DC electric field, passing back into the BZ on the opposite side. This results in quantum oscillations of the electron--i.e., a high frequency AC current in response to a DC voltage. Thus, engineering a BO will yield continuously electrically tunable high-frequency sources (and detectors) for sensor applications, and be a physics tour-de-force. More than a decade ago, Bloch oscillation (BO) was observed in a quantum well superlattice (QWSL) in short-pulse optical experiments. However, its potential as electrically biased high frequency source and detector so far has not been realized. This is partially due to fast damping of BO in QWSLs. In this project, we have investigated the possibility of improving the stability of BO by fabricating lateral superlattices of periodic coupled nanostructures, such as metal grid, quantum (anti)dots arrays, in high quality GaAs/Al{sub x}Ga{sub 1-x}As heterostructures. In these nanostructures, the lateral quantum confinement has been shown theoretically to suppress the optical-phonon scattering, believed to be the main mechanism for fast damping of BO in QWSLs. Over the last three years, we have made great progress toward demonstrating Bloch oscillations in QDSLs. In the first two years of this project, we studied the negative differential conductance and the Bloch radiation induced edge-magnetoplasmon resonance. Recently, in collaboration with Prof. Konos group at Rice University, we investigated the time-domain THz magneto-spectroscopy measurements in QDSLs and two-dimensional electron systems. A surprising DC electrical field induced THz phase flip was observed. More measurements are planned to investigate this phenomenon. In addition to their potential device applications, periodic arrays of nanostructures have also exhibited interesting quantum phenomena, such as a possible transition from a quantum Hall ferromagnetic state to a quantum Hall spin glass state. It is our belief that this project has generated and will continue to make important impacts in basic science as well as in novel solid-state, high frequency electronic device applications.

Terahertz-based target typing.

The purpose of this work was to create a THz component set and understanding to aid in the rapid analysis of transient events. This includes the development of fast, tunable, THz detectors, along with filter components for use with standard detectors and accompanying models to simulate detonation signatures. The signature effort was crucial in order to know the spectral range to target for detection. Our approach for frequency agile detection was to utilize plasmons in the channel of a specially designed field-effect transistor called the grating-gate detector. Grating-gate detectors exhibit narrow-linewidth, broad spectral tunability through application of a gate bias, and no angular dependence in their photoresponse. As such, if suitable sensitivity can be attained, they are viable candidates for Terahertz multi-spectral focal plane arrays.