A.G. Davies

High resistivity annealed low-temperature GaAs with 100 fs lifetimes

We demonstrate the critical effect of postgrowth annealing temperature on the properties of low-temperature-grown GaAs. By using annealing temperatures substantially below the 500–600 °C commonly reported, GaAs with high resistivity and with carrier lifetimes as short as 100 fs can be routinely obtained. We discuss the optimum, but different, anneal conditions required for terahertz photoconductive emitters and detectors, and illustrate their use in a continuous-wave system.

The development of terahertz sources and their applications

The terahertz region of the electromagnetic spectrum spans the frequency range between the mid-infrared and the millimetre/microwave. This region has not been exploited fully to date owing to the limited number of suitable (in particular, coherent) radiation sources and detectors. Recent demonstrations, using pulsed near-infrared femtosecond laser systems, of the viability of THz medical imaging and spectroscopy have sparked international interest; yet much research still needs to be undertaken to optimize both the power and bandwidth in such THz systems. In this paper, we review how femtosecond near-infrared laser pulses can be converted into broad band THz radiation using semiconductor crystals, and discuss in depth the optimization of one specific generation mechanism based on ultra-fast transport of electrons and holes at a semiconductor surface. We also outline a few of the opportunities for a technology that can address a diverse range of challenges spanning the physical and biological sciences, and note the continuing need for the development of solid state, continuous wave, THz sources which operate at room temperature.

Enhanced coherent terahertz emission from indium arsenide in the presence of a magnetic field

We demonstrate enhancement of terahertz (THz) emission from indium arsenide at 170 K in magnetic fields (B) up to 8 T. An order of magnitude increase in visible to terahertz conversion efficiency was observed, with no suggestion of saturation of the TE polarization at higher magnetic fields. Free-space electro-optic sampling measurements confirmed the coherent nature of this radiation over the field range investigated, and gave an insight into the carrier motion subsequent to photoexcitation, which may be responsible for the observed THz power enhancement.

The growth and physics of high mobility two-dimensional hole gases

Abstract Modulation-doped p-type GaAs-Al x Ga 1−x As heterojunctions have been grown by molecular beam epitaxy (MBE) on the (311)A GaAs surface using silicon as the acceptor. Two-dimensional hole gases with mobilities as high as 570,000 cm 2 V −1 s −1 at 50 mK have been obtained. It is shown that the sample mobility is dependent upon the direction of orientation of the fabricated Hall bar on the MBE wafer which could be due to the anisotropic nature of the Fermi surface in p-type systems. Experimental results on the fractional quantum Hall effect (FQHE) in these high mobility hole gases are also presented.

Magnetic-field-induced enhancement of terahertz emission from III–V semiconductor surfaces

Abstract We discuss the origins of the magnetic-field-induced enhancement of terahertz (THz) emission from bulk semiconductor surfaces. The principal effect of the magnetic field is to rotate the THz dipole and hence dramatically increase the THz power radiated through the semiconductor surface. It also significantly affects the ability of the photo-created carriers to screen surface electric fields. The sensitivity of THz emission to the motion of photo-created carriers makes this an ideal probe of hot carrier dynamics both in bulk semiconductors and sophisticated heterostructures.

The fractional quantum Hall effect in high mobility two-dimensional hole gases

We have studied the fractional quantum Hall effect (FQHE) as a function of temperature and magnetic field in a series of high mobility two-dimensional hole gases. The magnetic field dependence of the FQHE structure 1 < v < 2 has been investigated by tilting the heterojunction samples in the applied magnetic field. The behaviour of the v = 34 FQHE state upon tilting is suggestive of a change in the spin polarization of the ground state. Temperature activation analyses have been used to quantify this spin transition.

Dependence of fractal conductance fluctuations on soft-wall profile in a double-layer semiconductor billiard

We present a semiconductor system featuring two billiards located one on top of the other. We use this system to study the dependence of fractal conductance fluctuations on soft-wall potential profile and show the fluctuations to be surprisingly robust to changes in profile.

Geometry-independence of fractal ballistic processes

Abstract When semiconductor billiards are cooled to milli-Kelvin temperatures to preserve electron wave coherence, quantum interference effects generate reproducible fractal fluctuations in the billiards magneto-conductance. Here we show the fractal scaling properties to be remarkably robust to billiard shape.

MBE growth of terahertz quantum cascade semiconductor lasers

We report here the growth, fabrication, and operation of two different quantum cascade laser structures that emit in a single cavity mode in the terahertz regime; Both laser designs employ a chirped superlattice active region in a QC structure, comprising seven GaAs quantum wells separated by Al/sub 0.15/Ga/sub 0.85/As barriers, with a novel type of waveguide that provides confinement of the very long wavelength radiation to a very thin, highly doped buried layer.

Discrete energy level spectrum dependence of fractal conductance fluctuations in semiconductor billiards

Abstract We present experimental results demonstrating that fractal conductance fluctuations are remarkably robust to deviations from the semiclassical conduction regime. We find that the fractal dimension depends solely on an empirical parameter that quantifies the resolution of the billiards discrete energy level spectrum. Non-fractal fluctuations are obtained only in the extreme classical and quantum-mechanical conduction regimes.