Prof Michael Johnston

Abstract:

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.

Abstract:

A compact, high-power emitter of half-cycle terahertz (THz) radiation is demonstrated. The device consists of an epitaxial InAs emitter upon a GaAs prism and produces THz pulses that are 20 times more powerful than those from conventional planar InAs emitters. This improvement is a direct result of reorienting the transient THz dipole such that its axis is not perpendicular to the emitting surface. © 2002 Optical Society of America.

Abstract:

We present a theoretical treatment of surface-field THz generation in semiconductors, which explains the power enhancement observed when a magnetic field is applied. Our model consists of two parts: a Monte Carlo simulation of the dynamics of carriers generated by a subpicosecond optical pulse, and a calculation of the resulting THz radiation emitted through the semiconductor surface. The magnetic field deflects the motion of the carriers, producing a component of the THz dipole parallel to the surface. This causes the power transmitted through the surface to be increased by more than one order of magnitude. © 2002 American Institute of Physics.