Laser requirements for the designof fast laser-driven semiconductor switches for THz and mm-waves.

Abstract

A reduced parameter model of fast laser-driven semiconductor switches of THz and mm-waves has been developed. The model predicts peak reflectivity and minimum transmissivity of switches, showing good agreement with experimental data, while requiring fewer inputs than published models. This simplification facilitated a systematic survey of laser parameters required for efficient switching. Laser energy density requirements are presented as a function of laser wavelength, laser pulse width, switched frequency, reflection angle, and semiconductor material (silicon or gallium arsenide). Analytical expressions have been derived to explain the dependence of laser requirements on switch parameters and to derive practical minima of required laser energy density. Diffusion is shown to quickly negate the shallow absorption advantage of laser wavelengths shorter than about 500nm in silicon or 800nm in gallium arsenide. Decreasing laser pulse width, to a derived limit, and switching S-polarized THz or mm-wave signals are shown to be means of lowering required laser energy. This is an especially useful result for devices operating at high power levels or THz frequencies, where extended switches are used in quasioptical systems.