When a semiconductor substrate is illuminated by an optical beam whose photon energy exceeds its band-gap energy, free carriers are produced in the substrate, which provides a means of control for the substrate reflectance and transmittance of submillimeter electromagnetic waves. This paper is concerned with submillimeter wave modulation by an optically-excited semiconductor. High-resistivity silicon and GaAs are used as the substrate. Continuous radiation with a wavelength of 214 μm is used as the submillimeter wave. A Q-switched Nd:YAG laser is used to excite the free carriers. The experimental results are presented. Effects of semiconductor substrate material, wavelength of the excitation beam, and incident surfaces of the submillimeter wave and the substrate excitation beam, on the modulation waveforms are analyzed. As an application of the proposed modulation technique, submillimeter wave short pulses with variable duration are successfully generated.
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Nozokido et al. (1997) studied this question.