Using a mixed type-I/type-II GaAs/AlAs multiple-quantum-well sample, we have demonstrated an optically controllable and tunable terahertz (THz) filter. Long-lived electron–hole pairs in the quantum wells allow for efficient THz attenuation over a large THz spot size (2 mm) for extremely low optical cw power. This sample can also be used as an optically tunable THz phase shifter. The optically induced change of the GaAs quantum wells from a dielectric to a conducting material leads to the observed attenuation and the shifting of the THz wave forms.
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There has recently been alot of discussion on the superluminal behavior of the shifting of the THz wave forms in highly absorbing medium (see Ref. 17). While it appears at first glance that the peak velocity passes through the sample with speeds greater than that of light, the shifting is actually a consequence of a phase shift of the THz wave as it enters a highly conducting medium. This phase shift is small and therefore generally not observable, but it does dramatically effect the behavior of the THz pulses when they are measured in the time domain.
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