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The effect of the photoinduced absorption of terahertz (THz) radiation in asemi-insulating GaAs crystal is studied by the pulsed THz transmissionspectroscopy. We found that a broad-band modulation of THz radiation may beinduced by a low-power optical excitation in the spectral range of the impurityabsorption band in GaAs. The measured modulation achieves 80\%. The amplitudeand frequency characteristics of the resulting THz modulator are criticallydependent on the carrier density and relaxation dynamics in the conduction bandof GaAs. In semi-insulating GaAs crystals, the carrier density created by theimpurity excitation is controlled by the rate of their relaxation to theimpurity centers. The relaxation rate and, consequently, the frequencycharacteristics of the modulator can be optimized by an appropriate choice ofthe impurities and their concentrations. The modulation parameters can be alsocontrolled by the crystal temperature and by the power and photon energy of theoptical excitation. These experiments pave the way to the low-power fastoptically-controlled THz modulation, imaging, and beam steering.

Impurity-induced modulation of terahertz waves in optically excited GaAs