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Intracavity modulation of THz p-Ge laser gain by interband optical excitation

Optical quenching of the THz inter-sub-band p-Ge laser (tunable in the wavelength range 70-200 micron with ~1W output power) by Nd:YAG laser radiation has been investigated. YAG laser pulses were coupled into a p-Ge laser cavity through a SrTiO3 laser mirror, which is highly reflecting at cryogenic temperatures for THz frequencies and transparent for visible and near-IR light. Fast quenching of the p-Ge laser emission intensity was observed and attributed to free carrier absorption by optically generated electron-hole pairs in a thin layer of the active p-Ge crystal end surface. The effect also occurs when the interband absorption is confined to optically stimulated intracavity Si or GaAs spacers, which are transparent in the far-IR, placed between the SrTiO3 laser mirror and the active crystal end face. Such fast quenching of the p-Ge laser might be used to sharpen the trailing edge of the far-IR emission pulse for time-resolved or cavity-ring-down spectroscopic applications. Direct-gap semiconductor spacers might be used as fast, optically controlled intracavity modulators for active mode-locking.