Coherent phonon emission in the supersonic expansion of photoexcited electron-hole plasma in Ge

Coherent subnanosecond acoustic pulses generated by a laser-excited electron-hole plasma in a germanium single crystal are detected by a time-resolved pump-probe photodeflection technique. It is found that the front duration of these hypersound pulses is controlled by the time of plasma diffusion at supersonic velocities. The characteristic velocity of plasma diffusion evaluated from the experiments exceeds the longitudinal sound velocity in germanium by a factor of 1.5. The hypersound pulse shapes provide evidence for supersonic diffusion of the electron-hole plasma in a semiconductor at room temperature on a subnanosecond time scale.