Observation of nonequilibrium carrier distribution in Ge, Si, and GaAs by terahertz pump–terahertz probe measurements

We compare the observed strong saturation of the free-carrier absorption in n-type semiconductors at 300 K in the terahertz (THz) frequency range when single-cycle pulses with intensities up to 150 MW/cm² are used. In the case of germanium, a small increase in the absorption occurs at intermediate THz pulse energies. The recovery of the free-carrier absorption was monitored by time-resolved THz pump--THz probe measurements. At short probe delay times, the frequency response of germanium cannot be fitted by the Drude model. We attribute these unique phenomena of Ge to dynamical overpopulation of the high mobility Γ conduction-band valley.