  We compare the observed strong saturation of the free carrier absorption in
n-type semiconductors at 300 K in the terahertz frequency range when
single-cycle pulses with intensities up to 150 MW/cm2 are used. In the case of
germanium, a small increase of 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 gamma conduction band valley.
