We have observed optical birefringence in liquids induced by single-cycle THz pulses with field strengths exceeding 100 kV/cm. The induced change in polarization is proportional to the square of the THz electric field. The time-dependent THz Kerr sig
nal is composed of a fast electronic response that follows the individual cycles of the electric field and a slow exponential response associated with molecular orientation.
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 increas
e 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.
