  Pumping n-type GaAs and InSb with ultrafast THz pulses having intensities
higher than 150 MW/cm2 shows strong free carrier absorption saturation at
temperatures of 300 K and 200 K respectively. If the energy imparted to the
carriers exceeds the bandgap, impact ionization processes can occur. The
dynamics of carrier cooling in GaAs and impact ionization in InSb were
monitored using THz-pump/THz probe spectroscopy which provides both sub-bandgap
excitation and probing, eliminating any direct optical electron-hole generation
that complicates the evaluation of results in optical pump/THz probe
experiments.
