Ultrafast electron cascades in semiconductors driven by intense femtosecond terahertz pulses

An ultrafast terahertz (THz) source has been employed to study the nonlinear response of semiconductors to near-half-cycle femtosecond pulses in the THz regime. We report nonlinear absorption and self-phase modulation of the THz pulses associated with ultrafast impact ionization processes and the development of an electron cascade on femtosecond time scales by THz pump--THz probe measurements.