  We use an open-aperture Z-scan technique to show how intense few-cycle
terahertz pulses can experience a nonlinear bleaching of absorption in an
n-doped semiconductor due to terahertz-electric-field-driven intervalley
scattering of electrons in the conduction band. Coherent detection of the
transmitted terahertz pulse waveform also allows the nonlinear conductivity
dynamics to be followed with sub-picosecond time resolution. Both the Z-scan
and time-domain results are found to be in agreement with our theoretical
analysis.
