  Using two-dimensional THz spectroscopy in combination with numerical models,
we investigate the dynamics linked to carrier multiplication caused by
high-field THz excitation of the low-gap semiconductor InSb. In addition to
previously observed dynamics connected with quasi-ballistic carrier dynamics,
we observe other spectral and temporal features that we attribute to impact
ionization for peak fields above 60 kV/cm, which continue up to the maximum
investigated peak field of 430 kV/cm. At the highest fields we estimate a
carrier multiplication factor greater than 10 due to impact ionization, which
is well-reproduced by a numerical simulation of the impact ionization process
which we have developed.
