  Using reflective cross-polarized 2D THz time-domain spectroscopy in the range
of 1-12 THz, we follow the trajectory of the out-of-equilibrium electron
population in the low-bandgap semiconductor InSb. The 2D THz spectra show a set
of distinct features at combinations of the plasma-edge and vibration
frequencies. Using finite difference time domain simulations combined with a
tight binding model of the band structure, we assign these features to
electronic nonlinearities and show that the nonlinear response in the first
picoseconds is dominated by coherent ballistic motion of the electrons. We
demonstrate that this technique can be used to investigate the landscape of the
band curvature near the Gamma-point as illustrated by the observation of
anisotropy in the (100)-plane.
