  Intense multi-THz pulses are used to study the coherent nonlinear response of
bulk InSb by means of field-resolved four-wave mixing spectroscopy. At
amplitudes above 5 MV/cm the signals show a clear temporal substructure which
is unexpected in perturbative nonlinear optics. Simulations based on a
two-level quantum system demonstrate that in spite of the strongly off-resonant
character of the excitation the high-field pulses drive the interband
resonances into a non-perturbative regime of Rabi flopping.
