Non-perturbative Interband Response of InSb Driven Off-resonantly by   Few-cycle Electromagnetic Transients
  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. 
F. Junginger, B. Mayer, C. Schmidt, O. Schubert, S. M\"ahrlein, A.   Leitenstorfer, R. Huber, A. Pashkin
physics.optics cond-mat.other physics.atom-ph