Strong-field terahertz-optical mixing in excitons
  Driving a double-quantum-well excitonic intersubband resonance with a terahertz (THz) electric field of frequency \omega_{THz} generated terahertz optical sidebands \omega=\omega_{THz}+\omega_{NIR} on a weak NIR probe. At high THz intensities, the intersubband dipole energy which coupled two excitons was comparable to the THz photon energy. In this strong-field regime the sideband intensity displayed a non-monotonic dependence on the THz field strength. The oscillating refractive index which gives rise to the sidebands may be understood by the formation of Floquet states, which oscillate with the same periodicity as the driving THz field. 
M. Y. Su, S. Carter, M. S. Sherwin, A. Huntington, L. A. Coldren