Terahertz wave generation from hyper-Raman lines in two-level quantum   systems driven by two-color lasers
  Based on spatial-temporal symmetry breaking mechanism, we propose a novel scheme for terahertz (THz) wave generation from hyper-Raman lines associated with the 0th harmonic (a particular even harmonic) in a two-level quantum system driven by two-color laser fields. With the help of analysis of quasi-energy, the frequency of THz wave can be tuned by changing the field amplitude of the driving laser. By optimizing the parameters of the laser fields, we are able to obtain arbitrary frequency radiation in the THz regime with appreciable strength (as strong as the typical harmonics). Our proposal can be realized in experiment in view of the recent experimental progress of even-harmonics generation by two-color laser fields. 
Wei Zhang, Shi-Fang Guo, Su-Qing Duan, and Xian-Geng Zhao
physics.optics cond-mat.other physics.atom-ph