  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.
