Vilnius university, Lithuania

Difference-frequency generation in the three-level quantum system having forbidden in dipole approximation electronic and optically active vibration transitions was analyzed theoretically. Pump and injection laser pulses of duration short compared to the phase relaxation time of electronic and vibration coherences in the medium were considered. The wavelength of the pump pulse was taken as being in the visible or near infrared spectral regions, while the frequency of the injection pulse was almost twice higher than that of the pump. Non-perturbative treatment based on self-consistent system of modified Maxwell - Bloch equations was used to evaluate efficiency of THz radiation arising during excitation of both the electronic and vibration coherences at two-photon and combined resonances.