From backward THz difference-frequency generation to parametric oscillation

We have observed the backward difference-frequency generation (DFG) in two GaSe crystals in the terahertz (THz) region by mixing two infrared laser beams. For a 47-mm-long crystal, the output wavelength can be tuned in a wide range of 167.6-2060 /spl mu/m (0.146-1.79 THz), whereas the peak output power can be as high as 217 W. The corresponding power conversion efficiency is determined to be about 0.03%. On the basis of these experimental results, our calculations show that peak output intensity can be scaled up to a level which is sufficiently high for exploring novel nonlinear effects in the THz region. Furthermore, we have discussed the possibility of including a cavity for one of the mixing beams to enhance the conversion efficiencies for the backward DFG. After taking into consideration all the important issues, we have shown that the backward parametric oscillation is feasible in the THz region by using a few crystals under the optimum pumping conditions.