Enhancement of terahertz-wave output from LiNbO_3 optical parametric oscillators by cryogenic cooling

In recent years widely tunable terahertz- (THz-) wave generation from ${\mathrm{LiNbO}}_3$ optical parametric oscillators (OPO's) has been successfully demonstrated by use of the prism output-coupler method. However, there remains a problem of large absorption loss for generated terahertz waves inside the crystal, so we investigated the cryogenic characteristics of the OPO. We achieved 125-times-higher THz-wave output and 32% reduction of the generation threshold by cooling the crystal to 78 K. This scheme also provides direct loss measurement at THz frequency, and we found that the THz-wave enhancement mechanism is improvement of the gain as well as the reduction of the absorption coefficient.