Frequency down-conversion of multiline CO laser into the THz range with ZnGeP2 crystal

Frequency down-conversion (difference frequency generation) of a multiline Q-switched CO laser with a ZnGeP2 crystal was numerically studied. Two options of the CO laser spectrum were considered: narrow (44 lines) and broad (150 lines) ones. An influence of the ZnGeP2 crystal absorption on THz generation was taken into account and was shown to be significant. It affected optimal phase-matching angle, THz spectrum and frequency conversion efficiency, which were optimized by the crystal length. Maximal frequency conversion efficiency of 0.007% was numerically obtained for the narrow spectrum due to less influence of the crystal absorption.