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.