Widely tunable continuous-wave mid-infrared radiation (5.5-11 (mu)m) by difference-frequency generation in LiInS2 crystal

The first demonstration, to the best of our knowledge, of continuous-wave (cw) difference-frequency generation (DFG) in LiInS2 crystal is reported. Wide spectral coverage (5.5-11.3 (mu)m) has been obtained with angle and wavelength tuning for type II (eoe) critically phase-matched parametric interaction. The phase-matching conditions in cw DFG have been investigated, which allowed us to improve the Sellmeier parameters by use of a two-pole dispersion equation. An effective nonlinear coefficient d_eff = 6.9 +- 0.8 pm/V has been determined at ~7 (mu)m relative to the well-known nonlinear coefficient d_36 of AgGaS2, which yields a power-conversion efficiency of ~12.4 (mu)W/(W^2 cm). We evaluated the high-resolution spectral characteristics of the DFG source by recording C2H2 and SO2 spectra.