Possibilities and experimental investigation of parametric frequency converters with LiInSe2, AgGaGeS4, HgGa2S4 and Hg0.65Cd0.35Ga2S4 crystals

Second harmonic generation of 9 μm emission band of CO₂ laser is for the first time realized with a number of new nonlinear crystals: biaxial LiInSe₂, AgGaGeS₄ (that is solid solution AgGaS₂:GeS₂ or Ag_xGa_xGe_(1-x)S₂, or AgGaGe_xS_2(1+x) at x=0.5) and uniaxial HgGa₂S₄, Cd_0.35Hg_0.65Ga₂S₄ (solid solution HgGa₂S₄:CdGa₂S₄ with mixing ratio x=0.35). For LiInSe₂ and AgGaGeS₄ it is first type of frequency conversion realized at some time or other. The main feature of these crystals is a wide range transparency from 0.4-0.6 to 12.5-16.2 μm. Their damage thresholds for 30 ns TEA CO₂ laser pulses at 9.55 μm is determined as 1.7 to 2.3 times higher than for popular middle IR crystals such as ZnGeP₂ or AgGaSe₂. Parameters of second harmonic generation were determined correctly in comparison with ZnGeP₂ crystals at 33 ns pump pulses and intensities that are about 28.5% (40 MW/cm²) of damage thresholds of ZnGeP₂. As high as 7% external peak power efficiencies (4.7% in energy) is typical for orange and yellow phases of 3.1 mm HgGa₂S₄ crystal such pumping that is only 13% of its damage threshold. Efficiencies as high as 0.45% and 0.3% are realized in 2.1 mm AgGaGeS₄ crystal at pump intensities-that is only 11% of the damage threshold. Phase-matching angles, spectral dependence of second harmonic generation efficiencies, and angular and spectral phase-matching widths are investigated in detail.