Growth of new quaternary nonlinear optical crystals for 1-micron-pumped mid-IR generation

Crystal growth of two new quaternary crystals, AgGaGeS4 and AgGaGe5Se12 , was performed in order to evaluate the usefulness of these materials for nonlinear optical frequency conversion of one-micron solid state lasers into the mid-infrared spectral region beyond 4 microns. Each compound was synthesized by vapor transport in sealed ampoules from high purity elemental starting materials, and crystals were grown by the gradient freeze technique in horizontal transparent furnaces. AgGaGe5Se12 exhibited incongruent melting behavior, and small optical samples extracted from an as-grown polycrystalline boule had high scattering losses. AgGaGeS4 crystal growth proved to be far more favorable, resulting in a crack-free single crystal measuring 19mm in diameter and >80mm in length with as-grown 2.05-μm absorption losses < 0.05 cm-1. The measured laser damage threshold of an uncoated AgGaGeS4 crystal at 2.05μm was 1.1 J/cm2, and room-temperature measurements of thermal diffusivity, heat capacity, and thermal conductivity yielded values of 0.224 mm2/s, 0.448 J/g/K, and 0.399 W/mK respectively for the sulfide. The growth and properties of AgGaGeS4, despite being a quaternary compound with complex phase equilibria, appear indeed to be promising for shifting Nd-laser output directly into the mid-IR.