Recent progress on new infrared nonlinear optical materials with application prospect

Nonlinear optical (NLO) crystals can convert the frequencies of existing lasers to other favorable wavelength ranges, and are hence important material basis of photoelectron technology. Currently, there is an urgent need to investigate new NLO materials in the deep UV and IR range. In this review, we focus on the recent progress on new IR NLO materials with application prospect. The main challenge of discovering new practically usable IR NLO crystals is to synthesize new materials that can achieve the coexistence of various desirable properties including sufficient SHG coefficients, wide mid-far IR transparency range, high laser damage threshold, moderate birefringence, favorable single-crystal growth habit, and stable physicochemical properties. Here, we highlight that the research strategy combining structural design with exploratory synthesis can greatly increase the efficiency of identifying new practical IR NLO materials in metal chalcogenides. Several classes of IR NLO materials with excellent overall performance have been found, such as BaGa4Se7 and BaGa2GeSe6. Moreover, the research on these materials has gone beyond crystal structure and preliminary characterization. Great efforts have been carried out in the bulk crystal growth, in the thorough measurement of their physical properties using bulk crystals, in the evaluation of their IR laser output performance, and eventually in the fabrication of laser device. In the frequency down-conversion OPOs pumped by the conventional 1 mu m laser, BaGa4Se7 has realized the stable output of mid-far IR laser output with the highest power and widest wavelength range so far. In this review, instead of covering all compounds with IR NLO properties, our discussion will only focus on those showing good overall IR NLO performance and application prospect, especially on those of which large size single crystals have been already obtained. This review then concludes with an outlook on what efforts could be made in the future.