One promising approach to light generation in the sought-after mid-infrared region of the spectrum (3-30 μm) is to use nonlinear materials to efficiently down-convert light from powerful near-IR sources to the mid-IR. The development and characterization of non-oxide nonlinear crystals for this purpose has recently garnered great interest. In this work, Valentin Petrov and coauthors review several Ba compounds, including BaGa₄S₇ (BGS), BaGa₄Se₇ (BGSe), BaGa₂GeS₆ (BGGS), and BaGa₂GeSe₆ (BGGSe), and demonstrate their utility as nonlinear crystals for mid-IR light generation. They are shown to have broad transparency windows, from approximately 0.4 μm to 12 μm for BGS and BGGS and from approximately 0.6 to 17 μm for BGSe and BGGSe. Compared to the more mature Ag compounds, AgGaS­₂ (AGS) and AgGaSe₂ (AGSe), which are used in numerous commercial applications, the Ba compounds are shown to have higher damage thresholds, chemical stability, and relatively straightforward growth techniques despite exhibiting weaker nonlinearities. A summary of experimental works utilizing Ba compounds shows efficient generation of light in the 2.6-17 μm region through various nonlinear processes, including optical parametric oscillation and amplification as well as difference frequency generation, indicating their potential for future commercial application.

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