Generation of spontaneous parametric down conversion from BaGa₂GeS₆ crystal

We report the generation of highly non-degenerate spontaneous parametric down conversion (SPDC) in a novel mid-infrared nonlinear crystal BaGa₂GeS₆, and provide a direct experimental comparison with the widely used AgGaS₂ crystal. Using a continuous-wave near-infrared (near-IR) pump laser, we generate correlated photon pairs with signal wavelengths in the near-IR ∼815–900 nm and idler wavelengths extending deep into the mid-infrared (mid-IR) up to 22–24 µm. Wavelength-angular SPDC spectra are measured for both crystals under identical experimental conditions and analyzed using available dispersion data. Our results demonstrate, for the first time, the efficient generation of correlated photon pairs in BaGa₂GeS₆ via SPDC. A quantitative comparison shows that BaGa₂GeS₆ exhibits approximately twice the nonlinear conversion efficiency of AgGaS₂, which is in good agreement with values reported in the literature. In addition, BaGa₂GeS₆ produces a narrower external SPDC emission cone due to its lower refractive indices, which is advantageous for photon collection and nonlinear interferometric applications. Due to its large effective second-order nonlinearity and broad mid-IR transparency BaGa₂GeS₆ emerges as a highly promising platform for bright and stable sources of correlated photon pairs. These results establish BaGa₂GeS₆ as a strong candidate for mid-IR quantum spectroscopy, imaging, and gas-sensing applications.