Dispersion Analysis of Silver Thiogallate in a Broad Mid-Infrared Range

Nonlinear optical materials designed for mid-infrared applications gain increasing interest in photonics and quantum-optical research. However, many of these materials remain unexplored in the broad mid-infrared range due to the limited availability of suitable equipment. Silver thiogallate (AgGaS₂) is one of the widely known nonlinear crystals used in mid-infrared frequency-conversion applications. While its transmission and refractive indices are well characterized up to 10.6 μm, dispersion properties beyond this wavelength remain unreported. In this work, a quantum-optical approach is employed to determine the refractive indices of AgGaS2 across an extended mid-infrared wavelength range. Correlated photon pairs are generated in the crystal, where one photon in a pair is generated at a near-infrared wavelength and the other in the mid-infrared. Due to the correlations, detecting the spectrum of the near-infrared photons allows inferring the crystal's dispersion properties in the mid-infrared region up to 21 μm, without the need for infrared light sources or detectors. Sellmeier equations are further proposed that accurately describe both ordinary and extraordinary refractive indices across the entire transparency range of the crystal. The results are consistent with previously reported data below 10 μm and demonstrate the suitability of AgGaS₂ for frequency-conversion processes over an extended wavelength range.