Аннотация:Nonlinear optical materials designed for mid-infrared applications gainincreasing interest in photonics and quantum-optical research. However, manyof these materials remain unexplored in the broad mid-infrared range due tothe limited availability of suitable equipment. Silver thiogallate (AgGaS2) is one ofthe widely known nonlinear crystals used in mid-infrared frequency-conversionapplications. While its transmission and refractive indices are well characterizedup to 10.6 μm, dispersion properties beyond this wavelength remain unreported.In this work, a quantum-optical approach is employed to determine the refractiveindices of AgGaS2 across an extended mid-infrared wavelength range. Correlatedphoton pairs are generated in the crystal, where one photon in a pair is generatedat a near-infrared wavelength and the other in the mid-infrared. Due to thecorrelations, detecting the spectrum of the near-infrared photons allows inferringthe crystal’s dispersion properties in the mid-infrared region up to 21 μm,without the need for infrared light sources or detectors. Sellmeier equationsare further proposed that accurately describe both ordinary and extraordinaryrefractive indices across the entire transparency range of the crystal. The resultsare consistent with previously reported data below 10 μm and demonstratethe suitability of AgGaS2 for frequency-conversion processes over an extendedwavelength range.
