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Applied Physics B

Characterization of tunable mid-infrared spontaneous parametric down-conversion in AgGaS2

Filip Krajinić, M. M. Ćurčić, Marina M. Lekić, Branislav M. Jelenkovic

Published Mar 25, 2026• 0 citations
Publisher

Abstract

Abstract The nonlinear process of spontaneous parametric down-conversion in the silver-gallium-sulfide crystal can generate highly non-degenerate photon pairs in near-infrared and mid-infrared spectral ranges. In this work, we demonstrate and characterize the tunable mid-infrared photon source in silver-gallium-sulfide. By angle tuning the phase matching condition of the nonlinear crystal, we can tune the wavelength of the generated photons in mid-infrared from 5.661 to 10.885 μm (from 1766 to 919 cm −1 ), while the correlated photons in the near-infrared are from 747.1 to 702.6 nm. The phase matching curve and the frequency-angular spectrum of mid-infrared photons were experimentally validated by measuring the properties of near-infrared photons. The brightness of the photon pair source was found to be (29.2± 1.1) $$\frac{\text{MHz}}{\text{mW}}$$ in the low-gain regime, exceeding the values reported in previous studies using silver-gallium-sulfide crystals. The wide tunability of the photon source in the mid-infrared range is important for hyperspectral imaging and sensing, and it is of significant interest for biomedical, chemical, and environmental applications.

DOI
10.1007/s00340-026-08663-8

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Abstract The nonlinear process of spontaneous parametric down-conversion in the silver-gallium-sulfide crystal can generate highly non-degenerate photon pairs in near-infrared and mid-infrared spectral ranges. In this work, we demonstrate and characterize the tunable mid-infrared photon source in silver-gallium-sulfide. By angle tuning the phase matching condition of the nonlinear crystal, we can tune the wavelength of the generated photons in mid-infrared from 5.661 to 10.885 μm (from 1766 to 919 cm −1 ), while the correlated photons in the near-infrared are from 747.1 to 702.6 nm. The phase matching curve and the frequency-angular spectrum of mid-infrared photons were experimentally validated by measuring the properties of near-infrared photons. The brightness of the photon pair source was found to be (29.2± 1.1) MHzmW\frac{\text{MHz}}{\text{mW}}mWMHz​ in the low-gain regime, exceeding the values reported in previous studies using silver-gallium-sulfide crystals. The wide tunability of the photon source in the mid-infrared range is important for hyperspectral imaging and sensing, and it is of significant interest for biomedical, chemical, and environmental applications.
We demonstrate nondegenerate photon pair generation by spontaneous parametric downconversion in a silver gallium sulfide AgGaS2 crystal. By tuning the pump wavelength, we achieve phase matching over a large spectral range. This allows to generate idler photons in the mid-infrared spectral range above a 6 μm wavelength with corresponding signal photons in the visible. Also, we show photon pair generation with broad spectral bandwidth. These results are a valuable step toward the development of quantum imaging and sensing techniques in the mid-infrared.
Many quantum optics applications, especially in sensing and imaging, are based on correlated photon pairs with non-degenerate wavelengths. Often, one photon in an easily accessible spectral range like the visible is measured, whereas the object under test is probed by the correlated partner photon in a different wavelength range [1] . However, in most typically used crystals, the wavelength range of the long-wavelength photon is limited by absorption in the nonlinear medium which prohibits efficient photon-pair generation. Here, we demonstrate photon-pair generation in silver gallium sulfide AgGaS 2 (AGS), a nonlinear crystal with a transparency range spanning from 0.5 μm to 13 μm, which covers the molecular fingerprint region relevant for many spectroscopy applications.
Abstract We report the generation of highly non-degenerate spontaneous parametric down conversion (SPDC) in a novel mid-infrared nonlinear crystal BaGa 2 GeS 6 , and provide a direct experimental comparison with the widely used AgGaS 2 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 µ m. 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 BaGa 2 GeS 6 via SPDC. A quantitative comparison shows that BaGa 2 GeS 6 exhibits approximately twice the nonlinear conversion efficiency of AgGaS 2 , which is in good agreement with values reported in the literature. In addition, BaGa 2 GeS 6 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 BaGa 2 GeS 6 emerges as a highly promising platform for bright and stable sources of correlated photon pairs. These results establish BaGa 2 GeS 6 as a strong candidate for mid-IR quantum spectroscopy, imaging, and gas-sensing applications.
We report an experimental demonstration of wavelength variable generation and detection of photon pairs in the visible and mid-infrared (MIR) regions over a wide spectral range of 2–5 µm via spontaneous parametric downconversion. Despite the recent increase in interest in such a photon-pair source, there have been few detailed evaluations of emitted photons generated via the downconversion process in the low gain regime, due to the lack of suitable single-photon detectors. By changing the angle of the nonlinear crystal, we continuously tune the phase-matching condition and generate photon pairs as signal photons in the wavelength range of 600–965 nm and idler photons in the wavelength range of 1186–4694 nm. We evaluate the generated photon pairs using single and coincidence counts by superconducting nanowire single-photon detectors up to a wavelength of 2 µm and detect the intensity using an InSb detector with a lock-in detection system up to 5 µm. From this analysis, a pair generation rate of 10 5 s − 1 per mW of pump power is experimentally obtained for this wavelength range. This work provides a basis for the realization of applications such as heralded MIR single-photon sources, infrared imaging, and infrared spectroscopy based on quantum technologies in the MIR region.
Spontaneous parametric down-conversion is an essential tool for a quantum light source in the infrared region ranging 2-5 µm for the purpose of material identification, chemical analysis, and gas sensing. So far, photon pairs from the process in a nonlinear crystal have low tunability and a narrow spectral range because of the phase-matching condition. Here, we propose a novel type of spontaneous parametric down-conversion processes that overcomes these challenges, where two photon pairs are simultaneously produced in the visible and infrared regions in periodically poled stoichiometric lithium tantalite. It allows broadband and tunable generation of infrared photon pairs that can be employed as an alternative light source for quantum infrared spectroscopy.