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Laser Physics Letters

Generation of spontaneous parametric down conversion from BaGa 2 GeS 6 crystal

Э. С. Зацепин, A V Veselovskiy, Artem Sabanin, D V Badikov, G. Kh. Kitaeva, S. P. Kulik, Anna V. Paterova

Published Dec 24, 2025• 1 citation

Abstract

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.

DOI: 10.1088/1612-202x/ae2df9

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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.

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.

Photon pairs from spontaneous parametric down-conversion (SPDC) are important for a wide range of quantum optics experiments with spectral properties such as their bandwidths often being a crucial concern. Here we show the generic existence of particular phase-matching conditions in quasi-phase-matched KTP, MgO:LN, and SLT crystals that lead to ultra-broadband, widely nondegenerate photon pairs. It is based on the existence of group-velocity-matched, far apart wavelength pairs and, for 2 mm long crystals, results in SPDC bandwidths between 15 and 25 THz for photon pairs with the idler photon in the technologically relevant mid-IR range of 3-5 μm and the signal photon in the near-IR below 1100 nm. We experimentally demonstrate this type of broadband phase matching in ppKTP crystals for photon pairs centered at 800 and 3800 nm and measure a bandwidth of 15 THz. This novel method of generating broadband photon pairs will be highly beneficial for SPDC-based imaging, spectroscopy, refractometry, and optical coherence tomography with undetected mid-IR photons.

Recent years have seen the development of quantum sensing concepts utilizing nonlinear interferometers based on correlated photon pairs generated by spontaneous parametric down-conversion (SPDC). Using SPDC far from frequency degeneracy allows a “division of labor” between the mid-infrared photon for the strongest sample interaction and the correlated near-infrared photon for low-noise detection. The small number of photons provided by SPDC and the resulting inferior signal-to-noise ratio are, however, a limiting factor preventing the wide applicability of the novel sensing concept. Here, we demonstrate a nonlinear interferometer based on pump-enhanced SPDC with strongly improved emission rates while maintaining broadband spontaneous emission. For validation of the concept, we demonstrate high-resolution mid-infrared spectroscopy with near-infrared detection, showcasing improved accuracy. Although the number of mid-infrared photons is about five orders of magnitude smaller than in classical spectrometers, the sensitivity of the quantum spectrometer becomes comparable, marking an essential step toward real-world applications.

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Quantum states of light are opening new paradigm for imaging, enabling imaging with photons that didn’t interact with the object. Quantum imaging methods discussed here explore quantum correlated photon pairs, generated via spontaneous parametric down conversion (SPDC) in the nonlinear crystal (NLC). We demonstrate generation of highly non-degenerate photon pairs, via SPDC in AGS crystal, with the photon interacting with the object in the mid-infrared (MIR) and the non-interacting in the near-infrared spectral region. Results revealed that the wavelength of the interacting photon can be tuned in a wide spectral range, between 5 μm and 11 μm, covering most of "fingerprint" spectral region. This makes quantum imaging with AGS an alternative to more costly FTIR spectrometers.

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.