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arXiv:2110.01511 · Applied Physics Letters

Mid-infrared photon pair generation in AgGaS2

Mohit Kumar, Pawan Kumar, Andres Vega, Maximilian A. Weissflog, Thomas Pertsch, Frank Setzpfandt

Published Dec 13, 2021• 17 citations
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Abstract

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.

DOI
10.1063/5.0074054

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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.
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.
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.
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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.
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.