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Mid-IR spectroscopy with NIR grating spectrometers

Mid-infrared (mid-IR) spectroscopy is a crucial workhorse for a plethora of analytical applications and is suitable for diverse materials, including gases, polymers or biological tissue. However, this technologically significant wavelength regime between 2.5-10$μ$m suffers from technical limitations primarily related to the large noise in mid-IR detectors and the complexity and cost of bright, broadband mid-IR light sources. Here, using highly non-degenerate, broadband photon pairs from bright spontaneous parametric down-conversion (SPDC) in a nonlinear interferometer, we circumvent these limitations and realise spectroscopy in the mid-IR using only a visible (VIS) solid-state laser and an off-the-shelf, commercial near-infrared (NIR) grating spectrometer. With this proof-of-concept implementation, covering a broad range from 3.2$μ$m to 4.4$μ$m, we access short integration times down to 1s and signal-to-noise ratios above 200 at a spectral resolution from 12cm$^{-1}$ down to 1.5cm$^{-1}$ for longer integration times. Through the analysis of polymer samples and the ambient CO$_2$ in our laboratory, we highlight the potential of this measurement technique for real-world applications.