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 µm 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 µm to 4.4 µm we demonstrate short integration times down to 1 s and signal-to-noise ratios above 200 at a spectral resolution from 12 cm^-1 down to 1.5 cm^-1 for longer integration times. Through the analysis of polymer samples and the ambient CO₂ in our laboratory, we highlight the potential of this measurement technique for real-world applications.