Broadband Quantum Spectroscopy at the Fingerprint Mid-Infrared Region

Numerous molecules exhibit unique absorption bands in the "fingerprint" infrared (IR) region, allowing chemical identification and detection. This makes IR spectroscopy an essential tool in biomedical diagnostics, sensing, and material characterization. However, measurements in the IR range are challenging due to the relatively poor performance, need for cryogenic cooling, and high cost of IR light sources and photodetectors. Here, we demonstrate that the mid-IR fingerprints of the sample can be revealed from measurements in the near-IR range using conventional silicon photodetectors. Our approach relies on the quantum interference of correlated photon pairs produced in strong frequency nondegenerate parametric downconversion. Our technique simultaneously measures the absorption coefficient and refractive index in the broad "fingerprint" region with high accuracy. As a proof-of-concept, we perform spectroscopy of nitrous oxide gas in the 7.4 to 8.4 mu m wavelength range, with the detection in the 865 to 877 nm range. This work extends the applicability of quantum interferometry to a range of broad practical appeals, such as detection of atmosphere pollutants, breath diagnostics, chemical safety, and others.