On-chip mid-infrared spectroscopy without mid-infrared photons

Mid-infrared (mid-IR) spectra revealing the distinct “fingerprint” of molecules are of critical importance for both science and technology in chemistry, physics, materials science, and biology. Mid-IR sensing and spectroscopy provide unique and powerful tools for crucial applications in greenhouse gas monitoring, hazardous emission detection, and medical breath diagnosis. However, the low-photon energy characteristic of the mid-IR spectrum poses significant challenges for both mid-IR light sources and detectors, hindering their compact and cost-efficient applications in various areas. Here, we experimentally demonstrate mid-IR gas spectroscopy without mid-IR photons solely using convenient near-IR optical components. This is realized by generating photon pairs (i.e., a mid-IR photon and a near-IR one) through a spontaneous parametric downconversion in a microscale optical cavity. The generation rate of near-IR photons strongly relies on the absorption of the mid-IR photon due to their quantum correlation. Through interacting with evanescent fields around the microcavity, the absorptive mid-IR resonances of the surrounding gas greatly affect the detected near-IR photons without harassing any mid-IR photon. This scheme presents a cost-effective and compact solution for on-chip quantum gas spectroscopy without requiring expensive mid-IR lasers and detectors or lasers, thereby paving new avenues for applications in environmental monitoring/safety, healthcare, and agriculture.