Gas spectroscopy circumvents drawbacks of mid-infrared methods for more compact design

Mid-infrared (IR) spectroscopy has demonstrated a wide-ranging ability to capture molecular “fingerprints” through absorption, emission, or reflection of infrared light, a feature that has become indispensable in diverse fields from greenhouse gas monitoring to medical breath diagnostics. Widespread use, however, continues to be hampered by a lack of compact, cost-effective, and efficient machines. Such issues are driven in part by low photon energies.

Xue et al. developed a mid-IR gas spectroscopy made with convenient near-IR optical components that avoid mid-IR photons. The group formed pairs of mid- and near-IR photons using an approach that capitalizes on absorption of the mid-IR photon due to their quantum correlation. Their device provides an option for quantum sensing in an optical microcavity that uses whispering-gallery mode (WGM) in which the waves travel around the perimeter of the cavity.

Additionally, it does so while removing mid-IR components entirely, which gives it an advantage over traditional mid-IR systems that require multiple light sources and detectors for different analytes.

“By replacing costly mid-IR components with near-IR parts, our work makes mid-IR sensing — which was previously expensive — accessible to labs, industries, and other applications,” said author Wenjie Wan. “Our integration of WGM microcavities provides a blueprint for compact quantum sensors. We solved a longstanding multi-gas detection bottleneck through tuning the signal detection channel, making it feasible to achieve portable multi-analyte sensors.”

By matching mid-IR “idler” photons with the absorption fingerprint of target gases, the group successfully detected the mid-IR absorption peaks of carbon dioxide and ammonia gases using only near-IR “signal” photons.

The group next looks to explore which parameters can boost sensitivity and how to adapt the system for high-resolution spectroscopy, while testing multi-gas simultaneous detection.

Source: “On-chip mid-infrared spectroscopy without mid-infrared photons,” by Boyi Xue, Renhong Gao, Yicheng Zhu, Hengzhe Yan, Jiankun Hou, Xianfeng Chen, Ya Cheng, Jintian Lin, Li Ge, and Wenjie Wan, Applied Physics Reviews (2026). The article can be accessed at https://doi.org/10.1063/5.0291359 .

This paper is part of the Quantum Sensing Collection, learn more here .