Mid-infrared supermirrors with finesse exceeding 400 000

• Published in: Nature communications Vol. 14; no. 1; p. 7846
• Main Authors: Truong, Gar-Wing, Perner, Lukas W, Bailey, D Michelle, Winkler, Georg, Cataño-Lopez, Seth B, Wittwer, Valentin J, Südmeyer, Thomas, Nguyen, Catherine, Follman, David, Fleisher, Adam J, Heckl, Oliver H, Cole, Garrett D
• Format: Journal Article
• Language: English
• Published: England Nature Publishing Group 06.12.2023; Nature Portfolio
• Subjects: Absorption; Cavity ringdown; Coatings; Electric fields; Environmental science; Gas analysis; Gas sensors; Interference; Molecular beam epitaxy; Near infrared radiation; Single crystals; Spectroscopy; Spectrum analysis; Substrates; Trace gases
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-023-43367-z

For trace gas sensing and precision spectroscopy, optical cavities incorporating low-loss mirrors are indispensable for path length and optical intensity enhancement. Optical interference coatings in the visible and near-infrared (NIR) spectral regions have achieved total optical losses below 2 parts per million (ppm), enabling a cavity finesse in excess of 1 million. However, such advancements have been lacking in the mid-infrared (MIR), despite substantial scientific interest. Here, we demonstrate a significant breakthrough in high-performance MIR mirrors, reporting substrate-transferred single-crystal interference coatings capable of cavity finesse values from 200 000 to 400 000 near 4.5 µm, with excess optical losses (scatter and absorption) below 5 ppm. In a first proof-of-concept demonstration, we achieve the lowest noise-equivalent absorption in a linear cavity ring-down spectrometer normalized by cavity length. This substantial improvement in performance will unlock a rich variety of MIR applications for atmospheric transport and environmental sciences, detection of fugitive emissions, process gas monitoring, breath-gas analysis, and verification of biogenic fuels and plastics.