  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 um, 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.
