Mid-infrared supermirrors with finesse exceeding 400 000

TL;DR

This paper reports the development of mid-infrared supermirrors with finesse exceeding 400,000 and optical losses below 5 ppm, enabling highly sensitive spectroscopy for environmental and industrial applications.

Contribution

The authors demonstrate substrate-transferred single-crystal MIR interference coatings achieving unprecedented finesse and low optical losses, surpassing previous MIR mirror performance.

Findings

Finesse values from 200,000 to 400,000 near 4.5 μm

Optical losses below 5 ppm (scatter and absorption)

Lowest noise-equivalent absorption in MIR cavity ring-down spectroscopy

Abstract

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.