Mid-infrared supermirrors with finesse exceeding 400 000.

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.

Direct frequency comb spectroscopy of HCN to evaluate line lists.

We report direct frequency comb spectroscopy of the 2ν1 band of H13CN in the short-wave infrared (λ = 1.56 µm) towards experimental validation of molecular line lists that support observatories like JWST. The laboratory measurements aim to test spectral reference data generated from an experimentally accurate potential energy surface (PES) and an ab initio dipole moment surface (DMS) calculated from quantum chemistry theory. Benchmarking theory with experiment will improve confidence in new astrophysics and astrochemistry inferred from spectroscopic observations of HCN and HNC. Here we describe our instrumentation and initial results using a cross-dispersed spectrometer with a virtually imaged phased array (VIPA).

Doppler-Free Two-Photon Cavity Ring-Down Spectroscopy of a Nitrous Oxide (N2O) Vibrational Overtone Transition.

We report Doppler-free two-photon absorption of N2O at λ = 4.53 µm, measured by cavity ring-down spectroscopy. High power was achieved by optical self-locking of a quantum cascade laser to a linear resonator of finesse F = 22730 , and accurate laser detuning over a 400 MHz range was measured relative to an optical frequency comb. At a sample pressure of p = 0.13 kPa, we report a large two-photon cross-section per molecule of σ 13 ( 2 ) = 8.0 × 10 - 41 cm4 s for the Q(18) rovibrational transition at a resonant frequency of ν 0 = 66179400.8 MHz.

Precision Spectroscopy of Nitrous Oxide Isotopocules with a Cross-Dispersed Spectrometer and a Mid-Infrared Frequency Comb.

As a potent greenhouse gas and an ozone-depleting agent, nitrous oxide (N2O) plays a critical role in the global climate. Effective mitigation relies on understanding global sources and sinks, which can be supported through isotopic analysis. We present a cross-dispersed spectrometer, coupled with a mid-infrared frequency comb, capable of simultaneously monitoring all singly substituted, stable isotopic variants of N2O. Rigorous evaluation of the instrument lineshape function and data treatment using a Doppler-broadened, low-pressure gas sample are discussed. Laboratory characterization of the spectrometer demonstrates sub-GHz spectral resolution and an average precision of 6.7 × 10-6 for fractional isotopic abundance retrievals in 1 s.