Broadband high-resolution molecular spectroscopy with interleaved mid-infrared frequency combs

TL;DR

This paper presents a high-resolution, broadband mid-infrared spectroscopic technique using interleaved frequency combs, enabling detailed molecular analysis with unprecedented spectral coverage and resolution in a single measurement.

Contribution

The authors demonstrate a novel dual-frequency-comb system that acquires millions of spectral points across a broad wavelength range with high resolution, resolving narrow molecular lines and detecting trace chemical reactions.

Findings

Resolved molecular spectra with <100 kHz resolution.

Achieved continuous spectral coverage from 3.2 to 5.1 microns.

Detected chemical oxidation products of CS2.

Abstract

Historically, there has been a trade-off in spectroscopic measurements between high spectral resolution, broadband coverage, and acquisition time. Optical frequency combs, initially envisioned for precision spectroscopy of the hydrogen atom in the ultraviolet region, are now commonly used for probing molecular rotational-vibrational transitions throughout broad spectral bands in the mid-infrared with superior accuracy, resolution and speed. Here we demonstrate acquisition of 2.5 million spectral points over the continuous wavelength range of 3.2 to 5.1 microns (frequency span 1200 cm^{-1}, resolution <100 kHz) via interleaving comb-tooth-resolved spectra acquired with a highly-coherent broadband dual-frequency-comb system based on optical subharmonic generation. With the original comb-line spacing of 115 MHz, overlaying spectra with gradually shifted comb lines we fully resolved amplitude and phase spectra of molecules with narrow Doppler-broadened lines, such as {\nu}1+{\nu}3 band of carbon disulfide (CS2) and its isotopologues. Also, we were able to observe the chemical oxidation reaction of CS2 by detecting traces of carbonyl sulfide (OCS) via its asymmetric {\nu}3 stretch band.