HIGH-PRECISION MID-IR MOLECULAR SPECTROSCOPY WITH TRACEABILITY TO PRIMARY FREQUENCY STANDARDS USING SUB-Hz FREQUENCY COMB-STABILIZED QCLS

High-precision measurements with molecules may refine our knowledge of various fields of physics, from atmospheric and interstellar physics to the standard model or physics beyond it. Many of them can be cast as absorption frequency measurements, particularly in the mid-infrared molecular fingerprint region, creating the need for narrow-linewidth lasers of well-controlled frequency. We present a new technology for precision vibrational spectroscopy using quantum cascade lasers (QCLs)\footnote{Argence et al., \emph{Quantum cascade laser frequency stabilization at the sub-Hz level}, Nature Photon. \textbf{9}, 456 (2015).}. Via an optical frequency comb, a QCL is stabilized at the sub-Hz level on an ultra-stable near infrared reference signal provided by the French metrology institute (SYRTE) to our laboratory and transferred through a 43-km long fiber cable\footnote{W.-K. Lee et al., \emph{Hybrid fiber links for accurate optical frequency comparisons}, Appl. Phys. B \textbf{123}, 161 (2017).}. The stability of the reference is transfered to the QCL, which therefore exhibits a relative frequency stability lower than $2\times10^{-15}$ between 1 and 100~s. Moreover its absolute frequency is known with an uncertainty below $10^{-14}$ thanks to the traceability to the primary standards of SYRTE. The setup allows the QCL to be widely scanned over $\sim1$~GHz while maintaining the highest stabilities and accuracies. We report saturated absorption spectroscopy investigations conducted around 10~$\mu$m on osmium tetroxyde in a Fabry-Perot cavity and methanol in a multipass cell at low pressures ranging from 0.01 to 10 Pa, allowing central frequencies, pressure shifts and broadenings to be determined with record uncertainties. By combining wide tuneability, high sensitivity and resolution, this setup constitutes a key technology for precise spectroscopic measurements with molecules