\documentclass[11pt]{article}

\usepackage{hrms}


%% *-----------------------------------------------------------------------------------------*

\title{MID-INFRARED SATURATION SPECTROSCOPY FOR VIBRATIONAL FREQUENCY MEASURMENTS AT THE SUB-KHZ LEVEL}

\author{\underline{\FirstName Y. \LastName LIU}, \FirstName N. \LastName CAHUZAC, \FirstName E. \LastName CANTIN, \FirstName O. \LastName LOPEZ, \FirstName D.B.A. \LastName TRAN, \FirstName R. \LastName SANTAGATA, \FirstName A. \LastName AMY-KLEIN, \FirstName M. \LastName MANCEAU, \FirstName B. \LastName DARQUIE}
\address{Laboratoire de Physique des Lasers, Universit\'e Sorbonne Paris Nord, CNRS, Villetaneuse, France}
\author{\FirstName M. \LastName TONNES, \FirstName B. \LastName POINTARD, \FirstName M. \LastName ABGRALL, \FirstName L. \LastName LORINI, \FirstName Y. \LastName LE COQ, \FirstName R. \LastName LE TARGAT, \FirstName D. \LastName XU, \FirstName P.-E. \LastName POTTIE}
\address{LNE-SYRTE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universit\'es, UPMC, Paris, France}
\author{\FirstName H. \LastName ALVAREZ-MARTINEZ}
\address{LNE-SYRTE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universit\'es, UPMC, Paris, France; Real Instituto y Observatorio de la Armada (ROA), C\'adiz, Spain}


%% *-----------------------------------------------------------------------------------------*

%% The macros below will change the page head and foot.
%% Please, do not modify the content of the \fancyhead macro nor the \fancyfoot line.
\usepackage{fancyhdr}
\pagestyle{fancy}
\newcommand{\upcite}[1]{\textsuperscript{\textsuperscript{\cite{#1}}}}
\fancyhead[C]{{\footnotesize \textit{HRMS, 28th Colloquium, Dijon, France,
  Aug. 28 - Sept. 1, 2023}}}
\fancyfoot[C]{}
\renewcommand{\headrulewidth}{0.6pt}
\renewcommand{\footrulewidth}{0.6pt}

%% WARNING: Make sure that this package comes LAST since it re-defines many LaTeX macros.
\usepackage[colorlinks=true,urlcolor=blue,linkcolor=blue,citecolor=blue]{hyperref}
\voffset -.65 cm

%% *-----------------------------------------------------------------------------------------*

\category {14,15}
\begin{document}

\maketitle
\index{QCL}
\index{Frequency comb}
\index{Mid-infrared}




Molecular systems, owing to their numerous degrees of freedom, offer promising perspectives for improving tests of fundamental physics and precision measurements in general\footnote{\label{r1}
\href{https://iopscience.iop.org/article/10.1070/QEL16880/meta}{doi:10.1070/QEL16880},
A. Cournol \textit{et al.}, Quantum Electronics \textbf{49}, 288 (2019)}$^,$\footnote{\label{r2}
\href{https://pubs.acs.org/doi/10.1021/acs.jpclett.2c02434}{doi:10.1021/acs.jpclett.2c02434},
M.R. Fiechter \textit{et al.}, J. Phys. Chem. Lett. \textbf{13}, 42, (2022)}$^,$\footnote{\label{r3}
\href{https://doi.org/10.1140/epjqt/s40507-022-00130-5}{doi:10.1140/epjqt/s40507-022-00130-5},
G. Barontini \textit{et al.}, Quantum Technol. 9, \textbf{12}, (2022)}$^,$\footnote{
\href{https://doi.org/10.1103/PhysRevLett.127.043201}{doi:10.1103/PhysRevLett.127.043201},
J. Lukusa Mudiayi \textit{et al.}, Phys. Rev. Lett. \textbf{127}, 043201, (2021)}$^,$\footnote{
\href{https://doi.org/10.1088/1681-7575/aaa790}{doi:10.1088/1681-7575/aaa790},
J. Fischer \textit{et al.}, Metrologia. 55, R1 (2018)}. In our group at Laboratoire de Physique des Laser, using an optical frequency comb, a 10 $\mu$m quantum cascade laser (QCL) is stabilized at the sub-Hz level to an ultra-stable near infrared reference signal operated at the French metrology institute. This signal is calibrated there to some of the best atomic clocks and transferred through a noise-compensated 26-km long fiber cable of the REFIMEVE infrastructure\footnote{
\href{https://www.refimeve.fr/}{www.refimeve.fr}, REseau FIbr\'e MEtrologique \`a Vocation Europ\'eenne}. This results in a record relative frequency uncertainty of$\ 10^{-14}$ and a 0.1 Hz QCL linewidth\footnote{
\href{https://doi.org/10.1038/nphoton.2015.93}{doi:10.1038/nphoton.2015.93},
B. Argence \textit{et al.}, Nature Photon. \textbf{9}, 456-460 (2015)}.

We have used this ultra-stable and frequency controlled mid-infrared source to perform high resolution spectroscopy on various molecular species of fundamental, atmospheric and astrophysics interest  such as methanol, ammonia or trioxane, with a few kilohertz uncertainties on line positions using sub-Doppler saturated absorption spectroscopy in a multipass-cell\footnote{
\href{https://doi.org/10.1364/OPTICA.6.000411}{doi:10.1364/OPTICA.6.000411},
R. Santagata \textit{et al.}, Optica \textbf{6}, 411 (2019)}.

Here, I will discuss our latest efforts in order to improve these spectroscopic measurements, by realizing cavity enhanced measurements. I will show results on methanol achieving a few hundreds of Hz uncertainties on molecular frequencies, I will then present investigations on various sources of noise currently limiting our signal-to-noise ratio and perspectives for improvements. These results enhance our ability to meet the needs for our ongoing efforts towards studying the variation of fundamental constants\textsuperscript{\ref{r3}} or testing fundamental symmetries, such as the measurement of the tiny parity-violating energy difference
between enantiomers of a chiral molecule\textsuperscript{\ref{r1},\ref{r2}}, via precise molecular spectroscopy. Furthermore, it can contribute to the enrichment and refinement of the HITRAN database\footnote{
\href{https://doi.org/10.1016/j.jqsrt.2021.107949}{doi:10.1016/j.jqsrt.2021.107949},
I. Gordon \textit{et al.}, J. Quant. Spectrosc. Radiat. Transfer \textbf{277}, 107949 (2022)}, which can be highly beneficial for the fields of astronomy\footnote{
\href{https://doi.org/10.1038/326049a0}{doi:10.1038/326049a0},
W. Batrla \textit{et al.}, Nature \textbf{326}, 49-51 (1987)} and atmospheric physics\footnote{
\href{https://doi.org/10.1029/98JD02747}{doi:10.1029/98JD02747},
D. Simpson \textit{et al.}, J. Geophys. Res. \textbf{104} (D7), 8113-8152 (1999)}, providing valuable data for accurate modeling and analysis.


\end{document}
