Plenary Session, Invited Lecture

High-precision molecular spectroscopy referenced to frequency standards at the 10-14 level

A. Amy-Klein1
1Laboratoire de Physique des Lasers, Université Sorbonne Paris Nord, CNRS, Villetaneuse, France

Coauthors : D.B.A. Tran1, R. Santagata1,2, E. Cantin1, O. Lopez1, M. Tønnes, N. Cahuzac1, M. Manceau1, M. Abgrall2, Y. Le Coq2, R. Le Targat2, H. Alvarez-Martinez2, D. Xu2, P.-E. Pottie2, B. Darquié1

1. Laboratoire de Physique des Lasers, Université Sorbonne Paris Nord, CNRS, Villetaneuse, France; 2. LNE-SYRTE, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Paris, France.

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.

In order to reach the higher precision, we need to go beyond a reference to GNSS. In my talk I will show recent developments on frequency dissemination by optical fiber links. They provide the opportunity to transfer an ultrastable frequency reference elaborated in a National Metrological Institute (NMI), where it is controlled to frequency standards, to any physics lab provided they are connected with an optical fiber. This signal can then be used to control the repetition rate of an optical frequency comb, and the spectroscopic laser is then phase-locked to one mode of this comb. This opens the way to SI-traceable high-resolution molecular spectroscopy, as already demonstrated using national metrological networks in France and Italy.

In France, we are building a national metrological network, called REFIMEVE, to disseminate an ultrastable laser at 1,5 µm generated at LNE-SYRTE to more than twenty laboratories over France [1]. It was used at LPL to perform precise spectroscopy in the mid-infrared with Quantum Cascade Lasers (QCL) emitting at 10 µm [2,3]. After frequency stabilisation using the REFIMEVE signal, the QCL exhibits a relative frequency stability lower than 2×10-15 between 1 and 100 s and its absolute frequency is known with an uncertainty below 10-14 thanks to the traceability to the primary standards of SYRTE [4]. The setup allows the QCL to be widely tuneable while maintaining the highest stabilities and accuracies. This is achieved by scanning the near-infrared reference laser over ⁓10 GHz using an electro-optic modulator which in turn allows the QCL to be tuned over ⁓1.4 GHz.

With this stabilised QCL we performed saturated absorption spectroscopy of some C-O stretching vibrational transitions of methanol contained in a multipass cell at low pressure around 1 Pa [3]. We reached a statistical uncertainty at the kHz level on the transition centre frequencies and determined the frequencies with uncertainties as low as 7 kHz limited by systematic effects. Some of the lines we measured were not reported so far and we were also able to resolve for the first time some K-doublets. We also performed precision spectroscopy of trioxane and ammonia. This stabilisation set-up to a remote frequency standard is also a key technology for our on-going measurement of the tiny energy difference between enantiomers of a chiral molecule induced by electroweak interactions, a signature of parity (left-right symmetry) violation [5].

[1] E. Cantin et al, New J. Phys. 23(5), 053027 (2021).

[2] B. Argence et al,  Nature Photonics 9(7), 456–460 (2015).

[3] R. Santagata et al, Optica 6(4), 411–423 (2019).

[4] J. Guena et al, IEEE Trans. on Ultrasonics, Ferroelectrics, and Frequency Control 59(3), 391–409 (2012).