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. Most of them can be cast as absorption frequency measurements, creating the need for narrow-linewidth lasers of well-controlled frequency. The mid-infrared ‘molecular fingerprint’ region (wavelength from 3 to 25 µm) is of particular interest, as it hosts many intense spectral signatures of molecular vibrations. However, quantum cascade lasers typically used in this spectral window show substantial free-running frequency fluctuations. Now, researchers of LNE-SYRTE (Observatoire de Paris, CNRS, UPMC Université Paris 6) and of the Metrology, Molecules et Fundamental Tests group of LPL (Université Paris 13 et CNRS) have demonstrated that the excellent stability and accuracy of an ultra-stable near-infrared laser, the signal of which is transferred from SYRTE to LPL through a fibre link, can be copied to a quantum cascade laser emitting around 10 µm using an optical frequency comb. The obtained relative stability and accuracy of 2 x 10−15 and 10−14 exceed those demonstrated to date with quantum cascade lasers by almost two orders of magnitude.
This work paves the way for precision spectroscopic measurements with molecules at the ultimate levels so far only accessible to experiments on atoms, in the visible or near-infrared range. The set-up already allowed molecular absorption frequencies to be measured with state-of-the-art uncertainties, confirming its potential for ultra high precision spectroscopy.
This work is published in Nature Photonics (Quantum cascade laser frequency stabilization at the sub-Hz level, B Argence, B Chanteau, O Lopez, D Nicolodi, M Abgrall, C Chardonnet, C Daussy, B Darquié, Y Le Coq and A Amy-Klein, Nature Photonics 9, 456–460, 2015).