Mid-IR frequency laser control with a comb for H₂⁺ spectroscopy

¹ Laboratoire Kastler Brossel, Sorbonne Université, ENS, CNRS ; 4 place Jussieu, Case 74, 75005 Paris, France
² Département de Physique, Univ EVRY Université Paris Saclay, rue du père André Jarlan, 91025 Evry, France

^* Corresponding author: laurent.hilico@lkb.upmc.fr

Published online: 18 October 2023

Abstract

We will report our recent progress toward H₂⁺ spectroscopy by use of a SI-referenced Mid-IR source laser. H₂⁺ molecular ions are very interesting candidates to improve the determination of fundamental constants, such as the proton to electron mass ratio m_p/m_e and search for new physics beyond the standard model. At LKB, an erbium fibered frequency comb is phase locked to the LNE-SYRTE frequency standards thanks to the T-REFIMEVE network. By sum frequency generation in a AgGaSe2 crystal between a CO₂ laser and an output of the comb at 1895 nm, a shifted frequency comb centered at 1560 nm is generated. The latter is then mixed with the original one to generate a beatnote used to stabilise the Mid-IR laser. As a first application, a narrow saturated absorption line in formic acid has been extensively studied. Pressure, power and modulation depth shifts and broadenings have been evaluated, leading to a determination of its central frequency at a sub ppt (10^-12) resolution, high enough for H₂⁺ spectroscopy and fundamental constant determination.