High-resolution spectroscopy on a micrometrie layer of molecular vapor

Molecular spectroscopy can potentially achieve very high frequency resolution, but transitions are generally very weak. The scientific community has tried to tackle this problem by increasing the interaction path length using extremely long cells, sophisticated multipass cells and Fabry-Perot cavities. As such, molecular spectroscopic methods do not lend themselves naturally to miniaturization. Compact systems, such as hollow core fiber cells [1] and scattering porous materials [2] have been presented, although in this case the interaction length with the molecular vapor remains macroscopic. Here, we present high resolution selective reflection measurements on NH3 and SFe rovibrational transitions at 10, 6μm. Selective reflection is a linear spectroscopic technique, sensitive to a vapor layer whose size is comparable to the wavelength of optical excitation λ/2π, which in our experiments corresponds to an effective confinement of 1-2μm. Our signals are as narrow as ~1MHz and clearly sub-Doppler (Doppler full width at half maximum is ~90MHz and ~30MHz for NH3 and SF6 respectively).