Backward-emitted sub-Doppler fluorescence from an optically thick atomic vapor

TL;DR

This paper investigates sub-Doppler fluorescence in dense Cs vapor, revealing how surface quenching and normal incidence illumination produce narrow velocity-selective features in backward fluorescence spectra.

Contribution

It demonstrates the conditions under which sub-Doppler features appear in backward fluorescence from dense atomic vapor, highlighting the roles of surface quenching and incident angle.

Findings

Sub-Doppler features are observed in backward fluorescence spectra.

Normal incidence irradiation is essential for sub-Doppler feature visibility.

Surface quenching and short absorption length contribute to narrow spectral features.

Abstract

Literature mentions only incidentally a sub-Doppler contribution in the excitation spectrum of the backward fluorescence of a dense vapor. This contribution is here investigated on Cs vapor, both on the first resonance line (894 nm) and on the weaker second resonance line (459 nm). We show that in a strongly absorbing medium, the quenching of excited atoms moving towards a window irradiated under near normal incidence reduces the fluorescence on the red side of the excitation spectrum. Atoms moving slowly towards the window produce a sub- Doppler velocity-selective contribution, whose visibility is here improved by applying a frequency-modulation technique. This sub-Doppler feature, induced by a surface quenching combined with a short absorption length for the incident irradiation, exhibits close analogies with the narrow spectra appearing with thin vapor cells. We also show that a normal incidence irradiation is essential for the sub-Doppler feature to be observed, while it should be independent of the detection geometry