Sub-Doppler spectroscopy in a thin film of resonant vapor

We report on the spectroscopy in a resonant vapor film, in a regime in whic h the inhomogeneous transit time induces a velocity selection. For cell thi ckness (10-1000 mu m) smaller than the transverse dimensions and for low en ough atomic density, the atomic trajectories are essentially free flights f rom wall to wall. Hence, a single laser beam irradiation is sufficient to o bserve sub-Doppler resonances because of the transient atomic response, whi ch enhances the response of the slowest atoms flying nearly parallel to the wall. The peculiar features of this velocity selection an presented in the linear regime as well as in the case of saturation induced by the optical pumping. Also, we report, in the case of the Cs D-2 line (852 nm), on the p assage from a two-level linear-response regime to an optical pumping regime when increasing the incident intensity. We also describe the influence of several other parameters, including cell thickness and laser beam size and shape, on the experimental line shapes. An extension to Doppler-free polari zation spectroscopy in a thin cell with a single beam, and to saturated abs orption, is also reported. [S1050-2947(99)03105-4].