Saturation effects in the sub-Doppler spectroscopy of cesium vapor confined in an extremely thin cell

Saturation effects affecting absorption and fluorescence spectra of an atomic vapor confined in an extremely thin cell (cell thickness L<1 μm) are investigated experimentally and theoretically. The study is performed on the D2 line (λ=852 nm) of Cs and concentrates on the two situations L=λ/2 and L=λ, the most contrasted ones with respect to the length dependence of the coherent Dicke narrowing. For L=λ/2, the Dicke-narrowed absorption profile simply broadens and saturates in amplitude when increasing the light intensity, while for L=λ, sub-Doppler dips of reduced absorption at the line-center appear on the broad absorption profile. For a fluorescence detection at L=λ, saturation induces narrow dips, but only for hyperfine components undergoing a population loss through optical pumping. These experimental results are interpreted with the help of the various existing models and are compared with numerical calculations based upon a two-level modeling that considers both a closed and an open system