Sub-Doppler resonances in free polarization decay in ultrathin gas cells

The nonlinear optical effect of free polarization decay (FPD) in an ultrathin gas cell, the inner thickness of which is less than or on the order of the wavelength of the exciting monochromatic laser radiation propagating orthogonally to the plane-parallel walls of this cell, is studied theoretically. The considered coherent FPD signal is emitted immediately after abrupt termination of this comparatively weak stationary radiation. The sub-Doppler resonances appearing at the central frequencies of the studied quantum transitions in the FPD intensity and energy spectra due to the transit-time relaxation of atoms in the gas cell are detected and studied. It is shown that these resonances and the FPD dynamics strongly depend on the ratio between the inner thickness of the gas cell and the wavelength of the exciting monochromatic radiation. The narrow high–contrast sub-Doppler resonances of FPD observed and studied in a linear optical regime can be used in atomic (molecular) high-resolution spectroscopy and as references for compact frequency standards.