Abstract

In c.w. resonant transmission spectroscopy of ultra-thin vapor cells (10-100µm thick cells), we observe a characteristic deviation from the expected Doppler-broadened absorption when irradiation is under normal incidence. For very weak incident intensities, narrow sub-Doppler contribution appears on line center, which can be singled out a with FM derivation technique (see Fig. 1). In these rather peculiar vapor "microcells” characterized by an anisotropic 2D geometry, the thickness is much smaller than the atomic dipole free path, so that only atoms moving nearly parallel to the cell windows can reach a steady-state regime: their contribution, which is hence dominant, is basically unsensitive to the Doppler-broadening. Note that the velocity selection is related to $v_z^{\max }=\Omega L$ (Ω: optical coupling rate, L: cell length) so that it can be made arbitrarily narrow.

© 1996 IEEE