Collapse and revival of a Dicke-type coherent narrowing in a sub-micron thick vapor cell transmission spectroscopy

In a thin cell of dilute vapour, the absorption spectrum exhibits sub-Doppler features due to the relative enhancement of the slow atom contribution, with respect to the transient nature of the interaction with moving atoms. For a two-level system in the linear regime, the narrowest response is predicted to be found for lambda/2 thickness, as an effect of the coherent character of the dipole response as early described by Romer and Dicke (Phys. Rev., 99 (1955) 532) in the microwave regime. We report here on the direct observation of this effect in the optical regime in an ultra-thin vapour cell. This effect is shown to vanish for a thickness equal to., and a revival is observed at 3lambda/2, as expected from the predicted lambda-periodicity. The experiment is performed on the D-1 resonance line of Cs vapour (lambda=894 nm), in a specially designed cell, whose thickness varies locally.