Coherent Doppler narrowing in a thin vapor cell: Observation of the Dicke regime in the optical domain

Single light beam transmission spectroscopy in an ultrathin dilute vapor cell (10–100 μm) reveals sub-Doppler features. Indeed, atoms with fast (normal) velocity undergo a short interaction between the cell walls, and are shown to be less absorbing because they have not reached their steady state of interaction with light. The effect is observed in the linear-response regime with a simple two-level system, as ensured from the ultralow level of light irradiation (<~1μW/cm² for the Cs D₂ line). The effect, known for a long time in the microwave domain, is demonstrated here in the optical domain, and is extended to cell thicknesses large relative to the optical wavelength.