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 vap or cell (10-100 mu m) reveals sub-Doppler features. Indeed, atoms with fast (normal) velocity undergo a short interaction between the cell w alls, 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 ensur ed from the ultralow level of light irradiation (less than or equal to 1 mu W/cm(2) for the Cs D-2 line). The effect, known for a long time in the microwave domain, is demonstrated here in the optical domain, a nd is extended to cell thicknesses large relative to the optical wavel ength.