Atomic vapor confined in a nanoscale geometry: From mesoscopic to collective effects

When many light emitters are subjected to an electromagnetic field with a wavelength λ, they may react collectively to the field. The resonant resulting, light-induced dipole-dipole interactions between atoms should lead to a collective frequency shift of the atomic lines. This shift, unfortunately named the cooperative or collective Lamb shift (CLS) despite its classical nature, depends on the shape of the sample. I will present a new investigation of the origin and validity of the CLS. To do so, we have measured the transmission resonance line shape of a dense hot vapor of potassium atoms confined in a slab with nanometer thickness. Using a complete model, we extract a density-dependent shift and width of the bulk atomic medium resonance (See Fig. 1(a)), deconvolved from the cavity effect [1].