Collective effects in light scattering have gained a renewed interest recently with the recognition that they can bias the accuracy of atom-based sensors such as optical clocks by introducing unwanted energy level shifts. The resonant 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.
Here, I will present our recent measurements of the near-resonant transmission of light through a dense vapour of potassium confined in a slab cell with nanometer thickness in order to investigate the origin and validity of the collective Lamb-shift. A complete model including the multiple reflections in the nano-cell accurately reproduces the observed strong asymmetry of the line shape and allows extraction of a density dependent shift of the atomic resonance.
Finally, I will present a new generation of glass nano-cells with super-polished surfaces. These cells are promising tools for revisiting atom-surface interactions with thermal vapours.