Steady-state signatures of radiation trapping by cold multilevel atoms
  In this paper, we use steady-state measurements to obtain evidence of radiation trapping in an optically thick a cloud of cold rubidium atoms. We investigate the fluorescence properties of our sample, pumped on opened transitions. The intensity of fluorescence exhibits a non trivial dependence on the optical thickness of the media. A simplified model, based on rate equations self-consistently coupled to a diffusive model of light transport, is used to explain the experimental observations in terms of incoherent radiation trapping on one spectral line. Measurements of atomic populations and fluorescence spectrum qualitatively agree with this interpretation. 
Quentin Baudouin, Nicolas Mercadier and Robin Kaiser
physics.atom-ph