Rb optical resonance inside a random porous medium
  We studied resonant laser interaction with Rb atoms confined to the interstitial cavities of a random porous glass. Due to diffusive light propagation, the effect of atomic absorption on the light scattered by the sample is almost entirely compensated by atomic fluorescence at low atomic densities. For higher densities, radiation trapping increases the probability of non-radiative decay via atom-wall collisions. A simple connection of the fluorescence/absorption yield to the sample porosity is given. 
S. Villalba, H. Failache, A. Laliotis, L. Lenci, S. Barreiro and A.   Lezama
physics.atom-ph