Two-Photon Excitation Of Rubidium Atoms Inside Porous Glass

We study the two-photon laser excitation to the 5D(5/2) energy level of Rb-85 atoms contained in the interstices of a porous material made from sintered ground glass with typical pore dimensions in the 10-100 mu m range. The excitation spectra show unusual flat-top line shapes, which are shown to be the consequence of wave-vector randomization of the laser light in the porous material. For large atomic densities, the spectra are affected by radiation trapping around the D-2 transitions. The effect of the transient atomic response limited by the time of flight between pores walls appears to have a minor influence in the excitation spectra. It is however revealed by the shortening of the temporal evolution of the emitted blue light following a sudden switch-off of the laser excitation.