Radiation trapping in a cold atomic gas

We experimentally study radiation trapping of near-resonant light in a cloud of laser-cooled rubidium atoms. Unlike in most previous studies, dealing with hot vapors, collisional broadening is here negligible and Doppler broadening due to the residual atomic velocity is narrower than the homogeneous broadening. This is an interesting new regime, at the boundary between coherent and incoherent radiation transport. We analyze in detail our low-temperature data (quasi-elastic regime) and then provide some experimental evidence for Doppler-based frequency redistribution. The data are compared with an analytical model valid for coherent transport and a Monte Carlo simulation including the Doppler effect.