Expansion and cooling of a bright rubidium three-dimensional optical molasses

We have studied the expansion of a bright rubidium three-dimensional optical molasses realized with an intense laser light. In this regime, in addition to the laser friction force, the atoms strongly repel each other due to the radiation trapping force, which is analogous to the Coulomb force. The experimental observations show that the cloud volume increases linearly in time and that, simultaneously, the cloud temperature decreases following the law (1+t/τ)^-4/3. These time evolutions are explained using a physical picture of Coulomb explosion in a viscous medium.