Light self-trapping in a large cloud of cold atoms.

We show that, for a near-resonant propagating beam, a large cloud of cold Rb87 atoms acts as a saturable Kerr medium and produces self-trapping of light. By side fluorescence imaging, we monitor the transverse size of the beam and, depending on the sign of the laser detuning with respect to the atomic transition, we observe self-focusing or self-defocusing, with the waist remaining stationary for an appropriate choice of parameters. We analyze our observations by using numerical simulations based on a simple two-level atom model.