Compression of a cold atomic cloud by on-resonance laser light

We analyze the light-induced atom-atom interactions in optically thick atom ic clouds and show that, when the laser frequency is on-resonance with the atomic transition, they become attractive. On the basis of this analysis we propose and demonstrate a novel scheme to compress a cold and dense atomic cloud with a short on-resonance laser pulse. The compression force arises from attenuation of the laser light by the atomic cloud. The following free propagation of the atoms shows a lenslike behavior that yields a transient density increase at the focal time, where neither laser nor magnetic held perturbations exist. A cooling pulse, which is applied at the focal Lime of this lens, restores the initial temperature of atoms, and hence the phase space density is increased. Finally, we adopt our compression scheme to a q uasi-steady-state mode by temporally chopping it with the cooling and trapp ing beams of a magnet-optical trap. (C) 1999 Optical Society of America [S0 740-3224(99)01905-0].