Collective self-focusing and atom-optical solitons

We describe collective atom-optical solitons and modulational instability which appear due to mutual self-focusing of light and cold atoms. It is normally impossible to achieve simultaneous focusing since a two-level atom gives self- focusing of light for blue detuning, whereas the dipole potential is attractive for red detuning. Due to saturation effects self-focusing occurs with an attractive dipole potential under conditions of red detuning. We present experimental signatures of this effect observed using cold Cs atoms in a MOT.