Self-generated cooperative light emission induced by atomic recoil

The interaction of an atomic gas confined inside a cavity containing a strong electromagnetic field is numerically and theoretically investigated in a regime where recoil effects are not negligible. The spontaneous appearance of a density grating (atomic bunching) accompanied by the onset of a coherent, back-propagating electromagnetic wave is found to be ruled by a continuous phase transition. Numerical tests allow us to convincingly prove that the transition is steered by the appearence of a periodic atomic density modulation. Consideration of different experimental relaxation mechanisms induces us to analyze the problem in nearly analytic form, in the large detuning limit, using both a Vlasov approach and a Fokker-Planck description. The application of our predictions to recent experimental findings, reported by Kruse [Phys. Rev. Lett., 91 183601 (2003) ], yields a semiquantitative agreement with the observations.