2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/85760The interaction of an atomic gas confined inside a cavity with 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 coherent backpropagation 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 in [Phys. Rev. Lett. {\bf 91}, 183601 (2003)], yields a semi-quantitative agreement with the observations.10 pages, 7 figures. Text format, latex (RevTex) Figure format, encapsulated postscriptOpticsAtomic Physicstext