Spontaneous generation of a density grating in atomic vapours interacting with a strong electromagnetic field

This study shows that, for appropriate parameter values, a stable density grating can spontaneously emerge and be sustained over arbitrarily long times in a generalized model. The onset of a non-zero bunching is accompanied by a nonzero value of the coherence parameter. This regime, occurring at sufficiently low temperatures, is therefore characterized by both coherence and bunching-values different from zero. In order to shed some light on the role played by the two order parameters, the authors have reconstructed a detailed phase diagram by exploring their dependence on the pump intensity and on the temperature.