Transition from boundary- to bulk-controlled regimes in optical pattern formation.
By increasing the Fresnel number F of the cavity of a photorefractive oscillator, we report the transition from a boundary-controlled regime, where the size of the transverse patterns scales as 1/√F, to a bulk-controlled regime where the pattern size is independent of F. In this new regime, the size corresponds to an intrinsic correlation length imposed by diffusion processes within the material. The wave number spectrum of the emitted field has a sharp cutoff, corresponding to the length scale of bulk correlations. Such a new behavior is explained by the wave number dependence of the gain within the instability region