Quantum and classical effects in non-linear optical patterns

Summary  We discuss spontaneous pattern formation in the radiation field emitted by a degenerate optical parametric oscillator and illustrate various quantum effects in these patterns. Above threshold, we find the presence of perfect photonic correlations in the spatial structure, and we describe a phenomenon that we call «quantum image». We show that these effects are related to the wave and particle aspects of the radiation. Below threshold, we study the critical behaviour of the spatial correlation function of the electric field.