Vortex dynamics of counterpropagting laser beams in photorefractive materials

We study vortex patterns of counterpropagating laser beams in a photorefractive crystal, with or without the background photonic lattice. The vortices are effectively planar and have two “flavors” because there are two opposite directions of beam propagation. In a certain parameter range, the vortices form stable equilibrium configurations which we study using the methods of statistical field theory and generalize the Berezinsky–Kosterlitz–Thouless transition of the XY model to the “two-flavor” case. In the nonequilibrium regime, the patterns exhibit an Andronov–Hopf bifurcation which may lead to oscillations (limit cycle), chaos or decay to zero intensity due to radiation losses. We show how to identify various pathways toward instability from intensity patterns, i.e. from experiment.