Quantum criticality in photorefractive optics: vortices in laser beams and antiferromagnets

Abstract

We study vortex patterns in a prototype nonlinear optical system: counterpropagating laser beams in a photorefractive crystal, with or without the background photonic lattice. The vortices are effectively planar and described by the winding number and the "flavor" index, stemming from the fact that we have two parallel beams propagating in opposite directions. The problem is amenable to the methods of statistical field theory and generalizes the Berezinsky-Kosterlitz-Thouless transition of the XY model to the "two-flavor" case. In addition to the familiar conductor and insulator phases, we also have the perfect conductor (vortex proliferation in both beams/"flavors") and the frustrated insulator (energy costs of vortex proliferation and vortex annihilation balance each other). In the presence of disorder in the background lattice, a novel phase appears which shows long-range correlations and absence of long-range order, thus being analogous to spin glasses. An important benefit of this approach is that qualitative behavior of patterns can be known without intensive numerical work over large areas of the parameter space. More generally, we would like to draw attention to connections between the (classical) pattern-forming systems in photorefractive optics and the methods of (quantum) condensed matter and field theory: on one hand, we use the field-theoretical methods (renormalization group, replica formalism) to analyze the patterns; on the other hand, the observed phases are analogous to those seen in magnetic systems, and make photorefractive optics a fruitful testing ground for condensed matter systems. As an example, we map our system to a doped $O(3)$ antiferromagnet with $\mathbb{Z}_2$ defects, which has the same structure of the phase diagram.