Selection between hexagonal, square and stripe patterns in a polarization instability: an experimental investigation

The paper reports on experimental results on pattern selection in a nonlinear optical system based on a single‐mirror feedback‐scheme. Zeeman pumping in sodium vapor is utilized as optical nonlinearity. Above a certain power threshold the unstructured state with defined polarization becomes simultaneously unstable against a pattern forming and a polarization instability. In the resulting patterns the right‐ and left‐hand circular polarization components of the light field tend to separate in space. The pattern selection depends crucially on the polarization ellipticity of the input beam. Transitions between positive and negative hexagons via stripes or squares are observed. They are determined by the symmetry of the interaction between the spin of the light field and the atomic spin and are considered as experimental demonstrations of general principles of pattern formation.