Hexagonal optical patterns in anisotropic non-linear media

We present experimental observations of hexagonal patterns in strongly anisotropic non-linear optical media. Close to threshold rolls are observed. As the non-linearity is increased a transition from rolls to hexagons, leading finally to a pure hexagonal state, is observed. A mean-field model, in agreement with experiments, shows that the anisotropy locks the far-field orientation of the hexagons, while leaving their rotational symmetry undisturbed.