HEXAGONAL OPTICAL-PATTERNS IN ANISOTROPIC NONLINEAR MEDIA

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