Observation of dihedral transverse patterning of Gaussian beams in nonlinear optics

We present experimental evidence that the spontaneous transverse filamentation of a laser beam back reflected onto a thin Kerr optical medium can be strongly affected by the actual boundary conditions, namely the Gaussian shape of the beam. As the filament spacing is made to increase by moving the feedback mirror, well-known hexagonal patterns no longer grow up but rather the laser spot breaks up into lower dihedral symmetry patterns, such as pentagonal, rhombic, or triangular ones, in very good agreement with previous numerical predictions. Further increasing the laser intensity after the symmetry breaking leads to secondary bifurcations. The multistability, itineracy, and turbulence of those structures were also observed.