Non- and nearly hexagonal patterns in sodium vapor generated by single-mirror feedback

In the transmitted beam of a sodium cell with a single feedback mirror highly modulated patterns appear. The patterns possess the dihedral symmetries D6, D2, and D3. More complex patterns consisting of more than one elementary cell show a local hexagonal arrangement. The topological characteristics are very similar to the one of boundary affected patterns predicted in a thin Kerr slice. The instability is due to the competition of diffraction, diffusion, and optical pumping which is influenced by a weak magnetic field. For higher cell-to-mirror distances the observation of an additional length scale indicates a competition between the feedback and a further instability mechanism, probably the counterpropagating beam instability