Constrained order in frustrated 2D optical patterns

In 2D optical patterns obtained in a Liquid Crystal Light Valve with optica l feedback, we show a new kind of geometrical frustration which comes from the imposed form of the boundaries. The circular section of the incoming la ser beam presents a symmetry which belongs to the O(2) group, whereas the o ptical feedback selects patterns with a symmetry restrained to a dihedral s ubgroup of O(2). By imposing boundaries which respect the symmetry of the d ihedral group, we lift the frustration and obtain perfectly ordered pattern s.