OPTICAL-PATTERN SELECTION BY A LATERAL WAVE-FRONT SHIFT

We report experimental evidence of a series of transitions over differ ent classes of two-dimensional patterns (hexagons, horizontal rolls, v ertical rolls, zigzags) observed in a nonlinear optical system in whic h nonlocal interactions have been introduced by a lateral wave-front s hift. At variance with similar scenarios of transitions observed in fl uid dynamics, here the pattern selection is operated by the nonlocalit y mechanism, and a simple theoretical analysis yields predictions in g ood agreement with the experiment.