Optical pattern selection by a lateral wave-front shift.

We report experimental evidence of a series of transitions over different classes of two-dimensional patterns ~hexagons, horizontal rolls, vertical rolls, zigzags! observed in a nonlinear optical system in which nonlocal interactions have been introduced by a lateral wave-front shift. At variance with similar scenarios of transitions observed in fluid dynamics, here the pattern selection is operated by the nonlocality mechanism, and a simple theoretical analysis yields predictions in good agreement with the experiment. @S1050-2947~96!10509-6#