Abstract

Nonlinear optical materials under optical feedback are ideal subjects for studies of spontaneous pattern formation processes. Beyond a certain instability threshold, counterpropagating beams in liquid crystals, atomic vapors or photorefractives become modulationally unstable against perturbations with a special wave number which can be determined by a linear stability analysis. The nonlinear stage of the growth process determines the geometry of the pattern output. The predominant pattern selected by an uncontrolled, free running nonlinear optical feedback system is a hexagonal one, but also non-hexagonal patterns were reported. In particular, a photorefractive single feedback system is known to produce a rich variety of different spatial patterns, among them hexagons, rolls, squares, rectangles, squeezed hexagons [1] or recently discovered dodecagonal quasipatterns (see Fig. 1). A multiple pattern region can be accessed in experiment, where different patterns can be found for the same parameter values. These patterns coexist or may alternate in time in a non-systematic manner due to diffraction length variations by thermal noise or other external influences.

© 2001 EPS