Abstract

The spontaneous formation of periodic spatial patterns is well-known for a variety of nonlinear optical materials, e.g. atomic vapours, liquid crystals (Kerr slices), organic films or photorefractives, where squares and squeezed hexagons were first observed in experiment. Photorefractive materials are well-suited for pattern observation since their intrinsically slow dynamics offers the opportunity to perform real-time measurements and observations. A single-feedback configuration creating two counterpropagating beams in the nonlinear optical medium gives rise to transverse modulational instability above a certain threshold. This instability generally leads to the formation of hexagonal patterns. In the case of the photorefractive feedback system, patterns of non-hexagonal geometry can be excited by changing the distance between the crystal and the feedback mirror [1]. In addition, the powerful tool of spatial filtering can be applied to manipulate the system in a way that e.g. non-hexagonal patterns become dominant in a parameter region where the hexagon is the natural output of the system.

© 2000 IEEE