Multiple-pattern stability in a photorefractive feedback system

We report on the observation of a multiple-pattern stability region in a ph otorefractive single-feedback system. Whereas hexagonal patterns are predom inant for feedback with positive diffraction length we show that a variety of stable non-hexagonal patterns are generated for certain negative diffrac tion lengths. For the same values of the control parameters square, rectang ular, or squeezed hexagonal patterns are found alternating in time. Besides these pure states, we found a number of different mixed-pattern states. We review the linear stability analysis for this system and show that the spe cial shape of the threshold curves in the investigated parameter region giv es a first explanation for the occurrence of a multiple-pattern region.