Transition between Positive and Negative Hexagons in Optical Pattern Formation

Abstract

We report the observation of the transition between positive and negative hexagons by using optical pattern formation in a sodium vapor cell with a feedback mirror. We show that because of a secondary instability of a developed hexagon against its spatial ultraharmonics a new pattern (ultrahexagon) appears. Pattern formation for an incident Gaussian light beam takes place in combination with switching effects that result in flowerlike structures. The data from analytical investigations and numerical simulations based on a microscopic theory are in good accordance with experiment.