TRANSITION BETWEEN POSITIVE AND NEGATIVE HEXAGONS IN OPTICAL-PATTERN FORMATION

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