POLARIZATION PATTERNS IN ALKALINE VAPORS

The occurrence of polarization instabilities in alkaline vapours and t heir coupling with spatial degrees of freedom are considered. We discu ss experiments in which a laser beam is transmitted through a vapour c ell without external feedback or with feedback by a single mirror. In both cases a lateral separation of the light field into domains of ort hogonal circular polarization is found. Without feedback, relatively s imple structures like rings or two spots are obtained. With feedback, more complicated polarization patterns result from spontaneous symmetr y breaking. We emphasize the crucial role of the polarization elliptic ity of the input beam. Changing this control parameter we observe a tr ansition from positive to negative hexagons in one of the circular pol arization components, while simultaneously the opposite transition occ urs in the other one. The transition is mediated by square patterns oc curring for linear input polarization. This metamorphosis is governed by symmetry principles and therefore provides an example of the ease o f influencing the symmetry properties of systems consisting of multile vel atoms interacting with a vector light field.