Optical pattern formation in alkali metal vapors: Mechanisms, phenomena and use

The phenomena in optical pattern formation are reviewed using a single mirr or feedback experiment as an example. The nonlinear medium is sodium metal vapor. We discuss the mechanism responsible for the spontaneous formation o f spatial structures and the selection between simple periodic patterns as hexagons and squares. Introducing a polarization changing element in the fe edback loop, quasiperiodic patterns with a twelve-fold rotational symmetry are obtained and their properties are studied by filtering methods in Fouri er space. Finally, we discuss the formation of localized states as spatial solitons due to a self-induced lensing effect. Their interaction is studied and the formation of clusters provides a link between localized states and extended patterns. Genuine optical features, such as the polarization degr ees of freedom, the experimental accessibility of the Fourier space, and th e possibility to control the optical susceptibility of atomic vapors by mea ns of an external magnetic field, provide very powerful tools for investiga ting the principles of pattern formation.