Spatial and Temporal Behaviors of Instabilities Generated by Counterpropagating Laser Beams in Rubidium

Pattern formation in nonlinear systems is a very active subject in many areas of physics. For example, in hydrodynamics the arrangement of the convective cells of the Rayleigh-Bernard instability [1] constitutes a model problem for pattern formation. In optics, important advances have been recently obtained in the case of lasers [2]. In contrast, transverse pattern formation in mirrorless optical passive systems [3] is no so wellknown because of the intrinsic complexity of a system without cavity and of the fact that the first experiments were done with pulsed lasers [4]. We present here results obtained with c.w. lasers and rubidium atoms. In particular, the relationship with hydrodynamics observations, suggests the possible existence of some general rule for pattern formation.