Growth laws, pinning and localized structures: an experiment in sodium vapor

The nucleation problem in systems with two equivalent or nearly equivalent phases has attracted a lot of attention in equilibrium - and more recently - non-equilibrium physics. In two or more spatial dimensions the dynamics of a nucleation bubble is governed by curvature effects and possibly a ´pressure force´ if the two states are not exactly equivalent. So far, however, there has been no experimental demonstration of this fact in a nonlinear optical system. We are going to address the question of front dynamics experimentally in a 2D nonlinear feedback system. We find a t^1/2 growth law if two homogeneous states coexist due to a symmetry-breaking pitchfork bifurcation. The implications of a slight and a deliberate breaking of the symmetry are also considered.