ROLL-HEXAGON TRANSITION IN AN ACTIVE OPTICAL-SYSTEM

We study the formation of hexagonal patterns and the roll-hexagon tran sition in an active nonlinear optical system, namely a two-level laser with an injected signal, which displays distinctive and noteworthy fe atures from the most commonly encountered pattern-forming systems with broken inversion symmetry;such as convective hydrodynamic systems and passive nonlinear optical systems. With respect to these systems, we show here that the roll-hexagon transition is accompanied in the Fouri er space by a spontaneous breaking of the inversion symmetry k-->-k, w hich is a signature of the different physical mechanisms involved in t he elementary process of pattern formation.