Polarization degrees of freedom in optical pattern forming systems: alkalimetal vapour in a single-mirror arrangement

The use of polarization degrees of freedom in optimizing the conditions for optical pattern formation is investigated by discussing the particular exa mple of single-mirror feedback systems. It is demonstrated that in a quasi- scalar saturable medium pattern formation may be completely suppressed by t he interplay of saturation and diffusion. This effect takes place for the o ptical pumping nonlinearity between Zeeman ground states in alkaline vapour cells. It is shown that these limitations can be circumvented by introduci ng additional polarization components either in the input beam or in the fe edback beam. The latter is achieved by introducing polarization-changing el ements such as a quarter-wave plate in the feedback loop. In the optimized scheme, pattern formation is obtained with rather low thresholds. Thus, inv estigations with a rather high aspect ratio and far beyond threshold become feasible.