PHASE SINGULARITIES VIA NONLINEAR BEAM-PROPAGATION IN SODIUM VAPOR

The creation of phase singularities in a nonlinear optical system without feedback is observed experimentally after a single-pass of a focused laser beam through a high-density sodium vapor. Their appearance is reproduced in a numerical model which allows for a slight astigmatism of the incoming beam and can be qualitatively understood by the astigmatic deformation of the phase fronts, to first order by means of self-induced astigmatic lensing. Alternatively, the phase singularities can be interpreted to result from the interference of symmetry broken modes whose mutual phases are determined by the astigmatic lens.