Critical foliations.
Critical foliations are special two-dimensional slices (planes of observation) of a three-dimensional optical field in which an infinitesimally small change in the angle of observation produces major qualitative differences in the observed field structure. They are common, but previously unrecognized, features of optical fields that contain vortices. An experimentally realizable, on-axis example of such a foliation is described for a paraxial Gaussian laser beam.
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