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Momentum paradox in a vortex core

Zambrini, R.;Thomson, L.C.; Barnett, S.M.; Padgett, M.
Journal of Modern Optics 52, 1135-1144 (2005)

The identification of angular momentum lh per photon with optical vortices of charge l appears to require that the field amplitude be zero within a finite distance of the vortex. This, however, is not compatible with the known form of beams such as the Laguerre-Gaussian and Bessel beams. We resolve this paradox by analysing the propagation of a Bessel beam through a small circular aperture, showing that the resulting field has evanescent components.