  We demonstrate the generation of far-field propagating optical beams with a
desired orbital angular momentum by using a smooth optical mode transformation
between a plasmonic vortex and free space Laguerre-Gaussian modes. This is
obtained by means of an adiabatically tapered gold tip surrounded by a spiral
slit. The proposed physical model, backed up by the numerical study, brings
about an optimized structure which is fabricated by using highly reproducible
secondary electron lithography technique. Optical measurements of the structure
excellently agree with the theoretically predicted far-field distributions.
This architecture provides a unique platform for a localized excitation of
plasmonic vortices followed by its beaming.
