  We demonstrate that nanostructures carefully designed on both sides of a thin
suspended metallic membrane couple light into a chiral near field and transmit
vortex beams through a central aperture that connects the two sides of the
membrane. We show how far-field orbital angular momentum (OAM) indices can be
tailored through nanostructure designs. We reveal the crucial importance of OAM
selection rules imposed by the central aperture and derive OAM summation rules
in perfect agreement with experimental data.
