Title: Helicity locking in light emitted from a plasmonic nanotaper,
Abstract: Surface plasmon waves carry an intrinsic transverse spin, which is locked to
its propagation direction. Apparently, when a singular plasmonic mode is guided
on a conic surface this spin-locking may lead to a strong circular polarization
of the far-field emission. Specifically, an adiabatically tapered gold nanocone
guides an a priori excited plasmonic vortex upwards where the mode accelerates
and finally beams out from the tip apex. The helicity of this beam is shown to
be single-handed and stems solely from the transverse spin-locking of the
helical plasmonic wave-front. We present a simple geometric model that fully
predicts the emerging light spin in our system. Finally we experimentally
demonstrate the helicity-locking phenomenon by using accurately fabricated
nanostructures and confirm the results with the model and numerical data.