  We present scanning near-field images of surface plasmon modes around a
single elliptical nanohole in 88 nm thick Au film. We find that rotating
surface plasmon vortex modes carrying extrinsic orbital angular momentum can be
induced under linearly polarized illumination. The vortex modes are obtained
only when the incident polarization direction differs from one of the ellipse
axes. Such a direct observation of the vortex modes is possible thanks to the
ability of the SNOM technique to obtain information on both the amplitude and
the phase of the near field. The presence of the vortex mode is determined by
the rotational symmetry breaking of the system and it can be considered the
counterpart of the photonic spin Hall effect. Finite element method
calculations show that such a vorticity originates from the presence of nodal
points where the phase of the field is undefined, leading to a circulation of
the energy flow. The configuration producing vortex modes corresponds to a
nonzero total topological charge (+1).
