Near-field imaging of surface-plasmon vortex-modes on a gold film with a single elliptical nanohole

Nanostructured thin films that support plasmon resonances are interesting systems as they allow to investigate exotic optical phenomena related to the excitation of surface plasmon polaritons (SPPs), and to the spin-orbit interactions (SOIs). These optical features are very relevant at the subwavelength scale. SOIs phenomena allow to control the spatial degrees of freedom of light selecting the spin states of incident photons. Spatially inhomogeneous or anisotropic materials enhance the optical effects due the SOIs, leading to the observation of the photonic spin Hall effect (SHE), and of plasmonic vortex modes. In this context, we study the SOI effects of an evanescent field around an isolated elliptical nanohole in an 88nm thick Au film using a near-field scanning optical microscope (SNOM) working in transmission mode. Exploiting the rotational symmetry breaking due to the elongated shape of the nanohole, we generate a plasmonic vortex mode by illuminating the hole with an incident light beam without a spin state (linearly polarized beam). We show that a direct observation of the vortex mode is possible thanks to the ability of the SNOM technique to obtain information on both the amplitude and the phase of the near field. Interestingly, the rotation direction of the vortex (right- or left-hand rotation) depends on the angle between the polarization direction and the axis of the nanohole (±45°, respectively). This behaviour can be considered a photonic SHE generated in absence of the spin state of the light and caused by the rotational symmetry breaking of the nanostructure. This interpretation is supported by Finite Element Method (FEM) simulations, which reproduce the plasmonic vortex mode of the scattered field around the nanohole at 2nm from the sample surface. Due to the geometrical anisotropy of the nanohole, both the number and the distribution of the phase singularities change: when the linear polarization direction of incident field and the symmetry axes are tilted, phase singularity points are odd and the system acquires a topological charge ±1, which generates a spiral-like flow of the Poynting vector around the nanohole and, hence, of the scattered field.