Near-field imaging of surface-plasmon vortex-modes around a single elliptical nanohole in a gold film.

Light carries both spin and momentum. Spin−orbit interactions (SOIs) of light come into play at the subwavelength scale of nano-optics and nanophotonics, where they allow to control the spatial degrees of freedom of light selecting the spin states of incident photons. However, due to the small momentum carried by photons, the SOIs of light are exceedingly small and their experimental observation is challenging. In order to explore such weak processes, plasmonic metamaterials (nanoparticles or nanostructured thin films that support plasmon resonances) are largely used, thanks to the flexibility of their structural design and the enhanced subwavelength local field. In this context, we discuss about the SOIs effects generated in near-field region by means the excitation of surface plasmon polaritons (SPPs) around a single elliptical nanohole in a gold thin film. The optical setup is a Near-field Scanning Optical Microscope (SNOM) working in transmission mode. Exploiting the rotational symmetry breaking due to the elongated shape of the nanohole, a plasmonic vortex mode is generated by illuminating the hole with an incident light beam without a spin state (linearly polarized beam) able to excite SPPs and localized plasmon resonances. SNOM technique allows to obtain information on both the amplitude and phase of the electromagnetic near-field distribution and, thanks to this feature, a direct observation of the vortex mode is possible. Interestingly, the rotation direction of the vortex (right- or left-hand rotation) depends on the angle that the linear polarization direction forms with the major axis of the nanohole (±45°, respectively). This behaviour can be considered as a counterpart of the photonic spin Hall effect generated in absence of the spin state of the light and caused by the rotational symmetry breaking of the elliptical nanostructure. Results are supported by Finite Element Method (FEM) simulations, which reproduce the plasmonic vortex mode of the scattered field around the nanohole at 2 nm from the sample surface. Due to the geometrical anisotropy of the nanohole, both number and the distribution of the phase singularities change. Especially, 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 optical momentum vector around the nanohole and, hence, of the scattered field.