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

TL;DR

This paper demonstrates near-field imaging of surface plasmon vortex modes around an elliptical nanohole in a gold film, revealing polarization-dependent vortex excitation and linking it to symmetry breaking and topological charge.

Contribution

It provides the first direct observation of surface plasmon vortex modes around a nanohole, showing their dependence on polarization and symmetry, and connects these modes to topological charge and energy circulation.

Findings

Vortex modes are induced only when incident polarization differs from ellipse axes.

SNOM technique captures both amplitude and phase of near fields.

Vortex modes have a nonzero topological charge (+1).

Abstract

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).