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

Claudia Triolo,Rosalba Saija,Salvatore Savasta,Alessio Settineri,Salvatore Patanè,Sebastiano Trusso,Nisha Rani Agarwal

doi:10.1038/s41598-019-41781-2

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. 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).

Highlights

Subwavelength holes play an important role in several advanced techniques in nano-optics
We study the spin-orbit interactions (SOI) effects of an evanescent field scattered by an isolated elliptical nanohole in a 88 nm thick Au film by using a near-field scanning optical microscope (SNOM) working in transmission mode
In order to explore the contribution related to the scattered optical field around the elliptical nanohole to the SOI phenomena, the SNOM measurements are performed using two linear polarizers with crossed optical axes: the first (#1) placed in the optical path of the incident field before the sample allows to polarize the incident beam; the second one (#2) filters the electromagnetic field collected by the probe after the interaction with the sample

Summary

Introduction

Subwavelength holes play an important role in several advanced techniques in nano-optics. For linearly polarized incident light, it has been shown that the refracted beam can display a transverse splitting of the two spin components[16] In this context, we study the SOI effects of an evanescent field scattered by an isolated elliptical nanohole in a 88 nm thick Au film by using a near-field scanning optical microscope (SNOM) working in transmission mode. We demonstrate that the geometric anisotropy plays a key role in the near-field SOI and that it is responsible for the generation of a plasmonic vortex mode from a single elongated nanohole. The rotation direction of the vortex (right- or left-hand rotation) depends on the angle θ between the polarization direction of the incident field and the symmetry axes of the ellipse, which induce a spin-dependent splitting in the scattered field and controls its spatial distribution in near-field

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Conclusion