Sub-wavelength confinement of the orbital angular momentum of light probed by plasmonic nanoantennae resonances

We study how the Orbital Angular Momentum (OAM) of a plasmonic vortex (a surface plasmon-polariton with nonzero topological charge) can be probed by the Localized Surface Plasmon Resonance (LSPR) of rod-shaped nanoantennae. The nanorods are appropriately arranged and integrated within a Plasmonic Vortex Lens (PVL), a plasmonic nanofocusing device that converts a circularly polarized plane wave into a plasmonic vortex. We show that the LSPR of the nanorods is either excited or not, according to the OAM of the plasmonic vortex. In particular, we consider a specific type of PVL (single-arm Archimedean spiral) and we investigate the two cases that can be obtained by illuminating it with circularly polarized light with different handedness, corresponding to different values of the OAM of the generated plasmonic vortex. Four nanorods are placed in the center of the PVL and their response is studied by means of numerical simulations. We then present the fabrication of the simulated device and we discuss its characterization by Scanning Near-field Optical Microscopy (SNOM).