Nanofocusing on circularly distributed tapered metallic waveguides by means of plasmonic vortex lenses

We report our experimental results on the nanofocusing effect at the apex of planar nanotips placed at the center of a plasmonic vortex lens (PVL). PVLs are helical gratings that are able to generate surface plasmon polaritons (SPPs) carrying orbital angular momentum. A specific design allows us to couple the PVL with nanostructures placed at its center. The proposed configuration allows a simultaneous nanofocusing effect on four facing planar nanotips, showing efficient condensation of SPPs at the metal\u2013air interface toward the end point of the tips. An optimized fabrication process allows us to prepare high-quality structures with a sharp tip apex. Near-field scanning optical microscopy has been used to demonstrate the nanofocusing effect