Strong spin-orbit interaction of light in plasmonic nanostructures and nanocircuits

The coupling between the spin and orbital degrees of freedom of photons is usually very weak, but recent studies have shown that this spin-orbit interaction (SOI) can be easily detected in metal structures. Here we show how the SOI of light is enhanced in plasmonic metal nanostructures, explore the underlying mechanism for this effect, and further demonstrate how it could potentially be harnessed for nanophotonic applications. Specifically, we show that the scattering of circularly polarized photons by a single metal nanosphere causes light to propagate along sharply twisted chiral trajectories near the nanosphere, thus revealing a strong SOI in the near field of surface plasmons. We find similar spin-dependent trajectories of light induced by a strong SOI also in the near field of surface plasmons generated on the tip of a metal nanowire. We utilize this strong SOI to for the first time experimentally realize spin sorting of photons in a compact plasmonic nanocircuit. The findings offer insights into how the SOI of light can be enhanced and explored for a new degree of freedom in plasmonic nanocircuits and future spin-controlled nanophotonic devices.