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.

Figure 1

A strong SOI in the scattering of circularly polarized photons on a single gold nanosphere. (a) Calculation results for streamlines of the orbital momentum density ${\mathbf{p}}^o$ during the scattering. (b) The surface charge-oscillation wave circulating around the nanosphere showing the orbital AM of the generated SPs. Six segments with equal intervals during one period $T$ are shown. (c) The circular polarization degree $C$ for the field near the nanosphere (solid curve) and the field enhancement (dashed curve). The wavelength is 785 nm, and the particle is a gold nanosphere with a radius $r=80\mathrm{nm}$ embedded in a homogeneous dielectric medium ($n=1.5$).

Figure 2

The SOI and resulting spin-controlled propagation of SPs on a gold nanowire. (a) Schematic illustration for the excitation of SPs on a nanowire using circularly polarized photons. (b) The distributions of surface charges and the streamlines of ${\mathbf{p}}^o$ for the near field on the tip of a gold nanowire with a radius of 80 nm in a homogeneous dielectric medium ($n=1.5$). The wavelength is 785 nm. (c) Sectional view of the electric field distribution on the $x\text{−}y$ plane across the center of the nanowire. The scale bar is $1\mu \mathrm{m}$. The white dashed lines mark the nodes in the periodic zigzag pattern. (d) Field distribution of two lowest order eigenmodes of the nanowire, with the arrows denoting the directions of the electric field. (c) and (d) share the same color bar with normalized color range. (e) Sketch for understanding the field distribution in (c) from the perspective of the ray trajectory.

Figure 3

Spin-dependent directional propagation of SPs in a gold branched nanowire. (a) Scanning electron microscopy (SEM) image of the fabricated gold branched-nanowire structure. The gold nanowire is about 260 nm wide and 150 nm thick. The length of the main wire is about $3.7\mu \mathrm{m}$. (b) The SEM image in (a) overlaid with the simulated near-field distributions for excitations of 785 nm wavelength with different circular polarizations denoted by the circular arrows. A uniform dielectric environment ($n=1.5$) is used for the simulation, and the field distributions are extracted from the central horizontal section of the structure.

Figure 4

An experimental demonstration of spin sorting in a plasmonic nanocircuit. (a) Scattering images for excitation by focused laser beams of 785 nm wavelength with opposite circular polarizations denoted by the circular arrows. The color scale is saturated to clearly show the outputs (the nonsaturated figure is shown in Supplemental Material [20], Sec. XII). (b) Measured intensities from the two output ports of the branched-nanowire structure for varying angle $\theta$ between the optical axis of the quarter-wave plate and the linear polarization of the laser beam. The $\theta =0^o$, 90°, 180°, and 270° in (b) correspond to linear polarization parallel to the main nanowire of the $Y$-branch structure, while $\theta ={45}^o$ and 135° correspond to circularly polarized light with opposite spins as denoted by the circular arrows.