Spin–orbit coupling in surface plasmon scattering by nanostructures

The spin Hall effect leads to the separation of electrons with opposite spins in different directions perpendicular to the electric current flow because of interaction between spin and orbital angular momenta. Similarly, photons with opposite spins (different handedness of circular light polarization) may take different trajectories when interacting with metasurfaces that break spatial inversion symmetry or when the inversion symmetry is broken by the radiation of a dipole near an interface. Here we demonstrate a reciprocal effect of spin–orbit coupling when the direction of propagation of a surface plasmon wave, which intrinsically has unusual transverse spin, determines a scattering direction of spin-carrying photons. This spin–orbit coupling effect is an optical analogue of the spin injection in solid-state spintronic devices (inverse spin Hall effect) and may be important for optical information processing, quantum optical technology and topological surface metrology.