Free-Space Skyrmions Radiated from a Geometric Phase Aperture.

The topological properties of optical spin skyrmions provide an additional degree of freedom for data encoding in photonic networks. Although optical spin skyrmions can be realized at subwavelength scales by surface plasmonics, they fail to radiate into free space as information carriers due to spatial confinement to metal-dielectric interfaces. To date, free-space radiative spin skyrmions have relied on cascaded, bulky optical elements. Here, we report a direct approach to generating free-space optical spin skyrmions using a single surface plasmonic device named a plasmonic geometric phase aperture. Distinct from surface-bound modes, the skyrmion textures of the radiated fields are engineered by spin-orbit interaction in metallic nanoslits via Pancharatnam-Berry geometric phases. It transforms a portion of the circularly polarized waves into a vortex beam that, nested with the residual light, forms spin skyrmion fields, which can be experimentally visualized through spin-selective, phase-resolved scanning near-field optical microscopy. Importantly, the spin skyrmions are generated in the intermediate field region several micrometers above the device surface, bridging the crucial spatial gap between the optical near-field and far-field demanded for on-chip interconnection. The findings provide an ideal solution for high-capacity and robust chip-to-chip optical communications using optical skyrmions.