Observation of the Orbit–Orbit Interaction of Light in Plasmonics

Light can carry three types of angular momenta: a spin angular momentum, an intrinsic orbital angular momentum (OAM), and an extrinsic OAM [1]. These momenta are associated with the handedness of the circular polarization, optical vortex beams with helical phase fronts, and varying beam trajectories, respectively [1]. The interplay between these momenta yields the spin–orbit interaction (SOI) of light [2], in which the spin (circular polarization) controls the spatial (orbital) degrees of freedom of light: either the extrinsic OAM (trajectory) or the intrinsic OAM (vortex). By providing a toolbox for spin-controlled light manipulations, the SOI plays a crucial role in the new reality of nano-optics. However, while the SOI of light has been studied extensively [2], the interaction between the intrinsic OAM and the extrinsic OAM—the orbit–orbit interaction (OOI) of light—has remained elusive. In this nontrivial interplay, the helical phase fronts of optical vortices control the spatial trajectory of light, which gives rise to vortex-dependent shifts of optical beams. Strikingly, the OOI of light significantly enhances the toolbox available for controlling light by leveraging the manifold OAM states for vortex-controlled light manipulation, in contrast to SOI-based light manipulation [2], which exploits the binary polarization helicity.