Orbit–orbit photonics: harnessing vortex–trajectory interplay for light manipulation

Light can carry a spin angular momentum, an intrinsic and extrinsic orbital angular momentum, associated with circular polarization, optical vortex beams, and varying beam trajectories, respectively. The interplay between these momenta yields the spin–orbit interaction of light, in which the spin (circular polarization) controls the spatial (orbital) degrees of freedom of light: either the extrinsic (trajectory) or the intrinsic orbital angular momentum (vortex). While the well-known spin–orbit interaction of light plays a crucial role in nano-optics by providing spin-controlled light manipulation, the interaction between the intrinsic and the extrinsic orbital angular momentum— the orbit–orbit interaction of light —has remained elusive. In this interplay, the helical phase fronts of optical vortices control the spatial trajectory of light, giving rise to vortex-dependent shifts of optical beams. We report the orbit–orbit interaction of light in a plasmonic ellipse cavity, whose unique geometry facilitates the interplay when a vortex is considered in one of the foci of the ellipse. In this configuration, the orbit–orbit interaction is achieved by the interplay between the vortex of the source and the ellipse-induced transverse shift of the source beam, positioned at one of the focal points, thus inducing transverse vortex-dependent shifts at the second focal point. Strikingly, the orbit–orbit interaction of light significantly enhances the toolbox available for controlling light by leveraging the manifold orbital angular momentum states for vortex-controlled light manipulation—in contrast to light manipulation based on the spin–orbit interaction, which exploits the binary polarization helicity.