Spin-Orbit Interaction of Light in Plasmonic Lattices.

In the past decade, the spin-orbit interaction (SOI) of light has been a driving force in the design of metamaterials, metasurfaces, and schemes for light-matter interaction. A hallmark of the spin-orbit interaction of light is the spin-based plasmonic effect, converting spin angular momentum of propagating light to near-field orbital angular momentum. Although this effect has been thoroughly investigated in circular symmetry, it has yet to be characterized in a noncircular geometry, where whirling, periodic plasmonic fields are expected. Using phase-resolved near-field microscopy, we experimentally demonstrate the SOI of circularly polarized light in nanostructures possessing dihedral symmetry. We show how interaction with hexagonal slits results in four topologically different plasmonic lattices, controlled by engineered boundary conditions, and reveal a cyclic nature of the spin-based plasmonic effect which does not exist for circular symmetry. Finally, we calculate the optical forces generated by the plasmonic lattices, predicting that light with mere spin angular momentum can exert torque on a multitude of particles in an ordered fashion to form an optical nanomotor array. Our findings may be of use in both biology and chemistry, as a means for simultaneous trapping, manipulation, and excitation of multiple objects, controlled by the polarization of light.