Controlling light polarization by swirling surface plasmons

Light polarization is a key aspect of modern optics. Current methods for polarization control utilize birefringence and dichroism of anisotropic materials or of arrays of anisotropically shaped nanostructures. Based on collective optical effects, the resulting components remain much larger than the wavelength of light, which limits design strategies. Here, we present a travelling-wave plasmonic antenna that overcomes this limit using a gold-coated helical nanowire non-radiatively fed with a dipolar aperture nanoantenna. Our non-resonant hybrid nanoantenna enables tightly confined circularly polarized light by swirling surface plasmons on the subwavelength scale and taking advantage of optical spin-orbit interaction. Four closely packed circularly polarized light sources of opposite handedness and tunable intensities are demonstrated. By reaching near-field interaction between neighboring nanoantennas, we obtain a highly miniaturized wave plate whose polarization properties have never previously been demonstrated.