Imaging and Steering Unidirectional Emission from Nanoantenna Array Metasurfaces

Abstract

In radiofrequency antenna engineering, the array factor made long-distance communication with steerable transmission and receiving possible. At optical frequencies, low-loss signal transmission via free space by using nanoantennas is still in its infancy. Here, we suggest applying the array factor to the optical frequency regime by shaping the radiation pattern of plasmonic metasurfaces featuring nanoantenna arrays. We arrange dipolar gold nanoantennas operating at 785 nm wavelength in wavelength-sized arrays and control the phase that drives the antenna elements. We obtain collimated and unidirectional radiation from this metasurface upon illumination with circularly polarized light, which is not prone to major losses as in common plasmonic waveguide structures. We furthermore demonstrate switching the unidirectional emission to opposite directions with additional beamsteering by modifying the array factor. Our experiment corroborates the evidence for spin–orbit coupling between the helicity of light and suitably designed plasmonic metasurfaces, which can exhibit the spin-Hall effect for light.