Beam Steering of Nonlinear Optical Vortices with Phase Gradient Plasmonic Metasurfaces

The generation of photons with spin and orbital angular momentum is of great importance in the fields of classical and quantum optical communications. Recent studies show that optical vortices with on-demand angular momentum can be realized with geometric phase-controlled metasurfaces. However, such optical vortices have two spin-locked orbital angular momentum states, which are difficult to distinguish in the same propagating direction. While the beam steering of the optical vortices can be easily realized in the linear optical regime, it remains elusive in the nonlinear optical counterpart. Here, we propose to generate and spatially separate the spin-locked second-harmonic vortex beams through phase gradient plasmonic metasurfaces. Based on the concept of the nonlinear geometric phase, the fork-type phase distributions are encoded onto the metasurfaces by using gold meta-atoms with a threefold rotational symmetry. Under the pumping of fundamental waves in the near-infrared regime, the spin-locked optical vortices at second-harmonic frequency are generated and then projected to different diffraction orders. The proposed strategy may have important applications in high-dimensional optical information processing.