Beam Steering of Nonlinear Optical Vortices with Phase Gradient Plasmonic Metasurfaces

The generation of photons with spin and orbital angularmomentumis of great importance in the fields of classical and quantum opticalcommunications. Recent studies show that optical vortices with on-demandangular momentum can be realized with geometric phase-controlled metasurfaces.However, such optical vortices have two spin-locked orbital angularmomentum states, which are difficult to distinguish in the same propagatingdirection. While the beam steering of the optical vortices can beeasily realized in the linear optical regime, it remains elusive inthe nonlinear optical counterpart. Here, we propose to generate andspatially separate the spin-locked second-harmonic vortex beams throughphase gradient plasmonic metasurfaces. Based on the concept of thenonlinear geometric phase, the fork-type phase distributions are encodedonto the metasurfaces by using gold meta-atoms with a threefold rotationalsymmetry. Under the pumping of fundamental waves in the near-infraredregime, the spin-locked optical vortices at second-harmonic frequencyare generated and then projected to different diffraction orders.The proposed strategy may have important applications in high-dimensionaloptical information processing.