Creating Composite Vortex Beams with a Single Geometric Metasurface.

Composite vortex beams (CVBs) have attracted considerable interest recently due to the unique optical properties and potential applications. However, these beams are mainly generated using spatial light modulators, which suffer from large volume, high cost, and limited resolution. Benefiting from the ultrathin nature and unprecedented capability in light manipulation, optical metasurfaces provide a compact platform to perform this task. A metasurface approach to creating these CVBs is proposed and experimentally demonstrated. The design is based on the superposition of multiple circularly polarized vortex beams with different topological charges, which is realized based on a geometric metasurface consisting of metallic nanorods with spatially variant orientations. The effects of the initial phases, amplitude coefficients, incident polarization state, and propagation distance on the generated CVBs, which are in good agreement with the theoretical prediction, are experimentally analyzed. This work has opened a new avenue for engineering CVBs with a minimal footprint, which has promising applications ranging from multiple optical traps to quantum science.