Gap‐surface plasmon metasurfaces for broadband circular‐to‐linear polarization conversion and vector vortex beam generation

The ability to control and manipulate the polarization state of light is of crucial importance in many modern optical applications ranging from quantum technologies to biomedical sciences. Here, an ultrathin quarter‐wave plate (QWP) with a gap‐surface plasmon metasurface is designed, fabricated, and experimentally demonstrated, allowing for broadband and efficient conversion between circular and linear polarizations with ≈85% average reflectance across a 200 nm wide bandwidth in the near‐infrared range (750–950 nm). Based on the QWP design, a general method is further derived to generate vector vortex beams (VVBs) that possess spatially varied distributions of the polarization vector and carry specified orbital angular momentums by using space‐variant QWP unit cells. The fabricated metasurface exhibits highly efficient VVB generation over a wavelength range from 750 to 950 nm, with average efficiencies of ≈72% and ≈68% for the right circularly polarized and left circularly polarized incident light, respectively. The developed approach allows compact, cost‐effective, and high‐performance polarization converters to be realized, paving the way for the ultimate miniaturization of optical devices with arbitrary control of light fields.