Broadband reflective lens in visible band based on aluminum plasmonic metasurface

We demonstrate a flat optical lens based on plasmonic reflectarray metasurface, which consists of a planar array of hyperbolic-shaped aluminum (Al) nanoantenna separated from an Al ground plane by a SiO₂ spacer. The gradual change in the width of the Al nanoantenna enables unique broadband (400-700 nm) to focus on the visible band because of its hyperbolic reflection-phase profile. The focal length of metalens is quickly decreased with the increase of wavelength in the short wavelength region (400-550 nm), compensating the chromatic aberration in traditional lenses. In long wavelength region (550-700 nm), the focal length has only a slight change, thereby minimizing chromatic aberration. Furthermore, the proposed metalens creates a small focal spot beyond diffraction limit, while maintaining high focusing efficiency. Our method of simple and anisotropic nanoantenna is used to realize wide phase tuning range offers a novel strategy to design braodband metalens, and our metalens has widespread applications in compact camera, telescope, and microscope.