Ultrathin flat parabolic reflector based on gradient metasurface

This paper reports an ultrathin flat parabolic reflector to focus a laser beam of 1.47μm wavelength based on gradient metasurface. The metasurface consists of an array of sub-wavelength cross resonators fabricated on a continuous gold film spaced by a layer of SiO2. The cross resonators are arranged in a radially periodic manner following a hyperboloidal phase distribution, which is controlled by varying their arm lengths and widths. Transverse profiles of the laser spot were captured by an IR detector along the propagating direction, showing the minimum spot size to be 167μm at focal point with the focal length of 5.8 cm and the overall reflection efficiency of 44%. The symmetric design in the resonators also renders its performance of polarization independence to the incident light, which was also experimentally verified.