10.1117/2.1201309.005068 Highly efficient anomalous reflection by an optical metasurface Shulin Sun, Kuang-Yu Yang, Chih-Ming Wang, Ta-Ko Juan, Wei Ting Chen, Chun Yen Liao, Qiong He, Shiyi Xiao, Wen-Ting Kung, Guang-Yu Guo, Lei Zhou, and Din Ping Tsai A gradient metasurface guides a 750–900nm wavelength incident beam to non-mirror-like reflection or a surface wave, controlled by the incident angle, with around 80% efficiency. Photonics research and novel applications require an ability to manipulate light. Much can already be achieved with natural materials, but their permittivity and permeability are limited. However, artificial metamaterials (MTMs), which are made of electromagnetic (EM) microstructures in deep-subwavelength scales, operate as an effective medium with almost arbitrary permittivity and permeability and thus offer greater freedom to control light. Homogeneous MTMs have been shown to offer in- teresting effects such as negative refraction and a perfect lens. 1, 2 Slowly varying inhomogeneous MTMs have separately realized invisibility cloaking for propagating waves (PWs) 3–5 and surface waves (SWs). 6, 7 Recently, the extraordinary light-manipulation abilities of ultrathin MTMs (i.e., metasurfaces) with abruptly varying ma- terial properties have attracted much attention. A V-shaped antenna array metastring supports anomalous reflection and refraction of incident light but has the drawback of low con- version efficiency. 8, 9 In previous work, some of us introduced a new metasurface that functions as the perfect link between PWs and SWs. 10 Unlike with slowly varying inhomogeneous MTMs, 3–7 EM waves do not stay inside ultrathin gradient meta- surfaces for a long time. As a result there is less scattering, phase distortion, and dissipation. The metasurfaces offer great versatility for manipulating light, for instance, for anomalous reflection, 8, 9 an antireflection coating, and a highly efficient sur- face plasmon polariton (SPP) coupler (an SPP is a quasiparticle of light and electron waves). 10 Figure 1. Schematics of the designed sample with a unit cell (in- set) consisting of a gold (Au) nanorod (yellow) and a continuous Au film (yellow) separated by a magnesium fluoride (MgF 2 ) spacer (blue). L, w, d 1 : Length, width, and depth of the nanorod, respectively. L 1; 2 : x, y dimensions of the unit cell, respectively. d 2; 3 : Depth of the MgF 2 spacer and the Au film, respectively. L x;y : x, y dimensions of the repeating unit, respectively. However, all these MTMs either exhibit cross-polarized low- efficiency anomalous reflection/refraction or operate only in the microwave regime. We have designed and fabricated a gra- dient metasurface that can redirect incident light to a single anomalous reflection mode with conserved polarization: see Figure 1. 11 When the incident beam arrives at the metasur- face, the waves are re-radiated from different units with lin- ear reflection phases. They thus form an oblique wavefront: see Figure 2. Such anomalous reflected PWs carry a parallel wave- vector k x D  contributed by the radiation phase gradient of the metasurface. 8–10 For the oblique incident case, the wave-vector of the reflected beam becomes k r x D k 0 sin  i C ; Continued on next page