10.1117/2.1201304.004812 Metalens with convex and concave functionality Shuang Zhang, Xianzhong Chen, Lingling Huang, Holger M ¨ uhlenbernd, Guixin Li, Benfeng Bai, Qiaofeng Tan, Guofan Jin, Cheng-Wei Qiu, and Thomas Zentgraf A new, dual-polarity lens made of metamaterials offers extra flexi- bility for focusing and imaging devices and may find application in integrated nano-optoelectronics. Lenses are the key components of telescopes, microscopes, and cameras and are used by the semiconductor industry in opti- cal lithography to manufacture nano-electronic devices and in- tegrated circuits. A conventional lens, which is usually made of glass or other transparent materials, has a curved surface and a fixed focal length. It is either convex (that is, it converges light to a focused spot) or concave (it diverges light). The surface topol- ogy of a conventional lens provides the necessary phase accu- mulation for bending the light in a desired way. The progress in metamaterials has provided alternative routes for manipulating the propagation of light. Metamaterials are ar- tificial photonic structures with deep-subwavelength building blocks, the so-called meta-atoms or meta-molecules, which play a similar role to the constituent molecules or atoms in natural materials. Benefiting from flexibility in engineering the shape and structure of meta-atoms, researchers have manufactured lenses with various unconventional optical properties, including artificial optical magnetism, invisibility, and negative refractive index. Recently, scientists proposed a new class of metamaterial surfaces (or metasurfaces) capable of generating abrupt phase discontinuity. This effect occurs for transmitted light with a po- larization state orthogonal to that of the incident light. The meta- surface can be designed for linear polarizations, such as the ‘V’-shaped plasmonic antennas for the original implementation of the idea. 1, 2 In our work, we designed the metasurface for circular polarizations by using simple metallic nanorods with carefully designed orientations. 3, 4 In this case, the phase discon- tinuity only depends on the orientation of the nanorods, and consequently our lens represents a robust way of generating Figure 1. Schematic of a metalens with interchangeable polarity and a scanning electron microscopy (SEM) image of the fabricated plas- monic lens. (a) The focusing property of the same metalens can be in- terchangeable between a convex lens and a concave lens by controlling the helicity of the incident light. (b) SEM image of the 2D dual-polarity plasmonic lens (top view). RCP: Right circular polarization. LCP: Left circular polarization. desired phase profiles on the metasurface. More importantly, we can reverse the phase discontinuity by simply flipping the circular polarization of the incident beam. Based on the polarization-switchable phase discontinuity on a metasurface, we designed a dual-polarity planar ‘metalens’ for visible light: see Figure 1(a). 4 The device consists of an ar- ray of metallic nano-rods fabricated on top of a glass substrate: see Figure 1(b). Each rod, having a dimension of 40nm (width) by 200nm (length) by 40nm (height), is deep subwavelength in size and can thus be considered a dipole antenna. The entire rod pattern works like a lens because each rod controls the lo- cal wavefront of the input light beam through its orientation. The rod pattern thus generates a phase deformation of the light wave and can be used for focusing or imaging. Further, when the circular polarization of light is reversed, from a left circular polarization to right circular polarization or vice versa, the phase profile is also reversed. Thus, a concave lens designed for a par- ticular circular polarization would be transformed into a convex lens when illuminated by the opposite circular polarization: see Continued on next page