1. Physical concept Conclusions A gradient-index meta-surface is the very bridge to link PWs and SWs. Besides obvious interest in fundamental science, our findings may lead to many practical applications. The high PW-SW conversion efficiency can be used to make a surface plasmon coupler; the complete suppression of specular reflections is useful in antireflection and invisibility applications; redirecting a PW to bounded SWs can be very helpful in light absorption applications. Realizing these ideas, particularly at high frequencies, is exciting and challenging for future projects. References: [1] S. Sun, Q. He, S. Xiao, Q. Xu, X. Li & L. Zhou, Nature Materials 11, 426-431 (2012). Gradient-index meta-surfaces: A bridge linking propagating waves and surface waves Gradient-index meta-surfaces: A bridge linking propagating waves and surface waves Shulin Sun 1,2, Qiong He 1, Shiyi Xiao 1, Qin Xu 1, Xin Li 1, Lei Zhou 1 * 1 State Key Laboratory of Surface Physics, Key Laboratory of Micro and Nano Photonic Structures (MOE) and Physics Department, Fudan University, Shanghai 200433, China 2 National Center for Theoretical Sciences at Taipei (Physics Division) and Department of Physics, National Taiwan University, Taipei 10617, Taiwan * Email: phzhou@fudan.edu.cn The arbitrary control of electromagnetic (EM) waves is a key aim of photonic research. Although the control of freely propagating waves (PW) and surface waves (SW) has separately become possible using transformation optics and metamaterials (MTMs), a bridge linking both propagation types has not yet been found. Such a device has particular relevance given the many schemes of controlling EM waves at surfaces and interfaces, leading to trapped rainbows, lensing, beam bending, deflection, and even anomalous reflection/refraction. Here, we demonstrate theoretically and experimentally that a specific gradient index meta-surface can convert a PW to a SW with nearly 100% efficiency. Distinct from conventional devices such as prism or grating couplers, the momentum mismatch between PW and SW is compensated by the reflection-phase gradient of the meta-surface, and a nearly perfect PW–SW conversion can happen for any incidence angle larger than a critical value. Experiments in the microwave region, including both far-field and near-field characterizations, are in excellent agreement with full-wave simulations. Our findings may pave the way for many applications, including high-efficiency surface plasmon couplers, anti-reflection surfaces, light absorbers, and so on. FDTD and Far-field (FF) characterizations SW is the radiation from a special charge-density wave (CDW). A flat metal + a gradient MTM can support CDW, illuminated by a plane wave. What kind of MTM? [ε M (x)=μ M (x)] or [ε M = const, μ M (x)] SW  CDW Meta-surface Unit cell: metallic H + metal sheet Varying b  local reflection phase  (h)disappears Far-field scattering: (d, f) oblique PW (ξ k 0 ) Near field (NF) characterizations NF scanning demonstrates the generation of SW on the ξ=1.14k 0 meta-surface. Experiment matches well with FDTD simulation. Perfect conversion from PW to SW by gradient MTM. Physical picture: Cherenkov radiation ξ < k 0, radiations from different sub-cells construct an obliquely outgoing beam. > k 0 ξ > k 0, radiations from different sub-cells form a SW rather than a PW. Analogy to Cherenkov effect: Moving charge  CDW Dispersion relation: generalized Snell’s law Dispersion relation: Additional k vector contributed from gradient MTM. Experiments (NF+FF) and FDTD simulations verified this generalized law. ξ > k 0 or θ > θc, PW is converted to SW. Driven SW  SPP SWs generated on meta-surface are not SPP, existing only with the incident PW. A mushroom structure supports eigen SPP-like surface modes in microwaves. Attach a mushroom structure to meta- surface, the driven SWs can couple to SPPs on mushroom. Coupling is robust, insensitive to the incident angle (non-resonant scheme). Source Detector Meta-surface Far-field Measurement