title: Gradient-index meta-surfaces as a bridge linking propagating waves and surface waves.
abstract: The arbitrary control of electromagnetic waves is a key aim of photonic research. Although, for example, the control of freely propagating waves (PWs) and surface waves (SWs) has separately become possible using transformation optics and metamaterials, a bridge linking both propagation types has not yet been found. Such a device has particular relevance given the many schemes of controlling electromagnetic 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.
authors.forename: Shulin
authors.initials: S
authors.lastname: Sun
authors.forename: Qiong
authors.initials: Q
authors.lastname: He
authors.forename: Shiyi
authors.initials: S
authors.lastname: Xiao
authors.forename: Qin
authors.initials: Q
authors.lastname: Xu
authors.forename: Xin
authors.initials: X
authors.lastname: Li
authors.forename: Lei
authors.initials: L
authors.lastname: Zhou