梯度折射率meta-surfaces传播波和表面波之间的联系

The arbitrary control of electromagnetic waves is a key aim of

photonic research. Although, for example, the control of freely

propagating waves (PWs; refs 1–6) and surface waves (SWs;

refs 7–10) 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

rainbows11,12, lensing13–16, beam bending17, deflection18–20, and

even anomalous reflection/refraction21,22. Here, we demonstrate

theoretically and experimentally that a specific gradientindex

meta-surface can convert a PWto a SWwith nearly 100%

efficiency. Distinct from conventional devices such as prism23

or grating24–26 couplers, the momentum mismatch between PW

and SW is compensated by the reflection-phase gradient of

the meta-surface, and a nearly perfect PW–SWconversion 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.