Topologically protected broadband rerouting of propagating waves around complex objects

Zeki Hayran,Francesco Monticone,Seyyed Ali Hassani Gangaraj

doi:10.1515/nanoph-2019-0075

Abstract

Abstract Achieving robust propagation and guiding of electromagnetic waves through complex and disordered structures is a major goal of modern photonics research, for both classical and quantum applications. Although the realization of backscattering-free and disorder-immune guided waves has recently become possible through various photonic schemes inspired by topological insulators in condensed matter physics, the interaction between such topologically protected guided waves and free-space propagating waves remains mostly unexplored, especially in the context of scattering systems. Here, we theoretically demonstrate that free-space propagating plane waves can be efficiently coupled into topological one-way surface waves, which can seamlessly flow around sharp corners and electrically large barriers and release their energy back into free space in the form of leaky-wave radiation. We exploit this physical mechanism to realize topologically protected wave-rerouting around an electrically large impenetrable object of complex shape, with transmission efficiency exceeding 90%, over a relatively broad bandwidth. The proposed topological wave-rerouting scheme is based on a stratified structure composed of a topologically nontrivial magnetized plasmonic material coated by a suitable isotropic layer. Our results may open a new avenue in the field of topological photonics and electromagnetics, for applications that require engineered interactions between guided waves and free-space propagating waves, including for complex beam-routing systems and advanced stealth technology. More generally, our work may pave the way for robust defect/damage-immune scattering and radiating systems.

Highlights

The application of relevant ideas of topological physics from solid-state electronic systems to the realm of photonics and electromagnetics is opening a new landscape of opportunities to realize optical structures that are robust under defects, discontinuities, and deformations [1,2,3]
The broadband operation of the topological wave rerouting scheme can be appreciated in Figure 4A–C: high transmission is obtained at all frequencies within the bulk-mode bandgap for angles at which the dominant transverse wavenumber of the incident plane-wave beam matches the momentum of the topological surface mode
We have discussed how this exciting wave-rerouting behavior is enabled by the ability to control the propagation and radiation properties of one-way, topologically protected, leaky surface waves, supported by the surface of a topological photonic insulator, realized here in the form of a magnetically biased plasma

Summary

Introduction

The application of relevant ideas of topological physics from solid-state electronic systems to the realm of photonics and electromagnetics is opening a new landscape of opportunities to realize optical structures that are robust under defects, discontinuities, and deformations [1,2,3]. With few exceptions [10, 11], far, topological surface waves have been typically studied in “closed” systems, in which the surface states lie within the bulk-mode bandgaps of the surrounding media and cannot couple to free-space radiation. It would be highly desirable to bridge the gap between free-space propagating waves (the continuous plane-wave spectrum supported by the surrounding environment) and topological surface states confined and guided on a given structure This possibility may extend the realm of applications of topological wave physics to radiating and scattering devices and systems. We propose a general strategy to couple an incident propagating plane wave to a surface wave with topologically nontrivial properties and with controllable radiation loss We show that such a surface wave can flow around sharp edges and large scatterers without inducing any back-scattering. The proposed strategy is drastically different from any schemes based on artificial impedance surfaces, holographic surfaces, and metasurfaces supporting surface waves, which have been used to guide waves around certain smooth objects (e.g. Ref. [19]) but are typically inefficient and inadequate in the presence of large surface defects or sharp corners and bends, yielding strong back-scattering due to the inherent modal bidirectionality

Design and results

Conclusion