Free Extension of Topological States via Double-zero-index Media

TL;DR

This paper introduces a method using double-zero-index media to spatially expand topological states in photonic systems, overcoming traditional confinement limits and enabling more scalable topological devices.

Contribution

The authors propose and experimentally verify a novel approach to extend topological states using double-zero-index media, breaking conventional bulk-edge correspondence.

Findings

Numerical simulations confirm the expansion of topological states.

Microwave experiments demonstrate the practical feasibility.

The approach applies to various wave systems beyond photonics.

Abstract

Topological states, known for their robustness against disorder, offer promising avenues for disorder-resistant devices. However, their intrinsic spatial confinement at interfaces imposes geometric constraints that limit the scalability of topological functionalities. Here, we propose a strategy to overcome this limitation by using double-zero-index media to expand topological interfaces. Although occupying finite space, these media are optically equivalent to infinitesimal points, effectively altering the geometry of topological interfaces and breaking conventional bulk-edge correspondence. This strategy enables the spatial expansion of uniform topological states beyond their native interface, offering new possibilities for topological photonic devices. We have verified this behavior through numerical simulations and microwave experiments in a two-dimensional photonic Su-Schrieffer-Heeger lattice. Our findings offer a universal framework to overcome the inherent dimensional limitations of topological states, with implications extending to general wave systems such as acoustic metamaterials.