Observation of a higher-order topological bound state in the continuum

TL;DR

This paper experimentally demonstrates that higher-order topological insulators can host symmetry-protected corner states that remain localized without hybridizing with bulk states, even without a bulk bandgap, enabling new light control applications.

Contribution

It introduces the concept of symmetry-protected bound states in the continuum within higher-order topological insulators, expanding their potential functionalities.

Findings

Corner-localized modes are symmetry protected

States do not hybridize with bulk states without a bandgap

Potential for confining light in gapless systems

Abstract

Higher-order topological insulators are a recently discovered class of materials that can possess zero-dimensional localized states regardless of the dimension of the lattice. Here, we experimentally demonstrate that the topological corner-localized modes of higher-order topological insulators can be symmetry protected bound states in the continuum; these states do not hybridize with the surrounding bulk states of the lattice even in the absence of a bulk bandgap. As such, this class of structures has potential applications in confining and controlling light in systems that do not support a complete photonic bandgap.