Topological materials or structures: Origin of higher-order topological states

TL;DR

This paper experimentally investigates the origin of higher-order topological states in acoustic systems, demonstrating that local boundary configurations in hollow structures support topologically protected hinge and corner modes, advancing the understanding of HOTSs.

Contribution

It reveals that local boundary configurations are the source of HOTSs in acoustic structures, supported by experimental evidence in hollow and composite topological structures.

Findings

Hollow acoustic structures support HOTSs without bulk or surface states.

Experimental confirmation of hinge and corner states in designed structures.

Robust HOTSs observed in 2D and 3D composite structures.

Abstract

Higher-order topological states (HOTS) have been extensively investigated in classical wave systems. They do not exist in the band gaps of infinite materials, while exhibit as the in-gap localized modes once the infinite materials are truncated to be the finite structures. Here, we will experimentally reveal the origin of HOTSs in acoustic systems. We design the hollow acoustic structures exclusively composed of hinge and corner resonators. We present the experimental proof that, despite the lack of surfaces and bulks, the hollow acoustic structures can still support the topologically protected hinge and corner states, indicating that the local configurations of boundaries are the sources for the generation of HOTSs. We then get the composite structures by assembling the 2D and 3D topological hollow acoustic structures, and experimentally observe the robust HOTSs in them. Our results provide a fundamental perspective on HOTSs in periodic structures, and we foresee that these findings will pave the way toward designing new topological devices for energy recovering, information processing, non-destructive testing and acoustic sensing.