Experimental Observation of the Multiple Higher-Order Extensible Topological States in Acoustic Systems

Abstract

Recently, higher-order topological insulators with both gapped edge states and in-gap corner states, have been simulated and observed. However, multiple higher-order extensible topological states have not yet been witnessed. Here, we fabricate two acoustic samples, each consisting of double sandwichlike heterostructures, a sonic crystal with a gapless state is inset between two sonic crystals with opposite topological phases in each heterostructure. We theoretically simulate and experimentally obtain the higher-order extensible topological states, which are induced by the Jackiw-Rebbi mechanism, and also demonstrate the robustness of the higher-order extensible topological states via experiment and numerical simulations. The distributions of the pressure fields of the higher-order extensible topological states localized within a certain range of the vertical direction, rather than at a corner, and decay exponentially in the horizontal direction. At the same time, by adjusting the number of layers of the middle sonic crystal in the double acoustic sandwichlike structures, we witness the multiple higher-order extensible topological states protected by band topology. Although the width of the middle sonic crystal is limited by the size of the sample, we provide a feasible solution for different energy harvesting, and enrich the understanding of higher-order topological states.