Realization of multiple topological boundary states in phononic crystals

Acoustic topological insulators that host the topological boundary states, which is insensitive to defects and disorder, have become an important research topic and provide completely new ways to manipulate acoustic waves. However, most of the acoustic topological boundary states generally appear in a single or two band gaps, hindering the applications in multiband acoustic devices. Compared to previous work, in this paper, we experimentally observe the acoustic multiple topological boundary states in four band gaps, including the end (corner) states in one-dimensional (two-dimensional) phononic crystals, based on the acoustic quartic-root topological insulators. These topological boundary states originate from two consecutive square-root procedures, which is similar to the square-root topological insulators. Our paper provides a brand-new approach to achieve the multiple boundary states by simply inserting additional cavities without elaborate designing the structure, which makes the manipulation of the acoustic more flexible.

Automated summary

In this paper, the authors experimentally observe the existence of multiple acoustic topological boundary states in four band gaps, including end states in one-dimensional phononic crystals and corner states in two-dimensional phononic crystals, based on acoustic quartic-root topological insulators. These boundary states originate from two consecutive square-root procedures, similar to square-root topological insulators. The paper provides a simple approach to achieve multiple boundary states by inserting additional cavities, without the need for elaborate structure designing, making acoustic manipulation more flexible.