Demonstration of a quantized acoustic octupole topological insulator

TL;DR

This paper reports the experimental realization of a 3D acoustic octupole topological insulator, demonstrating higher-order topological states and phase transitions, advancing the understanding of complex topological phases in classical systems.

Contribution

The study successfully demonstrates the first experimental acoustic octupole topological insulator, overcoming previous challenges and revealing a hierarchy of topological states in 3D metamaterials.

Findings

Observation of 3rd order corner states

Detection of 2nd order hinge states

Identification of topological phase transitions

Abstract

Recently extended from the modern theory of electric polarization, quantized multipole topological insulators (QMTIs) describe higher-order multipole moments, lying in nested Wilson loops, which are inherently quantized by lattice symmetries. Overlooked in the past, QMTIs reveal new types of gapped boundaries, which themselves represent lower-dimensional topological phases and host topologically protected zero-dimensional (0D) corner states. Inspired by these pioneering theoretical predictions, tremendous efforts have been devoted to the experimental observation of topological quantized quadrupole phase in a variety of two dimensional (2D) metamaterials. However, due to stringent requirements of anti-commuting reflection symmetries in crystals, it has been challenging to achieve higher-order quantized multipole moments, such as octupole moments, in a realistic three-dimensional (3D) structure. Here, we overcome these challenges, and experimentally realize the acoustic analogue of a quantized octupole topological insulator (QOTIs) using negatively coupled resonators. We confirm by first-principle studies that our design possesses a quantized octupole topological phase, and experimentally demonstrate spectroscopic evidence of a topological hierarchy of states in our metamaterial, observing 3rd order corner states, 2nd order hinge states and 1st order surface states. Furthermore, we reveal topological phase transitions from higher- to lower-order multipole moments in altered designs of acoustic TIs. Our work offers a new pathway to explore higher-order topological states (HOTSs) in 3D classical platforms.