Mechanical behaviors are traditionally investigated with closed Hermitian systems. However, the Non-Hermiticity, such as the energy loss or gain, is everywhere when we study an open system that has energy exchange with the environment. In acoustic systems, the loss of sound energy to other types is generally treated as write-off. But if such loss or gain components can be intentionally designed and arranged, we may be able to turn waste into treasure, and realize non-conventional sound modulation such as topological sound energy and information guiding or localization.

Recently, research team led by Dr Jie ZHU, Associate Professor of the PolyU Department of Mechanical Engineering, collaborate with Dr Baile ZHANG’s team from Nanyang Technological University, to bring non-Hermiticity design together with topological insulator in acoustic system. They present the first experimental demonstration of a non-Hermitian route to higher-order topology in an acoustic crystal. Specifically, a two-dimensional acoustic coupled resonator lattice that is incorporated with deliberately introduced losses has been designed and constructed. Through direct acoustic measurements, the sound energy is highly localized in the corners and edges of the topological insulator structure at specific frequencies, which are hallmark features of the higher-order topology. This finding points to a fundamentally new direction beyond the framework of conventional topological physics and offers the possibilities for actively steerable topological wave manipulations. It may help the development of new acoustic sensor, detecting network and system.

This work has been recently published in Nature Communications [“Non-Hermitian route to higher-order topology in an acoustic crystal”, https://www.nature.com/articles/s41467-021-22223-y]. Dr He GAO, current PolyU ME Research Associate, also a PolyU ME PhD graduate, is the first author.