Nonlocal Acoustic Moiré Hyperbolic Metasurfaces.

The discovery of the topological transition in twisted bilayer (tBL) materials has attracted considerable attention in nano-optics. In the analogue of acoustics, however, no such topological transition has been found due to the inherent nondirectional scalar property of sound pressure. In this work, by using a theory-based nonlocal anisotropic design, we transform the in-plane acoustic pressure into a spatially distributed vector field using twisted multilayer metasurfaces. So-called "acoustic magic angle"-related acoustic phenomena occur, such as nonlocal polariton hybridization and the topological Lifshitz transition. The dispersion becomes flat at the acoustic magic angle, enabling polarized excitations to propagate in a single direction. Moreover, the acoustic topological transition (from hyperbolic to elliptic dispersion) is experimentally observed for the first time as the twist angle continuously changes. This unique characteristic facilitates low-loss tuneable polariton hybridization at the subwavelength scale. We also experimentally demonstrate a twisted trilayer acoustic metasurface, and find more possibilities for manipulating acoustic waves. These discoveries not only enrich the concepts of moiré physics and topological acoustics but also provide a complete framework of theory and methodologies for explaining the phenomena we observed. This article is protected by copyright. All rights reserved.