Acoustic Moiré Flat Bands in Twisted Heterobilayer Metasurface.

Twisted bilayer systems enable a fundamental platform for engineering novel physical phenomena, such as topological phase transitions, polaritons, flat bands, and superconductivity. However, previous reports mainly focused on homobilayer structures, where each layer has identical configurations. In this work, a twisted heterobilayer (tHB) metasurface with strong anisotropy is presented. It is shown that hybridizing the dispersion curves of two different profiles significantly enhances the application potential of bilayer structures. It is observed that when two stacked heterogeneous metasurfaces are rotated to a specific angle, a topological phase transition occurs, which is accompanied by hyperbolic to elliptical wave propagation characteristics. At a specific "magic angle", the dispersion curves of the two layers merge into a flat band with drastically reduced diffraction, enabling sound waves to propagate with minimal energy dissipation. Furthermore, the performance of the tHB system is robust to defects or disorders, manifested by minimal changes before and after introducing line defects into the layers. Our findings overcome the limitations of homogeneous metasurface and provide new ideas for controlling sound-material interactions.