Collaborative non-local topology optimization of gradient acoustic metasurfaces for broadband extreme reflection modulation

To control the complex energy flow between neighboring microstructures for realizing the extreme acoustic functionality using a large-scale non-local metamaterial, here this paper shows that a collaborative non-local topology-optimization methodology is developed to construct gradient acoustic metasurfaces with smooth boundaries for the broadband large-angle reflection. An inverse-designed non-local acoustic metasurface is numerically and experimentally validated to support the large reflection angles within the low-frequency broadband range while maintaining high efficiency greater than 98.9%. It is further revealed that the strong non-local energy exchange between the multiple adjacent optimized microstructures can enable arbitrary surface impedances, thus obtaining efficient large-angle anomalous reflection from 52° to 90°. The present collaborative non-local topology-optimization methodology is not only applicable to airborne acoustic waves, but also can be extended to the customization of efficient wave functionalities of elastic and underwater acoustic metasurfaces.