Underwater Willis lens for broadband low-frequency focusing

Broadband underwater sound focusing in the low-frequency range is essential for various applications such as battery-free environmental monitoring and sensing. However, achieving low-frequency underwater focusing typically necessitates bulky, heavy structures that hinder practical deployment. Here, we introduce a three-dimensional underwater lens comprising cavity-based locally resonant asymmetric structures, enabling the efficient manipulation of low-frequency waterborne sound through a densely packed lattice configuration. We experimentally validated its broadband focusing performance over a range of 20-35 kHz. In addition, we observed that our lens exhibits asymmetric backscattering-a distinctive effect arising from its bianisotropic nature-which we term the Willis lens. Unlike conventional underwater lenses that rely on fully filled structures, our design employs cavity-based scatterers, achieving a lighter yet robust focusing performance. With its lightweight, efficient, and reliable design, the Willis lens provides a promising platform for underwater sensor networks and future advancements in on-demand waterborne sound focusing.