Hybrid metalens for ultrawide-angle and high-efficiency focusing of waterborne sound

Acoustic metalenses that enable wide-incident-angle and high-efficiency focusing are in high demand for diverse applications, from underwater imaging to biomedical diagnostics. In this work, we introduce a design paradigm for an acoustic metalenses based on a quadratic phase profile, which can incorporate a synergistic combination of a central metasurface and a peripheral metagrating. By our integrating grating diffraction theory with phase engineering and applying an intelligent optimization algorithm, our design achieves high-efficiency, ultrawide-angle focusing of underwater sound across a continuous angular range from 0\ifmmode^\circ\else\textdegree\fi{} to \ifmmode\pm\else\textpm\fi{}85\ifmmode^\circ\else\textdegree\fi{}, with an average efficiency of 31.72%. Thanks to its wavelength-scale thickness and distinct design principle, this metalens provides a compact platform suitable for integration into advanced acoustic systems with complex functional requirements.