Mechanism-optimization of high-transmission underwater acoustic metasurfaces

Underwater acoustic metasurfaces face significant challenges in achieving simultaneous high transmission, full phase coverage, and operational robustness, particularly when constructed from high-impedance materials like steel. This work presents a novel mechanism-optimization collaborative design framework that systematically addresses these limitations. The key innovation lies in utilizing decoupled mass-stiffness substructures which independently tailor the effective mass and stiffness of each unit cell. This approach ensures high transmission (> 90 %) and continuous 2π phase modulation over a subwavelength thickness, demonstrating precise wavefront engineering, such as subwavelength-scale focusing. To validate our methodology, a steel prototype was fabricated and experimentally tested. The metasurface employed in experiment exhibits exceptional broadband robustness, with the acoustic pressure modulus at the subwavelength focusing spot (designed for 20 kHz) to be over 1.9 times the background sound field across a bandwidth equivalent to approximately 1/4 of the operating frequency range (from 15,680 to 20,540 Hz). Our robust design methodology establishes a new, practical paradigm for realizing high-performance underwater acoustic metasurfaces, with potential applications in underwater monitoring, energy harvesting, and communication systems.