Simplified metasurface design for MHz underwater acoustic field manipulation

Acoustic metasurfaces have been widely employed for manipulating audible sound and low-frequency ultrasound in the kHz range. However, their application to underwater high-frequency acoustic fields in the MHz range remains limited due to structural complexity and subwavelength feature constraints. Herein, a simple, broadly applicable, and fabrication-friendly acoustic metasurface is proposed to achieve transmission-based phase modulation of MHz-frequency acoustic waves, enabling continuous phase shifts spanning a 0-π range. By introducing a universal dimensionless area ratio γ as the core control parameter, the metasurface avoids reliance on complex subwavelength structures and realizes precise phase manipulation via a macroscopically tunable parameter. Polymethyl methacrylate (PMMA) is utilized as the substrate material, allowing for low-cost, scalable fabrication via CNC machining with sub-millimeter precision. Finite element analysis (FEA) and experimental results demonstrate a full width at half maximum (FWHM) of 1.3λ for the focal pressure profile, with focal lengths tunable from 0.5 to 5 mm. These findings confirm the metasurface's suitability for manipulating underwater high-frequency acoustic fields in the 1-10 MHz range. The proposed design balances physical mechanism innovation with practical engineering feasibility, offering a readily implementable solution for applications such as biomedical imaging and targeted acoustic therapy, where precise wavefront manipulation in underwater high-frequency regimes is essential.