Ultra-thin resonant Fresnel metalens for achromatic high-gain acoustic focusing

Abstract

The classical Soret-type acoustic Fresnel zone plate suffers from two major drawbacks: severe chromatic aberration and low energy efficiency. This work proposes a Subwavelength High-gain Achromatic Resonant Plate with a thickness of only 0.43λ0: straight/coiled-up units achieve π phase complementarity and near-total transmission through Fabry–Pérot resonance, while open cavities and radial λ/4 tubes cooperatively provide frequency-adaptive phase compensation to eliminate the focal length–frequency dispersion. Full-wave finite element simulations show that at 9 kHz the peak sound pressure level (SPL) at the focus reaches 160.6 dB, a 2.0 dB improvement over a Soret plate of the same size, with only a small spatial drift of the focus across 8–10 kHz. The polymethyl methacrylate substrate exhibits the lowest thermoviscous loss, with an SPL loss of only 1.86 dB under a ±3% scaling error. Combining compactness, high focusing performance, and manufacturability, it offers a feasible solution for broadband acoustic imaging and directional acoustic energy transmission.