Nonlocal inverse design of an ultrasonic lens for underwater manipulation of orbital angular momentum

Acoustic orbital angular momentum (OAM) beams hold potential for underwater communications, particle manipulation, and biomedical applications. However, adapting air-based OAM technologies to the underwater environment presents unique challenges. In the underwater environment, the medium cannot be considered acoustically soft, and commonly used ultrasound frequencies have shorter wavelengths compared with air, necessitating new design approaches. Conventional underwater ultrasonic lens design methods often rely on simplified models that neglect nonlocal interactions and diffraction within the structure, therefore leading to suboptimal performance. We introduce a novel nonlocal inverse design method by integrating the full-wave models with the nondominated sorting genetic algorithm II (NSGA-II). This approach diverges from conventional phase-based design by optimizing the physical structure of the lens to account for nonlocal interactions within the material. We experimentally demonstrate the performance when generating high-purity OAM beams underwater. Our experimental results show that this method can generate high-purity OAM beams underwater, achieving over 90% purity for the OAM beams of topological charges from m = 1 to m = 4. This is a significant improvement compared with traditional methods, which typically reach 70–89% purity. These findings highlight the practical applicability of our method for nonlocally designing the ultrasonic lens, paving the way for advancements in beam performance for various applications.