Generating high-purity underwater acoustic vortex beam based on bubble-containing soft media

<p indent="0mm">In recent years, vortex acoustic beams carrying orbital angular momentum have demonstrated pivotal application values in various scenarios spanning from acoustic communication to acoustic manipulation. However, the existing research about the vortex acoustic beam generation and manipulation mostly focuses on airborne acoustic. Despite recent advancements in the development of methods utilizing acoustic metamaterials to generate acoustic vortex fields in underwater environments, how to efficiently produce high-purity underwater vortex sound is still an immediate unresolved undertaking. Here we propose and numerically demonstrate a method for the high-purity generation of underwater acoustic vortices based on bubble-containing soft media that is optimized through simulated annealing algorithm. An underwater metamaterial of bubble-containing soft media with slow sound speed characteristics is rationally designed to modulate the transmission phase of underwater sound wave precisely within subwavelength spatial dimension. Using the equivalent medium theory, we theoretically derive the relationship between the equivalent sound speed and structural parameters (bubble filling ratio) of the bubble-containing soft medium. Based on this, a single-layered ultra-thin acoustic metasurface is built to generate underwater acoustic vortices. Furthermore, an optimization method based on the simulated annealing algorithm is developed to inversely design the all structural parameters of the metasurface in a global way and address the problem of low vortex mode purity caused by unit coupling effects in underwater environments. To validate the effectiveness of our designed underwater acoustic metasurface and the developed global optimization method, we numerically simulate the generation of single-mode vortex beams and multiple-mode vortex beams in an underwater waveguide system using a finite element method based on Comsol Multiphysics software. And the global optimization was performed using Comsol with MATLAB co-simulation software. The comparison between the results before and after optimization demonstrates significant improvements in terms of the quality of the generated vortex beam and mode purity. To be specific, the mode purity values of first-order and second-order modes in case of single-mode vortex generation increase from 0.58 and 0.61 to 0.85 and 0.92 respectively after optimization. And the improvement for the multiplexing case is also obvious. Although in the current study, we only demonstrate several simple examples of single-mode vortex beam generation and dual-mode vortex multiplexing, it is apparent that our mechanism is general and has no dependence on the order of vortex beams and mode number, ensuring its application in more complex systems calling for higher-order vortex generation or more-mode multiplexing. Besides, it can be further extended to other systems such as underwater free space. In summary, the designed metasurface built by bubble-containing soft media and optimized by simulated annealing algorithm in our study can effectively generate high-purity underwater acoustic vortices. We anticipate such a metasurface with exceptional advantages in terms of compact size, high mode purity and simple configuration to find important applications in many diverse fields, such as underwater acoustic communication, particle manipulation, and beyond.