Observation of merged Fabry-Perot bound states in the continuum within an underwater phononic crystal

Resonant modes within phononic crystals exhibit remarkable capabilities for the effective control of acoustic waves. In this study, we examine an underwater system comprising bilayer periodic polymethyl methacrylate rods and investigate the behaviors of Fabry-Perot bound states in the continuum (BICs) by adjusting the interlayer spacing parameter and the side length of rectangular rods. Additionally, we demonstrate the evolutionary trajectories of the Fabry-Perot BICs within a two-dimensional parameter space. During the evolution process, we observe fascinating phenomena such as merging, annihilation, and regeneration of BICs. Notably, the merging of BICs enables the achievement of high Q resonance across a broadened range of geometric parameters. The outcomes of this investigation present more opportunities for the design of robust acoustic devices.