Elastic bound states in the continuum by acoustoelastic interaction

One of the most widely-held beliefs was that no such thing as a bound state existed in a continuum. Notably, it only lately emerged to disclose such a state in several domains of wave physics. Here we report the presence of elastic quasi-bound states in the continuum (QBICs) by balancing local Fano and Fabry–Pérot resonant states, with vanishing linewidths nearby which indicate BICs. The rational design is made by directly integrating an acoustic cavity into an elastic bar, which enables the two to interact. The proposed continuum elastic bar is in analogy to the Fano–Anderson discrete model, which further satisfies a criteria for both Fano and Fabry–Pérot resonances to obtain optimal QBIC modes. Experiments are also carried out for proof-of-concept purposes to uncover these states with a good agreement. This platform would be versatile in illustrating generic Fano lineshapes and high Q-factors, bringing up a new avenue for using the acoustoelastic nature. We hope that our findings would also contribute to the understanding of quantum-driven phenomena in classical contexts with sharing the comparable underlying wave physics.