Elastic bound states in the continuum with multi-polarization hybridization

Bound states in the continuum (BICs) have garnered increasing interest for their outstanding ability to enhance wave-matter interactions and provide an infinite quality (Q) factor. However, elastic BICs have not been fully explored due to the complex polarization modes that differ from those of electromagnetic and acoustic waves. In this paper, we theoretically, numerically, and experimentally investigate elastic BICs with multi-polarization hybridization between flexural and longitudinal waves. Based on the hybridization, Friedrich-Wintgen (FW) BICs can be realized by degenerating two different-eigenmode resonances, unlike conventional realizations of two identical ones. Further, we demonstrate that the FW and accidental quasi-BICs can coexist in the multi-polarization elastic system by detuning two flexural resonances and simultaneously modulating the destructive interference. Our study reveals the rich properties of elastic BICs with multi-polarization hybridization and offers a method for achieving perfect mode conversion between flexural and longitudinal waves with a tunable Q factor. Elastic bound states in the continuum (BICs) with multipolarization hybridization, distinct from optical and acoustic cases, remain an open area of investigation. This study demonstrates the coexistence of Friedrich–Wintgen and accidental BICs in a multi-polarization elastic system and achieves perfect mode conversion via these BICs.