The partial or complete confinement of waves in an open system is omnipresent in nature and in wave-based materials and technology. Here, we theoretically analyze and experimentally observe the formation of a trapped mode with perfect mode conversion (TMPC) between flexural waves and longitudinal waves, by achieving a quasi-bound state in the continuum (BIC) in an open elastic wave system. The latter allows a quasi-BIC in a semi-infinite background plate when Fano resonance hybridizes flexural and longitudinal waves and balances their radiative decay rates. We demonstrate that when the Fabry-Pérot resonance of the longitudinal wave is realized simultaneously, the TMPC formed by the elastic BIC approaches infinite quality factor. Furthermore, we show that quasi-BIC can be tuned continuously to BIC through the critical frequency of mode conversion, which offers the possibility of TMPC with an arbitrarily high quality factor. Our reported concept and physical mechanism open new routes to achieve perfect mode conversion with tunable high quality factor in elastic systems.

开放系统中波的部分或完全限制在自然界和基于波的材料和技术中无处不在。在这里，我们通过在开放弹性波系统中实现连续体 (BIC) 中的准束缚态，从理论上分析和实验观察了弯曲波和纵波之间具有完美模式转换 (TMPC) 的陷获模式的形成。当 Fano 共振混合弯曲波和纵波并平衡它们的辐射衰减率时，后者允许在半无限背景板中出现准 BIC。我们证明，当同时实现纵波的法布里-珀罗共振时，由弹性 BIC 形成的 TMPC 接近无限品质因数。此外，我们表明准 BIC 可以通过模式转换的临界频率连续调谐到 BIC，这提供了具有任意高品质因子的 TMPC 的可能性。我们报告的概念和物理机制开辟了新的途径，以在弹性系统中实现具有可调高质量因子的完美模式转换。