Elastic Bound State in the Continuum with Perfect Mode Conversion

TL;DR

This paper demonstrates the theoretical and experimental realization of a quasi-bound state in the continuum in elastic systems, enabling perfect mode conversion with tunable high quality factors for flexural and longitudinal waves.

Contribution

It introduces a novel elastic wave system achieving quasi-BIC with perfect mode conversion, and shows tunability of the mode conversion via critical frequency control.

Findings

Achieved quasi-BIC in an elastic wave system.

Demonstrated perfect mode conversion with high quality factors.

Showed continuous tuning of quasi-BIC to BIC through mode conversion frequency.

Abstract

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-P\'erot 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.