Compact Metaplate with Bound State in the Continuum: From Quasisymmetry to Symmetry.

Building localized states with high quality factors in compact dynamic systems could enhance the performance of wave control devices such as elastic filters and high-precision sensing devices. Here, we report on the theoretical and experimental investigation of symmetry-protected bound states in the continuum (BICs) in a compressed metaplate. The proposed theory establishes a Bessel-zero-directed multipolarization design that enables precise modulation for the frequencies and modes of BICs. Experimental results reveal a critical threshold for symmetric protection, beyond which the polarization mode of BICs still exists despite leakage modes. The measured quality factor of BICs in the cylindrical-shelled metaplate rises to 720 via symmetry translation. The metaplate shows an enhanced performance over conventional defect and topological modes. This work opens a route toward compact elastic devices for applications in fields like on-chip communication, elastic wave filtering, and high-precision sensing within tight environments.