Merging mechanical bound states in the continuum in high-aspect-ratio phononic crystal gratings.

Mechanical bound states in the continuum (BICs) present an alternative avenue for developing high-frequency, high-Q mechanical resonators, distinct from the conventional band structure engineering method. While symmetry-protected mechanical BICs have been realized in phononic crystals, the observation of accidental mechanical BICs—whose existence is independent of mode symmetry and tunable by structural parameters—has remained elusive. This challenge is primarily attributed to the additional radiation channel introduced by the longitudinal component of elastic waves. Here, we employ a coupled wave theory to predict and experimentally demonstrate mechanical accidental BICs within a high-aspect-ratio gallium arsenide phononic crystal grating. We observe the merging process of accidental BICs with symmetry-protected BICs, resulting in reduced acoustic radiation losses compared to isolated BICs. This finding opens up new possibilities for phonon trapping using BIC-based systems, with potential applications in sensing, transduction, and quantum measurements. Mechanical bound states in the continuum (BICs) offer a novel approach for realizing high-frequency, high-Q mechanical resonators. Here, the authors theoretically predict and experimentally demonstrate mechanical accidental BICs in a high-aspect-ratio phononic crystal grating, and their merging with symmetry-protected BICs. [ABSTRACT FROM AUTHOR]