Acoustic bound states in continuum protected by crystalline symmetry

Bound states in continuum (BICs) are highly localized resonant states embedded in the continuum background. Recently, crystalline symmetries have been exploited as a new mechanism in generating BICs. Here we propose crystalline-symmetry–protected BICs in a two-dimensional bilayer phononic crystal. Our system possesses a D4 point group symmetry and provides isolated quadratic degeneracies in momentum space to support defect localized BICs. Such BICs are forbidden to leak to the background due to the symmetry mismatch between the defect and the bulk modes. We also provide a rigorous tight-binding model to demonstrate the crystalline-symmetry–protected BICs. Our findings enrich the formation approach of BICs in phononic crystals and may inspire possible applications related to wave confinement, such as energy harvesting and particle manipulation