Bound States in the Continuum Protected by Reduced Symmetry of Three-Dimensional Open Acoustic Resonators

Bound states in the continuum (BICs) have been demonstrated as a powerful tool for trapping acoustic fields in an acoustic resonator. It has been widely recognized that symmetry-protected (SP) BICs result from symmetry incompatibility of some eigenmodes of a resonator with propagating modes of waveguides. The most typical example of SP BIC is the odd eigenmode of the resonator with the eigenfrequency embedded into the propagating band of even propagating eigenmodes of the waveguide. In this work, we consider a more sophisticated case of an acoustic cuboid resonator that is opened by the attachment of two cylindrical waveguides. We show that BICs can be sustained in an open acoustic resonator with reduced symmetry. For symmetrical positions of waveguides, the eigenmodes of the cuboid can also be classified as SP BICs and show different stability against the shifts of waveguides from the positions of symmetry of the cuboid. We fabricate a series of coupled waveguide resonators and experimentally verify the existence of these BICs by identifying the vanished linewidth of Fano resonance in transmission spectra. Besides, we also show that evanescent modes of waveguides play a role in the formation of BICs in a nonaxisymmetric waveguide-resonator system by tuning the angle θ between two waveguides. Consequently, the eigenmodes remain SP BICs for θ = 0° and θ = 180° but convert into accidental BICs at θ ≈ 85°or θ ≈ 275°. Such accidental BICs are also experimentally verified. Our results enrich the understanding of SP BICs and accidental BICs, and provide alternative methods of routing acoustic waves and designing acoustic devices requiring fine spectrum features, such as filters and sensors.