Fig. 1: The concept and formation mechanism of different types of BICs. | npj Acoustics

Fig. 1: The concept and formation mechanism of different types of BICs.

From: Theoretical developments and experimental Insights of acoustic and elastic bound states in the continuum

Fig. 1: The concept and formation mechanism of different types of BICs.

a Two-dimensional phononic crystals and Brillouin zone. \({\boldsymbol{k}}\) denotes wave vector with the out-of-plane (\({{\boldsymbol{k}}}_{\perp }\)) and in-plane (\({{\boldsymbol{k}}}_{\parallel }\)) components. b Symmetry-protected BICs (red dot) and accidental BICs (green dots) in the momentum space. The blue shaded area above the sound line (\(\omega =c{{\boldsymbol{k}}}_{\parallel }\)) represents the radiation continuum. c Left panel: Fabry-Pérot BICs. \({\omega }_{0}\) and \(\delta\) represent the resonance frequency and loss coefficient, respectively. \(\varphi\) is the phase shift, while d is the distance of the two resonators. Right panel: Friedrich-Wintgen BICs. \(\omega\) and \(\delta\) with different subscripts represent the resonance frequencies and loss coefficients of the two modes, respectively.

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