Interference traps waves in an open system: bound states in the continuum

Abstract I review the four mechanisms of bound states in continuum (BICs) application microwave and acoustic cavities open to directional waveguides. The most simple are symmetry-protected BICs, which localized inside cavity because orthogonality eigenmodes propagating modes However, general interesting is Friedrich–Wintgen mechanism, when BICs result fully destructive interference outgoing resonant modes. third type Fabry–Perot occurs a double resonator system each can serve as an ideal mirror. Finally, accidental be realized with no symmetry like Sinai billiard eigenmode become orthogonal waveguide accidentally due smooth deformation eigenmode. We also one-dimensional systems occur owing two waves separated by spin or polarization paths Aharonov–Bohm rings. make broad use method effective non-Hermitian Hamiltonian equivalent coupled mode theory, detects finding zero-width resonances.