Bound states in the continuum induced via local symmetries in complex structures

Bound states in the continuum (BICs) defy the conventional wisdom that assumes a spectral separation between propagating waves, that carry energy away, and spatially localized waves corresponding to discrete frequencies. They can be described as resonance states with infinite lifetime, i.e., leaky modes with zero leakage. The advent of metamaterials and nanophotonics allowed the creation of BICs in a variety of systems. Mainly, BICs have been realized by destructive interference between outgoing resonant modes or by exploiting engineered global symmetries that enforce the decoupling of a symmetry-incompatible bound mode from the surrounding radiation modes. Here, we study BICs relying on a different mechanism, namely local symmetries that enforce a field concentration on a part of a complex system without implying any global symmetry. We experimentally implement these BICs using microwaves in a compact one-dimensional photonic network. We demonstrate that such BICs form an infinite ladder in $k$-space and emerge from the annihilation of two topological singularities, a zero and a pole, of the measured scattering matrix. This alternative for achieving BICs in complex wave systems may be useful for applications such as sensing, lasing, and enhancement of nonlinear interactions that require high-$Q$ modes.