Accidental and symmetry-protected bound states in the continuum in a photonic-crystal slab: A resonant-state expansion study

The resonant-state expansion (RSE) provides a precise and computationally cheap tool to find resonant states in complex systems using the optical modes of a simpler system as a basis. We apply the RSE to a photonic crystal slab in order to identify and analyze its bound states in the continuum (BICs). We show that the RSE is a useful and reliable method for not only finding the BICs but also for differentiating between accidental and symmetry-protected BICs, as well as for understanding their formation from the basis modes and evolution with structural and material parameters of the system. The high efficiency of the RSE allows us to track the properties of BICs and other high-quality optical modes, covering the full parameter space of the system in a reasonable time frame.

Automated summary

The resonant-state expansion (RSE) is a precise and computationally efficient tool for identifying resonant states in complex systems, particularly in photonic crystal slabs. It can differentiate accidental and symmetry-protected bound states in the continuum (BICs) and track their properties across the system's parameter space.