Global phase diagram of bound states in the continuum

Bound states in the continuum (BICs) in photonic-crystal slabs have been conventionally classified into three types: single-resonance parametric, symmetry-protected, and Friedrich-Wintgen BICs. Here, we show that the singleresonance parametric BICs come from the coupling between the guided resonance (GR) and Fabry-Perot (FP) modes, and the symmetry-protected BICs from the coupling between degenerate GR modes. Hence, the three types of BICs in photonic-crystal slabs can be classified by the three different Friedrich-Wintgen origins. Based on this universal classification, a global phase diagram of BICs can be obtained with each phase identified by the indices of the three different Friedrich-Wintgen-type BICs. When BICs are created or annihilated, a phase transition occurs and is experimentally observed, in which the FP modes play a significant role. Our work shows a clear physical picture on whether BICs exist and how sensitive they are to changes in the parameter space, and enables improvements in experiment design and applications.

Automated summary

This study classifies different types of bound states in the continuum (BICs) in photonic-crystal slabs and reveals the origin of these BICs. It shows that single-resonance parametric BICs are formed through the coupling of guided resonance and Fabry-Perot modes, while symmetry-protected BICs are formed through the coupling of degenerate guided resonance modes. A global phase diagram of BICs can be obtained based on this classification, and phase transitions can be observed when BICs are created or annihilated.