Global phase diagram of bound states in the continuum
Peng Hu, Jiajun Wang, Qiao Jiang, Jun Wang, Lei Shi, Dezhuan Han, Z. Q. Zhang, C. T. Chan, Jian Zi
Abstract
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 single-resonance 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.