In non-Hermitian systems, two distinct types of singularities are identified: exceptional points (EPs) and bound states in the continuum (BIC). Exceptional points are characterized by the simultaneous coalescence of eigenvalues and eigenstates, which exhibit phase singularity properties such as heightened sensitivity. The latter is characterized by purely real eigenvalues decoupled from the radiation channel and exhibits an infinite Q-factor. Through the analysis of resonant BICs, we have identified EP-BIC solutions within parity-time (PT) symmetric systems, where the solution condition is defined by a balance of gain and loss. Furthermore, we have discovered the existence of BICs in both symmetric and broken-symmetry phases. The parametric relationship between BICs and EPs in PT-symmetric systems was investigated, emphasizing the evolution and unique features of various BICs throughout the PT phase transition. Our theoretical predictions have been realized in a specifically designed metasurface. This study elucidates the parameter relationship between BICs and EP, as well as the evolution of their characteristics with varying parameters, and reveals the existence of EP-BIC in PT-symmetric systems. Furthermore, the behaviors of the two types of BICs during the PT-symmetric process were further explored, providing theoretical insights into BIC-related research in PT symmetry.
Han et al. (Mon,) studied this question.