Coexistence and interplay of Fabry–Pérot and twin Friedrich–Wintgen bound states in the continuum in a plasmonic double T-cavity

Bound states in the continuum (BICs) are typically investigated in terms of distinct formation mechanisms, such as symmetry-protected (SP), Friedrich–Wintgen (FW), or Fabry–Pérot (FP) BICs. However, their coexistence and mutual interaction within a single plasmonic architecture have not been systematically examined so far. In this work, we show that these distinct BIC classes can coexist and interact in a metal–insulator–metal waveguide incorporating a double T-shaped cavity. Using an analytically tractable Green’s function formalism supported by full-wave finite element simulations, we identify the simultaneous emergence of twin FW-BICs and FP-BICs. Unlike FW-BICs, which are independent of the separation between the two cavities, FP-BICs occur at a discrete set of cavity separations. We show that the interaction between the two types of BICs gives rise to multi-BICs, featuring highly confined, non-radiative modes. We further analyze how breaking the BIC condition leads to Fano-like and plasmon-induced reflection resonances as well as the Dicke effect. The combined analytical–numerical analysis provides physical insight into BIC formation in plasmonic waveguides and underscores the potential of these nanostructures for sensing and integrated optical filtering applications.