Mode coupling with Fabry-Perot modes in photonic crystal slabs

Abstract Fabry–Perot (FP) modes are a class of fundamental resonances in photonic crystal (PhC) slabs. Owing to their low quality factors, FP modes are frequently considered as background fields with their resonance nature being neglected. Nevertheless, FP modes can play important roles in some phenomena, as exemplified by their coupling with guided resonance (GR) modes to achieve bound states in the continuum (BIC). Here, we further demonstrate the genuine resonance mode capability of FP modes PhC slabs. Firstly, we utilize temporal coupled-mode theory to obtain the transmittance of a PhC slab based on the FP modes. Secondly, we construct exceptional points (EPs) in both momentum and parameter spaces through the coupling of FP and GR modes. Furthermore, we identify a Fermi arc connecting two EPs and discuss the far-field polarization topology. This work elucidates that the widespread FPs in PhC slabs can serve as genuine resonant modes, facilitating the realization of desired functionalities through mode coupling.