Tunable flatband Friedrich–Wintgen bound states in the continuum in graphene plasmonics

The flatband states, especially flatband quasi-bound states in the continuum (BIC) in periodic graphene nanostructures, are investigated. For a resonant plasmonic waveguide grating consisting of monolayer graphene and a binary dielectric grating with strong modulation, two types of resonant waveguide modes at the second stop band, characterized by a nearly flatband within the entire radiation continuum, are observed. One mode exhibits a high leakage rate, while the other features a low leakage rate and evolves into a symmetry-protected BIC at the band edge. A modified semianalytical Kazarinov–Henry model is developed to elucidate the underlying physics, which provides an excellent explanation of the results and can be applied to any plasmonic waveguide grating system. The model demonstrates that the flatband characteristic arises from strong, wavevector-independent coupling between eigenplasmons propagating in opposite directions. Through the strong interaction between the waveguide mode and a nearby dipolar plasmon on a graphene nanoribbon array, a flatband quasi-Friedrich–Wintgen BIC can be generated. Tuning the structural parameters enables the merging of these two types of BICs. This study holds potential applications in nanophotonics and low-intensity nonlinear optics, such as BIC-based nanophotonic devices exhibiting both tunability and ultra-wide-angle performances.