Tunable flatband plasmonic quasi-bound states in the continuum based on graphene-assisted metasurfaces

Bound states in the continuum (BICs) of plasmonic systems offer a powerful method for enhancing light-matter interaction at the nanoscale. The recent emergence of flatband quasi-BICs has alleviated the limitation of the incident angle of the excitation light on generating high-quality-factor (high-Q-factor) resonances, which makes it feasible to produce substantial near-field enhancement by focused light. However, the current works are limited to passive systems with fixed amplitude and Q-factor, hindering the dynamic tunability of light field enhancement. Here, we design a plasmonic metasurface integrated with monolayer graphene to achieve tunable flatband quasi-BICs. Under the illumination of a tightly focused transverse-magnetic wave, our simulations show that adjusting the chemical potential of graphene can increase Q-factor from 52.5 to 75.9 and improve absorption amplitude from 81% to 95%. These results pave the way for dynamically adjustable near-field enhancement with tightly focused light.