Flat bands in bilayer photonic crystal at large twist angle due to interlayer strong coupling

The twisted bilayer system provides an excellent platform for the study of flatbands. In this work, we propose a bilayer hexagonal boron nitride (h-BN)-like surface plasmon crystal at a large twist angle of 38.213° due to the interlayer strong coupling, in which the adjacent pillars are in different radii. We numerically and theoretically calculate the band structure while tuning the pillar radius ratio (PRR) and the interlayer separation distance. As a result, both increasing the PRR and decreasing the separation distance contribute to the transition from weak coupling to strong coupling, leading to the flatbands with slow velocity and large density of state. Consequently, the in-layer geometry as well as the separation distance offers the degree of freedom to achieve flatbands in the bilayer surface plasmon crystal. Our work provides a fundamental understanding of the band structure of the twisted bilayer photonic system, which enriches the methods to obtain flatbands at a large twist angle.