Moiré-enabled chiral plasmons in twisted bilayer graphene nanoislands
Twisting two stacked graphene layers superimposes a moiré superlattice that reshapes their electronic structure, yet how it shapes the collective optical response of finite, nanoscale bilayers remains largely unexplored. Using an atomistic tight-binding and self-consistent density-matrix approach, we compute the twist-dependent plasmonic response of twisted bilayer graphene nanoislands comprising thousands of atoms. The spectra are strongly tunable and split into two regimes at a critical angle where a single moiré wavelength spans the island; beyond it, an ordered ladder of resonances emerges whenever an integer number of moiré periods fits within the overlapping bilayer. In doped islands, the interlayer coupling also produces a moiré-enabled Born--Kuhn chiral plasmon: opposite transverse dipoles in the two layers form an electric quadrupole that hybridizes with the bright dipole, opening an anticrossing near the critical angle, with a handedness that reverses with the twist direction. Triangular islands confirm that the moiré lattice, not the flake boundary, governs both effects, establishing twist as a mechanical handle on chiral graphene plasmons.