Plasmonics in Large-Angle Twisted Bilayer Graphene Beyond the Decoupled Dirac Cones Paradigm

Over the last decade, magic-angle (≈1.1°) twisted bilayer graphene (TBG) has emerged as a versatile “all-in-one” platform for exploring a wide range of quantum phenomena arising from hybridized flat bands near the Dirac point. In contrast, other twist angles have received far less attention. Small angles are dominated by lattice reconstruction, while large angles are often dismissed as two decoupled, non-interacting Dirac cones—a view valid for transport at low doping. Yet at high carrier densities, remote bands interact and give rise to moiré minibands with striking electronic textures. Theory has even predicted the emergence of “cooked spaghetti” bands in this regime. Here we probe these unconventional states using gate-tunable hyperbolic phonon–plasmon polaritons (HP³s) in hBN-encapsulated large-angle TBG (3°–6°), studied via scattering-type near-field optical microscopy. Electrostatic gating reveals strong coupling between moiré-enhanced graphene plasmons and hyperbolic phonon polaritons of hBN. The crossover from “uncooked” to “cooked” bands manifests as a highly nonlinear shift in HP³ wavelength and an unusually large (~80%) modulation of the effective in-plane refractive index. Temperature- and gate-dependent HP³ decay rates further resolve miniband fingerprints at carrier densities associated with this transition. These results uncover a new regime of light–matter interaction in large-angle TBG, pushing plasmonics far beyond the conventional decoupled Dirac cones paradigm.