Photonic crystal for graphene plasmons

Recent advancements in high-quality graphene devices enable long-lived surface plasmon polaritons which propagate over several microns. Various tunable parameters including gate voltage, twist angle, and superlattice potential enable efficient control of device functionalities. We used near-field nano-imaging techniques to study graphene plasmonic crystals at cryogenic temperatures. High-mobility graphene is transferred to a periodically patterned SiO₂ substrate with Si back-gate. This heterostructure imprints the graphene with 80 nm-scale periodic variations in carrier density under application of a field effect, thus forming a gate-tunable photonic crystal for plasmons. We observed the formation of a selectively engineered full plasmonic bandgap where propagation of plasmons is strongly suppressed within the superlattice. Additionally, we implemented a designed domain wall within the superlattice which simultaneous supports strongly confined 1D plasmons within the plasmonic bandgap. These findings signify a new route towards designer-engineered band-structures to route and manipulate highly confined plasmons within high mobility graphene devices.