Polaritons in Moire Superlattices of Graphene and Boron Nitride

Electronic properties of graphene can be drastically altered when it is laid upon another graphene layer, resulting in a moiré superlattice. The relative twist angle between the two layers is a key tuning parameter of the interlayer coupling in thus-obtained twisted bilayer graphene (TBG). We studied the propagation of plasmon polaritons in TBG by infrared nano-imaging. The atomic reconstruction occurring at small twist angles transforms the TBG into a network of conducting channels that act as efficient reflectors of plasmon polaritons. The analysis of plasmonic images allows one to infer the conductivity of the domain walls in moire superlattices. TBG at small twist angles effectively acts a natural plasmon photonic crystal for propagating nano-light. We observed similar nano-optical effects in other twisted van der Waals crystals including hexagonal boron nitride. In hBN, domain walls modify the response of phonon polaritons.