Photonic crystal for graphene plasmons

Graphene surface plasmon polaritons (SPPs) are hybrid excitations of electrons and photons which can be controlled by the optical properties of graphene. Periodically varying the optical properties results in a photonic crystal for graphene SPPs. Here we utilize cryogenic near-field optical microscopy to study the band structure induced by a graphene photonic crystal consisting of a high-mobility graphene atop a patterned SiO₂ dielectric layer [Xiong, Nat. Commun. 10: 4780 (2019)]. Gating through the dielectric provides a periodic field effect that spatially modulates local carrier densities [Forsythe, Nat. Nanotech. 13, 566–571 (2018)] and the propagation of plasmon polaritons through the graphene. A full plasmonic bandgap and characteristic SPP propagation properties are revealed. Selective engineering of domain wall in the middle of the photonic crystal produces localized SPP modes propagating strictly along the domain wall. These findings signify a new route towards designer-engineered band-structures to route and manipulate highly confined plasmons within high mobility graphene devices.