Active Valley-topological Plasmonic Crystal in Metagate-tuned Graphene

A valley plasmonic crystal for graphene surface plasmons (GSPs) is proposed. We demonstrate that a designer metagate, placed within a few nanometers from graphene, can be used to impose a triangular periodic Fermi energy landscape on the latter. For specific metagate geometries and bias voltages, the combined metagate-graphene structure is shown to produce sufficiently strong Bragg scattering of GSPs to produce complete propagation bandgaps, and to impart the GSPs with nontrivial valley-linked topological properties. Valley-selective kink states supported by a domain wall between differently patterned metagates are shown to propagate without reflections along sharply curved interfaces owing to suppressed inter-valley scattering. Our approach paves the way for non-magnetic dynamically reconfigurable topological nanophotonic devices.