  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.
