Nanopolaritonic second-order topological insulator based on graphene plasmons

Ultrastrong confinement, long lifetime, and gate-tunability of graphene plasmon polaritons (GPPs) motivate wide-ranging efforts to develop GPP-based active nanophotonic platforms. Incorporation of topological phenomena into such platforms will ensure their robustness as well as enrich their capabilities as promising test beds of strong light-matter interactions. A recently reported approach suggests an experimentally viable platform for topological graphene plasmonics by introducing nanopatterned gatesmetagates. We propose a metagate-tuned GPP platform emulating a second-order topological crystalline insulator. The metagate imprints its crystalline symmetry onto graphene by modulating its chemical potential via field-effect gating. Depending on the gate geometry and applied voltage, the resulting two-dimensional crystal supports either one-dimensional chiral edge states or zero-dimensional midgap corner states. The proposed approach to achieving the hierarchy of nontrivial topological invariants at all dimensions lower than the dimension of the host material paves the way to utilizing higher-order topological effects for onchip and ultracompact nanophotonic waveguides and cavities.