  Topological optical states exhibit unique immunity to defects and the ability
to propagate without losses rendering them ideal for photonic applications.A
powerful class of such states is based on time-reversal symmetry breaking of
the optical response.However, existing proposals either involve sophisticated
and bulky structural designs or can only operate in the microwave regime. Here,
we propose and provide a theoretical proof-of-principle demonstration for
highly confined topologically protected optical states to be realized at
infrared frequencies in a simple 2D material structure-a periodically patterned
graphene monolayer-subject to a magnetic field below 1 tesla. In our graphene
honeycomb superlattice structures plasmons exhibit substantial nonreciprocal
behavior at the superlattice junctions, leading to the emergence of
topologically protected edge states and localized bulk modes enabled by the
strong magneto-optical response of this material, which leads to
time-reversal-symmetry breaking already at moderate static magnetic fields. The
proposed approach is simple and robust for realizing topologically nontrivial
2D optical states, not only in graphene, but also in other 2D atomic layers,
and could pave the way for realizing fast, nanoscale, defect-immune devices for
integrated photonics applications.
