  Topological insulators are unique devices supporting unidirectional edge
states at their interfaces. Due to topological protection, such edge states
persist in the presence of disorder and do not experience backscattering upon
interaction with defects. We show that despite the topological protection and
the fact that such states at the opposite edges of a ribbon carry opposite
currents, there exists a physical mechanism allowing to resonantly couple
topological excitations propagating at opposite edges. Such mechanism uses weak
periodic temporal modulations of the system parameters and does not affect the
internal symmetry and topology of the system. We study this mechanism in
truncated honeycomb arrays of microcavity pillars, where topological insulation
is possible for polaritons under the combined action of spin-orbit coupling and
Zeeman splitting in the external magnetic field. The temporal modulation of the
potential leads to a periodic switching between topological states with the
same Bloch momentum, but located at the opposite edges. The switching rate is
found to increase for narrower ribbon structures and for larger modulation
depth, though it is changing nonmonotonically with the Bloch momentum of the
input edge state. Our results provide a promising realization of a coupling
device based on topologically protected states. The proposed scheme can be used
in other topological photonic and condensed matter systems, including Floquet
insulators and graphene.
