Pseudospin-mediated optical spin–spin interactionin nonlinear photonic graphene

Dynamic manipulation of the spin of photons is important for manyapplications ranging from optical sensing to optical information processing.In the past, the study on controlling the spin of light has focused on chiralmaterials, three-dimensional structures lacking any mirror symmetries.However, the complexity of fabrication of such structures has hindered theirpractical applications. In this work, the dynamic switching of optical chiralityin two-dimensional honeycomb photonic crystal—the so-called photonicgraphene is explored. In particular, optical spin–spin interaction mediated bythe pseudospin states of the photonic graphene is proposed. A circularlypolarized pumping beam can lift the degeneracy at the Dirac cones, leading tochiral responses for a probe beam incident along the direction of the Diracpoints. Interestingly, the chirality is determined by the both the valley indexand the spin of the pumping beam. The proposed nonlinear photonicgraphene offers a new route to manipulate valley- and spin-dependentphenomena in 2D photonic systems.