Controlled generation of pseudospin-mediated vortices in photonic graphene

Abstract

We demonstrate controllable generation and destruction of pseudospin-mediated topological charges (vortices) in the photonic analogy of graphene—optically induced honeycomb lattices (HCLs). When only one of the two sublattices is selectively excited by the probe beams that are momentum-matched onto the Dirac points, a singly-charged optical vortex emerges in the output of the symmetric conical diffraction pattern. Furthermore, flipping of the topological charge is observed as the excitation shifts from sublattice A to sublattice B. On the other hand, when both sublattices are simultaneously excited, the conical diffraction pattern becomes highly asymmetric, accompanied by interesting phenomena related to the generation of half-integer vortices and line singularities. We present four different cases of selective excitation using two different approaches; one with three input probe beams that are momentum-matched to the three K valleys, and the other with only two probe beams while the Bloch modes surrounding the third valley are excited due to Bragg reflection. Our experimental results are confirmed by numerical simulation of the paraxial wave equation with a HCL potential as well as by theoretical analysis of the two-dimensional Dirac–Weyl equations directly. These studies indicate that the lattice pseudospin is not just a mathematical formality, but rather it can manifest through its angular momentum transferred to probing optical beams.