Self-trapping and stabilization of doubly-charged optical vortices in two-dimensional photonic lattices

We report our recent work on self-trapping of doubly-charged vortices (DCVs) in two-dimensional optically induced photonic lattices. We show that in an ideal isotropic nonlinear periodic medium self-trapping of a DCV can lead to formation of a rotating 'quasi-vortex' soliton with periodic charge-flipping, but the vortex breaks up into multiple singly-charged vortices in an anisotropic nonlinear medium. Such topological transformation of high-order vortices under conventional four-site excitation in a square photonic lattice has been observed in our experiment and corroborated with numerical simulations. With geometrically extended eight-site excitation, however, discrete self-trapping of a DCV into a stable high-order vortex soliton can be realized with both self-focusing and self-defocusing nonlinearities.