Stability of vortex solitons in a photorefractive optical lattice

Stability of onand off-site vortex solitons with unit charge in a photorefractive optical lattice is analysed. It is shown that both types of vortex solitons are linearly unstable in the highand low-intensity regimes. In the highintensity regime, the vortex appears as a familiar ring vortex, and it suffers oscillatory instabilities. In the low-intensity regime, the vortex suffers both oscillatory and Vakhitov–Kolokolov instabilities. However, in the intermediateintensity regime, both types of vortices could be stable if the lattice intensity is moderate, and the applied DC field is above a certain threshold value. Under the same physical conditions, on-site vortices are more stable than off-site ones. Increasing the DC field stabilizes both types of vortex solitons.