Stability of vortex solitons in a photorefractive optical lattice

Stability of on- and 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 high- and low-intensity regimes. In the high-intensity 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 intermediate-intensity 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.