Dark and bright photovoltaic spatial solitons in photorefractive crystals with positive refractive-index perturbation.

We have demonstrated, for what is believed to be the first time, that in a photorefractive crystal with positive refractive-index perturbation a two-dimensional dark or bright photovoltaic spatial soliton can be formed in the same crystal by a signal beam and a background beam of different wavelengths. We discuss the conditions on the effective Glass constants and the absorption coefficients of the background and signal beams in determining the formation of the spatial soliton.