Self-trapping of weak optical beams in photorefractive materials

Intensity-independent self-trapping of optical beams in photorefractive crystals was found recently. But we show that due to the existence of dark conductivity in some photorefractive materials or coherent and non-coherent background irradiance, the self-trapping of weak optical beams in photorefractive materials, such as SBN, becomes intensity dependent. The threshold condition of the applied external de electric field for self-trapping is determined as function of the material parameters as well as the initial transverse sizes and relative intensities of the beams.