Saturable nonlinearity in photovoltaic-photorefractive crystals under open-circuit condition

We show that the refractive index change induced by a focused incident beam with an additional incoherent uniform illumination in photovoltaic-photore fractive crystals under open-circuit condition has a saturable nonlinearity form. The incoherent uniform background illumination can be used to increa se the effective dark irradiance. The formation time of the photovoltaic so liton can be decreased by keeping the intensity of the soliton at ii higher value without over-saturation by use of the background illumination.