Photovoltaic saturable nonlinearity in 2D in photorefractive crystals

We formulate the equation describing the two-dimensional nonlinearity chang e in the refractive index induced by a focused beam under an incoherent uni form background illumination in photovoltaic photorefractive crystals. The analytical results indicate that under the appropriate condition the photov oltaic nonlinearity has a saturable Kerr-type form and uniform background i llumination plays a role of the dark irradiance. Local saturable Kerr-like nonlinearity suggests the possibility of the existence of circular symmetri c solitons in the crystals. (C) 2000 Elsevier Science B.V. All rights reser ved.