We present original data on experiments with cubic photorefractive cry
stal Bi12TiO20 under an external de electric field in the configuratio
n of Z-scan characterization technique. In particular, behavior of the
Z-scan curves versus the external dc field, light polarization, and t
he (focused beam) /(uniform sample illumination) intensity ratio is re
ported. The data obtained are interpreted in the framework of an origi
nal theoretical analysis based on the standard drift model of the phot
orefractive effect and assumption of a Gaussian profile of the propaga
ting laser beam.