DYNAMICS OF PHOTOINDUCED LENS FORMATION IN A PHOTOREFRACTIVE BI12TIO20 CRYSTAL UNDER AN EXTERNAL DC ELECTRIC-FIELD

We report experimental. results on the dynamics of formation of a phot oinduced lens produced by a two-dimensional cw He-Ne Gaussian laser be am (633 mn) in a cubic, Bi12TiO20 photorefractive crystal under an ext ernal de electric field. The main features of the effect, namely, asti gmatism of the dynamic lens formed, the pulse shape of the far-field r esponse after switching on of the focused beam, inverse linear depende nce of time of its growth on the beam intensity, and approximate const ancy of its peak level, are explained in the framework of the analytic al model for a one-dimensional Gaussian beam and numerical calculation s for a two-dimensional one. (C) 1996 Optical Society of America.