At low light intensity contrasts, the profile of the refractive index pattern under an ac field exhibits a gradient dependence on the light intensity. At high contrasts, however, the situation is dramatically different and new features arise. The light-induced space-charge density distribution exhibits a strong spatial localization or delta-type singularity, coupled with sharp discontinuities of the space-charge field and the index profiles. The effect shows a clear threshold behavior depending on the ratio of the width of the light beam and the mobility-lifetime-ac field product. The new features are even more striking when a narrow light beam propagates through such a self-generated discontinuous refractive index pattern. These include a combination of diffraction together with self-focusing and defocusing effects, and even with the self-trapping along the index discontinuity.
© 2003 Optical Society of America