Self-action of laser beams in photorefractive crystals with external applied AC or DC electric fields
The purpose of the dissertation work is to study theoretically and experimentally the influence of the additional photoelastic contribution to the modulation of the optical properties by the space-charge field and the effect of the optical activity on self-focusing and self-bending of laser beams. We have developed a theoretical model of self-action of one-dimensional light beams in (1 1 0) and (1 1 -2)-cut photorefractive sillenite crystals of point group 23 taking into account the additional contribution of the photoelastic effect to the optical properties of the medium, the induced birefringence and the optical activity. The additional photoelastic contribution to the optical properties of the photorefractive cubic medium in some cases can reduce the necessary value of the electric field amplitude by a factor of 1.7 to observe the same magnitude of the self-action effect as in existing theoretical models. The presented theoretical model predicts the possibility to observe quasi-solitons in cubic photorefractive crystals with a large magnitude of optical activity. The experimental and theoretical studies of the laser beam self-bending in bismuth titanium oxide crystals under applied external alternating fields showed that the self-bending magnitude, the self-bending direction and the form of the beam on the output face of the crystal can be controlled by the input polarization of propagated light if the bias field is applied along the direction [1 -1 1]. A theoretical model for the investigation of the self-bending effect in barium titanate crystal (BaTiO3) for a speckled extraordinary light beam taking into account the additional elasto-optic contribution was developed for the first time. The model shows that BaTiO3 crystals exhibit the property of positive or negative gradient lenses depending on whether the speckled laser beam propagates along the optical axis c or in the opposite direction. For the strontium barium niobate crystal we found experimentally that the output beam diameter of a propagating laser beam (633 nm) does not depend on the diameter of the input beam especially along the crystal axis if it lies between 18 m and 40 m. This effect is called optimal focusing.