Twin angle, Kerr effect and optical transients in barium titanate

We present new information regarding the ferroelectric barium titanate (BaTiO3) which was obtained using nondestructive optical techniques while the crystal was subjected to an externally applied electric field. The barium titanate sample was a (100) bulk single crystal in the tetragonal state and at room temperature (23.5°C). The application of electric fields of different magnitudes across the sample induced changes in both the surface and the volume of the crystal. The changes in the surface morphology were due to the phenomenon of twinning, which is characterized by a parameter called the twin angle alpha. The volume changes were caused by the electrooptic effect, which is characterized by the variations in the refraction indexes of the bulk crystal as functions of the applied electric field. In the first part of this thesis, a new method is presented for determining the electrically-induced twin angle alpha of BaTiO3 using a technique based on Fraunhofer diffraction. In the second part of this paper, we present a method used to measure directly some of the off-diagonal coefficients of the quadratic electrooptic (Kerr) tensor of barium titanate using polarized light transmission. The technique relies on designing an experimental optical configuration where the linear electrooptic (Pockels) effect does not contribute to the data. The power of this novel method relies in the fact that it can, in principle, be used to directly measure some of the individual off-diagonal second order coefficients belonging to the symmetry tensors not only for BaTiO3, but also for 90% of all the 20 point group symmetry crystal classes in which both the first and the second order effects coexist. Another important feature of this technique is that it allows, in principle, for an in situ observation of the optical time-resolved transient behavior of crystals during an externally induced phase transition. In the last part of this thesis dissertation we make use of this visual feature to obtain quantitative data regarding the optical transient time constants of excitation, switching and relaxation corresponding to our bulk barium titanate sample.