Optical Field Effects and Band Structure of Some Perovskite-Type Ferroelectrics

Investigation of critical points in the band structure of some ferroelectric materials of the perovskite type has been carried out by the electroreflectance method (ER). Conducting samples of BaTi${\mathrm{O}}_3$, KTa${\mathrm{O}}_3$, and KTa${\mathrm{O}}_3$-KNb${\mathrm{O}}_3$ mixtures have been studied. The light was reflected at the interface between the samples and an electrolyte, where high electric fields could be achieved. Very large changes in the reflectivity have been measured upon application of the field. The observed behavior corresponds to shifts of the critical-point energies and splitting of the degenerate levels associated with the transition-metal-oxygen octahedra. The effect is interpreted in terms of a large lattice polarization induced by the external field. The dependence of the ER on sample orientation and light polarization has also been studied. Crystal-field symmetry considerations have allowed tentative assignments of the ER singularities to critical points in the band structure. The ER behavior in the photon range where the extinction coefficient is small compared to the refractive index is consistent with the electro-optic effect observed in the visible. It is suggested that a single mechanism accounts for both effects. In KTa${\mathrm{O}}_3$ the fundamental edge has been additionally investigated by electro-absorption. The data suggest that quantum-mechanical tunneling of the Franz-Keldysh type plays only a minor role in the optical field effects of ferroelectric crystals.