The Photorefractive Effect in Ferroelectric Oxides

Abstract

Ferroelectric materials derive their name by analogy with ferromagnetic materials, of which have found great utility as memory storage media and magnetic field sensors. Crystals such as lithium niobate (LiNbO3), barium titanate (BaTiO3) potassium niobate (KNbO3) are ferroelectric crystals that have the Perovskite structure, ABO3. They undergo a structural phase transition in which the force of the local electric field caused by the ionic displacement is larger than the first order elastic restoring force [1]. This results in the formation of electric dipole moments, a net spontaneous dielectric polarization, and a noncentrosymmetric crystal. The change from a paraelectric to ferroelectric state is accompanied by either a continous (second-order) or discontinous (first-order, e.g. BaTiO3) change in the spontaneous polarization at the structural phase transition temperature called, (by analogy with ferromagnetic materials), the Curie temperature, TC. The polarity of these crystals results in strong anisotropic nonlinear properties, e.g., the electrooptic or Pockels effect. These large band-gap materials (Eg∼3 eV) are insulators, but intrinsic or extrinsic defects with energy levels in the band gap make these materials photoconductive at visible and near-infrared wavelengths. As described in the introductory chapter of this book, these defects help produce the photorefractive modulation of the real and imaginary parts of the index of refraction. Index modulations or phase modulations can be used for optical information processing, e.g. correlators, filters or neural networks, and potentially (as has been speculated since the discovery of the photorefractive effect) may be a method by which a ferroelectric inorganic crystal can become a memory storage media, i.e. for volume holographic data storage.