Third-Order Nonlinear Optical Properties and Dielectric Properties of Pb-Complex Perovskite Thin Films Prepared by Sol-Gel Method

The third-order nonlinear optical properties of the sol-gel derived Pb(Fe_1/2Nb_1/2)O₃-PbTiO₃ (PFN-PT) thin films have been investigated by the THG method. The χ⁽³⁾ values of PFN-PT thin films increased from 3.2×10⁻¹² esu (PT) to 8.5×10⁻¹² esu (PFN) with increasing content of Fe³⁺ ions which possessed very high second-hyperpolarizability, ψ(Fe_2/3O). It was experimentally confirmed that complex oxides such as ABO₃-type perovskites consisting of large non-transition metal A-site cations and small transition metal B-site cations exhibited high χ⁽³⁾ as expected from Lines' model. It was also found that the χ⁽³⁾ values of complex oxides could be estimated from the second-hyperpolarizabilities of the constituent single oxides reported so far. Because of the large remanent polarization, PT thin films may exhibit the highest χ⁽³⁾ among the ferroelectric PFN-PT thin films studied in the present work. The maximum dielectric constant, ε, of 1990 at room temperature was obtained for 0.5PFN·0.5PT thin films, whose composition corresponds to so-called morphotropic phase boundary (MPB).