Second-Order Optical Nonlinearity and Magnetic Order in Disordered Oxides

Abstract

Recent research activities of the author's group concerning second-order optical nonlinearity induced in amorphous oxides and magnetic order in a disordered crystalline lattice are described. For the second-order optical nonlinearity, an attention is paid to second-harmonic generation (SHG) in thermally or optically poled tellurite glasses. Some tellurite glasses which exhibit interesting properties such as very stable second-order nonlinearity and large second-order nonlinear susceptibility are presented. The former is realized in ZnO-TeO2 glass, and the latter is attained for Na+-doped WO3-TeO2 glass. Also, the mechanism for SHG in optically poled Nb2O5-TeO2 glasses doped with vanadium ions is argued. A model including two-step absorption at a wavelength of 532 nm and the presence of photo-induced electrons and positive holes trapped at localized states is proposed. On the other hand, magnetic and magnetooptical properties of disordered ZnFe2O4 thin film prepared by a sputtering method are reported as an interesting example of magnetic order in a disordered lattice. The resultant disordered thin film shows high magnetization even at room temperature and a spin-freezing temperature higher than room temperature. The temperature dependence of magnetization under the conditions of field cooling and zero-field cooling, the dependence of spin-freezing temperature on frequency of ac magnetic field, and temperature variation of ac nonlinear susceptibility suggest that the spin-freezing or magnetic transition of the disordered ZnFe2O4 thin film is explainable in terms of the superparamagnetism with strong intercluster interactions. It is also demonstrated that large Faraday effect is observed in a short wavelength range for the disordered ZnFe2O4 thin film.