Study On The Photorefractive Properties Of Doped Lithium Niobate

Mg:Ce:Fe:LiNbO₃ crystals are grown at the first time by Czochralski method with different concentration of Mg. According to the theory of crystals growth, growth parameters(temperature gradient, pulling speed and rotating speed) are modified. The crystals grown with appropriate parameters have no macroscopic defect and exhibit good optical homogeneity. Crystals'properties are studied by XRD analysis,infrared absorption spectra,UV-Visible absorption spectra and wave coupling experiments.XRD results show that the lattice defects increase with the increase of Mg contents when the concentration of Mg is lower than threshold and decreases when concentration of Mg reaches the threshold.The infrared absorption spectra shows that Ce⁴⁺ and Fe³⁺ occupy the Nb sites and Li sites, respectively, fallowed by the appearance of Ce_Nb^2- and Fe_Li²⁺ defects. Crystals compensate charges by increasing Nb_Li⁴⁺ and Li vacancies. Mg ions entering LiNbO₃ crystal firstly substitute Nb_Li⁴⁺ and then occupy Nb sites and Li sites when the Mg²⁺ concentration is up to the threshold, forming Mg_Nb^3- and 3 Mg_Li⁺ and self-charges-balance.The UV-Visible absorption spectra of Mg:Ce:Fe:LiNbO₃ crystals shows that the absorption edge of Mg:Ce:Fe:LiNbO₃ shifts towards violet band with the increase of Mg contents and shifts towards red band relatively when the Mg contents reach threshold concentration.Ion sites in the LiNbO₃ crystals are analyzed according to the XRD analysis,infrared absorption spectra and the UV-Visible absorption spectra. No new phases appears when the LiNbO₃ crystals are doped with Mg,Ce and Fe. All these crystals are still LiNbO₃ with trigonal system. The locations of Ce, Fe, Mg ions and the threshold concentration of Mg in doped LiNbO₃ crystals are confirmed.The photorefractive properties are measured by two wave coupling. The weak light nonlinear optical effects of Ce, Fe-series LiNbO₃ crystals are studied. The results show that the diffraction efficiency and dynamic range decreases, the sensitivity increases and the writing time is shortened with the increasing of the Mg doped concentration. The dependence of Mg contents on the low photorefractive resistant properties is studied by facula distortion method. The results show that the photo- damage resistance threshold increases with the increasing of Mg concentration. When Mg doping concentration is about 6mol%, the photodamage resistance threshold improves greatly to 2 orders of magnitude higher than than that of Ce:Fe:LiNbO₃.