Superlinear photovoltaic currents in LiNbO3: analyses under the two-center model

Superlinear photovoltaic currents in LiNbO₃ are theoretically studied by using a two active center model, with Fe²⁺/Fe³⁺ as primary center and Nb_Li⁴⁺/Nb_Li⁵⁺ as secondary center. Analytical instead of numerical results are provided, including close-form expressions for most common experimental situations. Recent photovoltaic parameters obtained for -phase proton-exchanged LiNbO₃ waveguides (very similar to the substrate) are used for applying the model and comparing with published experimental results. Thoroughly studied aspects are: the redistribution of donor/acceptor states for each species as a function of the light intensity, their contribution to the photovoltaic current density, the effect of the temperature, and the role of the center concentrations and their reduction state. This provides a detailed understanding of the photovoltaic current function shapes versus light intensity, predicts new features of interest for experimentalists and suggests further experiments to better determine the material parameters. Photovoltaic measurements and modeling appear a simpler and safer way of understanding the role of the two-center photovoltaic effect in photorefractive phenomena as well as for determining important photorefractive parameters. PACS 42.70.Nq; 72.40.+w.