Time evolution of photovoltaic fields generated by arbitrary light patterns in z-cut LiNbO:Fe: application to optoelectronic nanoparticle manipulation.

The bulk photovoltaic effect is a phenomenon that generates high electric fields in certain ferroelectric crystals under illumination, as iron doped lithium niobate (LiNbO:Fe). A variety of innovative applications of these electric fields require using of z-cut plates, in which the polar axis is normal to the larger crystal faces. However, the kinetics and distribution of the photovoltaic fields in this configuration have not been investigated in depth. In this work, the photovoltaic charge transport of z-cut configuration is studied through a complete finite element analysis. Light patterns commonly used for particle trapping applications are used to study the temporal evolution of the electric field developed by the crystal. Results show that photovoltaic currents perpendicular to the optical axis play a key role in the development of the final charge distribution. Moreover, there is a relevant, localized charge accumulation inside the crystal which is required to reach the saturation electric field in the whole illuminated volume. The role of crystal thickness and light absorption are analysed. It has been found that they are important to determine the time evolution of the process. The simulations are expected to be a key tool to analyse and improve photovoltaic optoelectronic tweezers.