Time evolution of photovoltaic fields generated by arbitrary light patterns in z-cut lithium niobate crystals: application to optoelectronic nanoparticle manipulation

Abstract

Currently, an optoelectronic technique for the manipulation of micro- and nanoparticles called photovoltaic tweezers (PVT) is experiencing a great development. Some applications include generation of structures of nanoparticles, droplet manipulation and liquid crystal orientation. All these applications are based on LiNbO3:Fe substrates due to the high electric fields this material is able to develop because of the bulk photovoltaic effect (BPE). In particular, z-cut crystals are preferred over x- or y-cut ones as they allow flexible 2D particle manipulation and patterning. Unfortunately, charge transport related to the BPE has been scarcely investigated in z-cut configuration. This work aims to fill this gap and shed light on the formation of the photovoltaic electric and space charge fields in z-cut LiNbO3:Fe crystals. Their time evolution and other related magnitudes relevant for particle manipulation, such as dielectrophoretic and electrophoretic potentials outside the crystal, have been studied in depth for different illumination conditions using the finite element method. It has been found that the components of the drift current perpendicular to the optical axis, usually neglected in simpler models, play an important role in the final space charge distribution. This is the main reason why the models already available in the literature are not able to properly describe the internal photovoltaic electric fields for high values of the light exposure. The results of this work should be a useful tool for designing experiments and improving the operation of PVT and other related techniques that are also based on these photovoltaic fields in z-cut crystals.