Origin of UV-induced poling inhibition in lithium niobate crystals

Abstract

Short-term exposure of the $+z$ face of ${\text{LiNbO}}_{3}$ crystals to focused UV laser light leads to persistent inhibition of ferroelectric domain reversal at the irradiated area, a phenomenon referred to as ``poling inhibition.'' Different types of crystals (stoichiometric, congruent, or Mg-doped ones) are exposed, creating the so-called ``latent state'' and domain growth during subsequent electric-field poling is visualized. The latent state is robust against thermal annealing up to $250\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and uniform illumination. With the tip of a scanning force microscope the coercive field is mapped, showing not only the expected resistance against domain reversal in the UV-irradiated region but also easier poling adjacent to the UV-irradiated section. These results and theoretical estimates point to the following mechanism of poling inhibition: the UV light-induced heating results in a local reduction of the lithium concentration, via thermodiffusion. The required charge compensation is provided by UV-excited free electrons/holes. After cooling, the lithium ions become immobile, and the reduced lithium concentration causes a strong local increase in the coercive field in the exposed area, while the increased Li concentration next to this area reduces the coercive field.