Origin of UV-induced poling inhibition in lithium niobate crystals

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 $250°\text{C}$ 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.

Figure 1

Piezoresponse force microscope images obtained on MgCLN after locally poling a horizontal stripe of $h=5\mu \text{m}$ height with the scanning force microscope tip. For the generation of the latent state, vertical laser tracks written with (a) 20 mW and (c) 40 mW power were used. The zoomed image (b) shows the generation of nanopoling-inhibited domains within the UV laser-irradiated track. In (d) the formation of nanodomains occurs only at the transition region between poled and poling-inhibited area. In the center of the UV-irradiated track the surface is strongly damaged. Vertical dimensions in this figure are referred to as height, while horizontal dimensions are referred to as width.

Figure 2

(Color online) Illustration of the hopping migration of Li ions driven by a temperature gradient. The central empty position corresponds to the vacancy so that the left/right ions can only hop to the right/left, respectively.

Figure 3

Dependence of the coercive field $E_{\text{c}}$ on the degree of stoichiometry of lithium niobate crystals [lithium concentration: $\text{Li}/\left(\text{Li}+\text{Nb}\right)$]. The dots are taken from the literature (Refs. 11, 12, 13, 14) and the dashed line is a linear fit. Note that due to a different manufacturer of the SLN or different determination methods of $E_{\text{c}}$, values for SLN differ from our measured value of 5 kV/mm.