Intrinsic defects pinned at the domain wall in lithium niobate

Ferroelectric domain engineering is a powerful technique that can be used to create nonlinear optical devices based on quasi-phase matching. However, creating domains smaller than 1μm poses significant challenges. This is partially due to an incomplete understanding of the properties of the ferroelectric domains; particularly in regards to the interaction of the domain wall and intrinsic defects, which determine the domain shapes and growth dynamics. Such defects are present in large numbers in congruent LiNbO₃ crystals due to lithium deficiency. Due to the ionic nature of these intrinsic defects, they constitute defect dipoles that can be oriented in different ways in respect to the ferroelectric axis. In particular, after a domain inversion at room temperature these dipoles may not reorient along with the ferroelectric polarization and find themselves in an energetically unfavorable, “frustrated,” alignment that creates strain in the material. It has been found that such strain is released over time and by thermal treatments as shown by the observation of characteristic shifts in the Raman spectra.