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The analysis of the shapes of Barkhausen pulses (BPs) was used for the detailed characterization of the domain merging process in congruent lithium niobate LiNbO 3 (CLN) crystals. The BPs in ferroelectrics manifest themselves as the sharp peaks in a switching current during polarization reversal by the application of a constant or slow-varying external electric field. Three mechanisms of the BPs were previously proposed: domain nucleation, interaction of the domain walls with the pinning centers, and domain merging. We have revealed the domination of the domain merging mechanism for the generation of the BPs in CLN and classified the scenarios of domain structure evolution after merging in terms of the appearance and transformation of short-lived fast and superfast domain walls. We have shown that the input of merging events reaches 80% of the whole switching process. Two revealed types of BPs corresponding to the merging events considerably differ by duration and shape, thus opening the way for solution of the inverse problem-extracting the quantitative information about the domain kinetics by the analysis of the BPs. This analysis allows extracting the velocities of the fast and superfast walls and provides the possibility to go beyond the temporal resolution of the in situ imaging system in studying the superfast domain wall motion. The proposed analysis is applicable for the faceted growth of polygonal domains in any ferroelectric. It is necessary to point out that the analysis of the BPs allowed characterizing the main part of the domain evolution process in CLN as the fast domain transformations after merging prevail during the polarization reversal.

Single crystals of potassium titanyl phosphate (KTiOPO4, KTP) family (MTiOXO4, where M is K, Rb, or Cs, and X is P or As) with periodical domain structures have emerged as one of the key platforms for enabling nonlinear photonics applications. Potassium titanyl arsenate (KTiOAsO4, KTA) crystals possess nonlinear optical properties outperforming those of KTP. However, domain kinetics in KTA, being the crucial element for periodical poling, lacks comprehensive studies. We present the results of in situ imaging of domain kinetics in KTA with high temporal resolution. The analysis of a set of instantaneous domain structure images (kinetic map) has allowed reliable revealing of the slow and fast domain walls, similar to KTP. The mobility and the threshold fields for the domain walls have been estimated. The main stages of the domain structure evolution have been revealed. The original hatching stage representing the formation of quasiperiodic structure of the narrow stripe domains has been discovered. The relative input of the hatching stage has increased with external field. The obtained qualitative difference in the domain structure evolution, compared with KTP, has been attributed to a six times larger ratio of fast to slow wall mobility in KTA. This fact results in suppression of the undesirable broadening of the stripe domains thus making KTA crystals very attractive for periodical poling.

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