We have studied the kinetics of domain walls in lithium niobate (LN) crystals with stoichiometric and magnesium oxide doped compositions. We have analyzed the kinetic maps of the switching process to separate the main types of domain walls, their relative input and their field- and orientation-velocity dependence. All obtained domain walls were divided into three groups characterized by orientations and velocity: “slow walls”, “fast walls” and “superfast walls”. The dominant role of the fast and superfast walls in the switching process in the whole field range for studied crystals was revealed. The measured kinetic Wulff plot demonstrates the six-fold symmetry for the slow walls and three-fold symmetry for fast and superfast walls. The measured field dependencies of slow, fast and superfast domain wall velocities in all studied crystals were analyzed using two alternative approaches by linear and exponential fittings. The obtained wall orientations were attributed to the wall structure, characterized by averaged step length (kink concentration). It is shown that the mobilities of all types of walls in stoichiometric LN are several orders of magnitude higher than in doped crystals. In contrast, the threshold fields demonstrate a pronounced dependence on composition, which was attributed to dependence on the deviation from the stoichiometry and the healing role of the doping by MgO. Detailed studies of the kinetics of domain walls of various orientations in the main representatives of the LN family are important for further development of domain engineering methods for creation of the nonlinear optical frequency conversion devices.

The formation of submicron stripe domain structures was studied during two-stage switching with a change in the dimensionality of domain growth in congruent lithium niobate with a dielectric layer on the polar surface at elevated temperature. At the first stage, a two-dimensional hexagonal microdomain lattice representing a structure of isolated domains was created using the photoresist mask. After that, the polar surface was covered by solid dielectric layer. Thus, switching at the second stage was carried out under inefficient screening of depolarization field leading to growth of the stripe domains only. Self-organized formation of domain stars and snowflakes was demonstrated.

We present the study of domain structure evolution during polarization reversal in lithium niobate crystal with an initial tailored polydomain state representing a matrix of isolated domains arranged in a square lattice. Domain kinetics during switching differs significantly from the initial single domain state. Growing domains elongate in Y crystallographic direction coinciding with the 2D lattice direction. The field dependence of main domain walls follows the activation law. Typical switching current consists of many Barkhausen pulses separated by low current regions. Korcak’s analysis of switching current has demonstrated the Hurst exponent below 0.5, which indicates an anti-persistent character of the process.

We present a Barkhausen pulse (BP) shape fitting method that allows extracting the domain wall velocities during a given fast domain shape transformation after merging during switching in lithium niobate. Two scenarios of hexagonal domain merging were distinguished: for domains with comparable sizes, and for large and small domains. The fitting was based on the knowledge about domain wall types and delay time of the external circuit. The derived expressions were successfully used for fitting the shapes of experimentally obtained BPs. Proposed fitting allowed extracting the velocities of fast and superrfast domain walls with high accuracy.

Barkhausen pulses (BPs) in ferroelectrics manifest themselves as sharp peaks in switching current during polarization reversal. The total input of BPs in previously studied ferroelectric crystals is about 0.03 of the switched charge. We have revealed a dominating role of BPs (up to 0.7 of the switched charge) during polarization reversal in lithium niobate crystals. The recording of the switching current simultaneously with in situ domain imaging allowed studying the individual giant BPs appearing as a result of domain merging. It has been demonstrated that analysis of few main merging events provides the important information about domain kinetics.

The analysis of the shapes of Barkhausen pulses (BPs) was used for the detailed characterization of the domain merging process in congruent lithium niobate LiNbO3 (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.