We have studied the self-organized dendrite domain structures appeared as a result of polarization reversal in the uniform field in lithium niobate single crystals with the artificial surface layer created by proton exchange. We have revealed the self-organized sub-micron scale dendrite domain patterns consisting of domain stripes oriented along the X crystallographic directions separated by arrays of dashed residual domains at the surface by scanning probe microscopy. Raman confocal microscopy allowed visualizing the quasi-regular dendrite domain structures with similar geometry in the vicinity of both polar surfaces. The depth of the structure was about 20 μm for Z+ polar surface and 70 μm for Z− one. According to the proposed mechanism, the dendrite structure formation at the surface was related to the ineffective screening of the residual depolarization field. The computer simulation of the structure formation based on the cellular automata model with probabilistic switching rule proved the eligibility of the proposed scheme, the simulated dendrite domain patterns at various depths being similar to the experimental ones.

The strong dependence of the domain kinetics on the applied field and the thickness of the surface layers produced by proton exchange has been revealed in congruent lithium niobate. The correlated nucleation leads to formation of self-assembled structures consisting of isolated domains. Formation of the nanodomains in front of the domain wall results in its continuous motion. The pronounced self-organization effect leads to formation of broad domain boundaries and ensembles of isolated nanodomains consisting of nanodomain chains and nets. The obtained effects were attributed to highly non-equilibrium switching conditions caused by retardation of the bulk screening of depolarization field.

Formation and evolution of the bands with nanodomain structure in front of the moving domain wall were studied in proton exchanged LiNbO3 in field range. The width of the nanodomain bands grew with field increasing. For switching in constant field three types of domain structure evolution were studied: 1) continuous growth of hexagon domains at 21.0 kV/mm, 2) formation and growth of wide domain wall at 21.5 kV/mm, 3) growth of micro- and nanodomain ensembles at 22.0 kV/mm. Discrete switching being the origin of obtained effects is caused by retardation of depolarization field screening in crystal with dielectric surface layer.

Formation of Nanodomain Structures in Lithium Niobate and Lithium Tantalate Crystals studied by Confocal Raman Microscopy

ФЦП «Исследования и разработки по приоритетным направлениям развития научно-технологического комплекса России на 2007-2013 годы»

The original technique based on the laser confocal microscopy and Raman spectroscopy was applied for the domain visualization in the bulk of lithium niobate LiNbO3 (LN) single crystal. The change of the Raman spectrum in the vicinity of the domain walls of LN was used. The technique capabilities were inspected in LN with different types of domain geometry. The estimation of the spatial resolution at different depth was carried out for periodically poled LN. The proposed analysis of the three-dimensional nanodomain image allows us to characterize the evolution of the self-assembled nanodomain structures produced in LN with surface layer modification.

Formation of abnormal domain shapes was studied in single crystalline LiNbO3 with surface layer modified by proton exchange. It has been shown that the isolated domain shape is very sensitive to the electric field: 1) three-rayed stars and concave polygons appear in low field, 2) hexagonal domains form in moderate field, 3) oriented domain rays grow in high field. The transformation from concave to convex polygon and the fast growth of narrow domain rays in front of the growing domain wall were investigated. The obtained abnormal behavior was attributed to non-effective external screening of depolarization field caused by artificial dielectric layer.

Comprehensive study of depolarization field bulk screening was carried out in lithium niobate and lithium tantalate single crystals with various stoichiometry. Three used complementary methods are based on: 1) dependence of coercive field on delay time; 2) decrease of optical contrast of domain wall trace; 3) relaxation of light diffraction intensity on domain walls. The following parameters of bulk screening process were obtained: relaxation time constants, type of relaxation law, maximal value of bias field and “true” value of coercive field. Advantages and disadvantages of used experimental methods were analyzed.

Anomalies of domain structure evolution in congruent lithium niobate with surface layers modified by implantation of Cu ions were investigated. Formation of residual domains and growth of areas occupied by domain patterns with high density of charged domain walls were studied during cyclic polarization reversal. Growth of unusual three-rayed star domains was observed during polarization reversal in low external field much less than the threshold field for conventional congruent lithium niobate. The complicated geometry of these domains in the bulk was revealed using scanning laser confocal microscopy combined with Raman spectroscopy.

ФЦП «Исследования и разработки по приоритетным направлениям развития научно-технологического комплекса России на 2007-2013 годы»

Pulsed implantation of Cu ions into single crystalline congruent lithium niobate resulted in extraordinary effects. First, surface nano-domain structures consisting of star-like domains with rays oriented along three Y directions were induced under the action of the pyroelectric field appeared during cooling after implantation pulse. These nano-domains served as nucleation sites for subsequent polarization reversal under external field. Second, jump-like domain wall motion led to formation of residual domain arrays along the staying positions of the wall. Third, the implantation led to suppression of screening of depolarizing field, due to formation of surface layer with suppressed polarization or non-ferroelectric.

Kinetics of bulk screening of depolarization field was studied in single crystalline lithium niobate and lithium tantalate of various compositions by analysis of the switching current, birefringence contrast, and diffraction of coherent light on the domain walls. The bulk screening process was quantitatively characterized by several essential parameters, such as a type of relaxation law, relaxation time constants, and maximum value of bulk screening field. Advantages and disadvantages of used experimental methods were discussed. The comparison of the obtained and previously published results was carried out.

The abnormal domain kinetics in congruent lithium niobate single crystals caused by surface modification using proton exchange was studied experimentally. The dependence of domain kinetics on polarity of the growing domains at the surface with proton exchange was revealed. The abnormal smooth motion of irregularly shaped domain walls with finger-like features and correlated nucleation was observed for growth of domains with Z(+) at surface with proton exchange. Such behavior drastically differs from jump-like motion of plane domain walls oriented along Y directions observed both in conventional lithium niobate and for growth of domains with Z(-) at surface with proton exchange.

ФЦП «Исследования и разработки по приоритетным направлениям развития научно-технологического комплекса России на 2007-2013 годы»