Material thermal conductivity is a key factor in various applications, from thermal management to energy harvesting. With microstructure engineering being a widely used method for customizing material properties, including thermal properties, understanding and controlling the role of extended phonon-scattering defects, like grain boundaries, is crucial for efficient material design. However, systematic studies are still lacking primarily due to limited tools. In this study, we demonstrate an approach for measuring grain boundary thermal resistance by probing the propagation of thermal waves across grain boundaries with a temperature-sensitive scanning probe. The method, implemented with a spatial resolution of about 100 nm on finely grained Nb-substituted SrTiO3 ceramics, achieves a detectability of about 2 x 10(-8 )K m(2) W-1, suitable for chalcogenide-based thermoelectrics. The measurements indicated that the thermal resistance of the majority of grain boundaries in the STiO3 ceramics is below this value. While there are challenges in improving sensitivity, considering spatial resolution and the amount of material involved in the detection, the sensitivity of the scanning probe method is comparable to that of optical thermoreflectance techniques, and the method opens up an avenue to characterize thermal resistance at the level of single grain boundaries and domain walls in a spectrum of microstructured materials.

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.

In this paper, we present the study of the shape change on the polar surface and in the bulk of the walls of lamellar domains as a result of local switching by focused ion beam. Periodical lamellar domain structures (PDS) are created alternatively by two methods: (i) electric-field poling using photolithographically defined electrodes and (ii) ion beam poling. The dot irradiation of Z[Formula: see text] areas near the walls of lamellar domains leads to the formation of faceted or rounded hexagonal domains. For e-field PDS additional formation of nanodomain ensembles was observed. We have revealed two types of domain wall shape changes induced by irradiation: (1) merging of the hexagonal domain with the domain wall for Z[Formula: see text] areas; (2) formation of rounded distortion of the domain wall for Z− areas. For Z[Formula: see text] areas irradiation, the domain wall distortion was described by a simple model of independent growth of isolated domain with its subsequent merging with a static domain wall. For Z− surface irradiation, the domain wall shift increases linearly with the distance between the irradiation dot and the wall. It was revealed that the merging between the growing hexagonal pyramid domain and lamellar domain can be obtained in the bulk even for absence of merging at the surface. All obtained results have been explained within a kinetic approach to the domain wall motion by step generation. The switching field consists of inputs produced by: (i) the charges injected during dot irradiation into the photoresist layer and crystal bulk, (ii) the charges injected during the creation of i-beam PDS, (iii) the depolarization fields. The transition of the shapes of isolated domains and wall distortions from faceted to rounded ones with field increase was attributed to the transition from determined step generation to stochastic one.

The formation of the ferroelectric domain structure as a result of irradiation by focused ion beam of [100]-cut 0.61Pb(Mg[Formula: see text]Nb[Formula: see text]O 3 –0.39PbTiO 3 (PMN–PT) single crystals covered by surface artificial dielectric layer and with free surface was investigated. The dot irradiation resulted in formation of the wedge-like domains grown along [00[Formula: see text]] direction. For irradiation of the free surface, the domains are mainly located under the surface, while at the irradiated surface with an artificial dielectric layer the domains are located at the surface. It was shown that the subsurface wedge-shaped part of the domain is unstable and completely disappears after a month due to spontaneous backswitching under the action of the residual depolarization field. The revealed nonlinear dose dependence of the domain sizes was attributed to the distribution of the electric field using the point charge model. The domain interaction for the distance between irradiated dots below 30[Formula: see text][Formula: see text]m has been revealed in all samples. It was shown that the decrease of the distance between irradiated dots in the created domain row leads to an increase in the length of the central domains, which is explained by the contribution of all injected charges to the switching field.

The ion beam-induced domain growth in Y-cut MgO-doped lithium niobate (MgOLN) covered by the resist layer was studied. A comparative study of domain structure in the samples with and without surface dielectric layer is crucial for periodical poling. Differences in the domain shape with changes in the irradiation dose, energy, and current of the ion beam were revealed. The maximum domain depth was 400 nm at 100 pC. A detailed analysis of interaction for different distance between irradiated dots in row revealed formation of 1 D arrays with variating length of the wedge-like domains. These findings allow optimizing the poling process.

1D domain patterning by electron beam in potassium titanyl phosphate crystals covered by an artificial dielectric layer were studied. The polygonal shape of the switched domains elongated along Y-crystallographic direction was obtained. The linear dose dependence of the domain size was attributed to screening of the depolarization field by injected charge. The domain size increase with electron energy is caused by increase in the electron penetration into the resist layer. Formation of an array of nanodomain streamers at the domain wall was observed. Creation of a through 1 D periodical domain pattern with a 10 mu m period was demonstrated.

The features of nanodomain growth during local switching in X-cut lithium niobate on insulator (LNOI) were comprehensively studied using the biased tip of a scanning probe microscope. The obtained results were discussed in terms of the kinetic approach. The revealed differences in domain growth in bulk LN and LNOI were attributed to the higher bulk conductivity of LNOI. The obtained influence of humidity on the shape and growth of isolated domains was attributed to the water meniscus. Analysis of the transition between the "forward growth" and "sideways growth" stages was performed by switching to the stripe electrode. A sand-glass-shaped domain was formed due to growth in the opposite direction after the domain touched the electrode. Stable periodical domain structures down to 300 nm were created and characterized in LNOI. Highly ordered comb-like domains of various alternating lengths, including four- and eight-fold increase periods, were produced by performing biased tip scanning along the Y axis. The obtained knowledge is important for the future development of nanodomain engineering methods in monocrystalline ferroelectric thin films on insulators.

We have studied the domain forward growth (growth in polar direction) on the non-polar cut of lithium niobate crystal as a result of focused ion beam irradiation. The nonlinear dose dependencies of the length and width of the wedge-like domains were observed. The domain growth was considered in terms of the kinetic approach. The spatial distribution of a polar component of the electric field produced by the injected charge was calculated using the point charge model. The proposed approach allowed estimating the amount of the effective charge at the irradiated surface about 1% of the irradiation dose.

The periodical domain structure with period 2 mu m was created in 1-mm-thick MgO doped lithium niobate single crystals by electron-beam irradiation. The second harmonic generation (SHG) around 370 nm was obtained during pumping the crystals by 100 mW continuous wave tunable Ti:Sapphire laser. The normalized efficiency was 0.05%/W. The dependence of the SHG power on the pump beam position has indicated that the periodical domain structure depth is about 300 mu m. The obtained results have demonstrated that the electron-beam poling can be used for creation of the short pitch periodical domain structures for light frequency conversion.

The results of comparative analysis of microstructure, crystal structure and mechanical properties in submicrovolumes of Fe-12%(wt.)Mn-1%(wt.)C alloys crystallized from the melt in different structural state: with destruction and without destruction of microheterogeneity are presented. Earlier, the authors found that overheating of the Fe-12%(wt.)Mn-1%(wt.)C melt to a temperature of 1700 °C leads to destruction of microheterogeneity, which changes the crystallization conditions of the ingot. Microheterogeneity means the presence in the melt of dispersed particles enriched in iron, which are suspended in an environment of a different composition and separated from it by a clear interfacial surface. The study of alloys was performed by scanning electron microscopy, energy dispersion analysis (EDS), the method of backscattered electron diffraction (EBSD) and nanoindentation. It was found that the destruction of the microheterogeneity of Fe-12%(wt.)Mn-1%(wt.)C melts during cooling and subsequent crystallization led to an increase in the dendritic parameter from 85 to 120 μm with increasing length of secondary branches of dendrites, crystallite size and small angle borders. Regardless of the crystallization conditions, manganese-enriched liquation layers with a thickness of L ~ 70–120 μm with a manganese content of 20 % (wt.) are formed on the surface of austenite dendrites, which leads to deformation inhomogeneity of the ingot. Based on the nanoindentation data, it is calculated that the adhesion of the manganese-enriched segregation layer to the body of austenite dendrites (Kint) for the sample crystallized after destruction of the microheterogeneous state increased by 1.2 times. The fracture energy along the boundary layer and the austenite dendrite body (Gc) also increased by 1.4 times. Average hardness values of the Young’s modulus of austenite dendrites did not change after the destruction of microheterogeneity. The mechanical characteristics of the ingot crystallized after the destruction of the microheterogeneity of the Fe-12%(wt.)Mn-1%(wt.)C melt under impact load generally improved. The equipment of the Ural Center for Shared Use "Modern nanotechnology" UrFU was used. The study was performed with the financial support of the Russian Foundation for Basic Research within the research project No. 19-33-90198.

In integrated photonics, the precise knowledge of the waveguides refractive index profile is mandatory for the modeling of photonic chips and therefore implementing innovative circuits. Usual index profile determination relies on effective index measurement of propagating modes in planar waveguides coupled with numerical fitting tools. In this paper we propose an alternative technique based on the characterization of the second harmonic generation signature of a nonlinear waveguide. We include the characterization of high-order spatial modes showing their relevance to probe both vertical and lateral distributions. We finally provide an explicit profile ready-to-use for modeling soft-proton exchanged waveguides in lithium niobate and we test its prediction capability.

Formation of the domain structure by ion beam irradiation was studied in thermally depolarized Ce-doped strontium barium niobate single crystals covered by a dielectric layer. Three types of irradiation regimes were used: dot exposure, stripe exposure, and line exposure. The dependences of the domain size and depth on the irradiated dose were measured. The circular shape of the isolated domains with partially switched broad domain boundary was obtained. Isotropic domain growth was attributed to the step generation at the wall by merging with the residual nanodomains that appeared after thermal depolarization. The obtained linear dose dependence of the switched area was attributed to the screening of the depolarization field by the injected charge. The shape distortion of the domains growing in the neighborhood with already created ones was attributed to the electrostatic interaction of the approaching charged domain walls. The obtained results can be applied for the creation of precise domain patterns with arbitrary orientation and shape to produce nonlinear optical devices with improved characteristics, including electrically tunable diffractive optical elements.

The domain switching by ion beam irradiation of single-domain Y-cut plates of congruent lithium niobate doped by 5 mol% MgO was studied. Dot irradiation resulted in the formation of wedge-like domains grown along Z- polar direction. The nonlinear dependences of the domain sizes on irradiation conditions were obtained. The model of domain growth on non-polar cut was proposed in frame of kinetic approach. The absence of the domain merging along Z- polar direction was attributed to the deceleration of the forward growth by electrostatic interaction of the positive charges at the charged domain walls and injected positive ions.

The growth of requirements to frequency mixers adapter for high-power lasers stimulates the studies of domain kinetics in nonlinear ferroelectric crystals with the slanted polar axis. Forward domain growth after local polarization reversal by focused electron beam irradiation and domain interaction were studied in 0.5-mm-thick 36 degrees Y-cut crystals of congruent lithium niobate. The dependences of domain length on the distance between domains and position in a block of domains were explained by the distribution of the space-charge field and domain-domain interaction. The stripe domains with length up to 500 mu m were produced.

The features of the domain patterning in thin lithium niobate crystals were studied. It was shown that decrease in crystal plate thickness led to domain size increase due to limitation of forward growth. The domain size weakly depended on electron energy due to equality of screening charge value at the same doses and various accelerating voltages. The weak dependence of domain sizes on pattern period was attributed to absence of electrostatic interaction of domain walls. The possibility of creation of the through periodical domain structures with the vertical domain walls and period down to 2 mu m in thin crystal was demonstrated.

We have studied experimentally the domain formation by electron beam irradiation in congruent lithium niobate crystals covered by a photoresist layer at the elevated temperatures up to 200 °C. The measured linear dose dependence of the switched domain area created by isolated dot irradiation has been attributed to external screening of the depolarization field by charges produced by incoming electrons. The significant decrease in the threshold field at the elevated temperatures has led to an increase in the domain area. The observed qualitative change of the domain shape with the temperature increase has been attributed to the highly nonequilibrium switching conditions caused by the existence of the artificial dielectric layer and domination of the stochastic nucleation. The optimal values of temperature and dose have allowed obtaining almost flat boundary of the switched area for line irradiation. The periodically poled structures of the through domains with periods from 10 to 40 μm have been created.

The equipment of the Ural Center for Shared Use “Modern nanotechnology” Ural Federal University was used. The research was made possible by the Russian Science Foundation (Grant № 17-72-10152).