Congruent Lithium Niobate Crystals Research Articles

Domain shape evolution under multiple IR laser irradiation in lithium niobate

In this work, we have studied the shape evolution of the isolated domains in domain array in congruent lithium niobate crystals under the action of pyroelectric fields arising during multiple irradiation by laser pulses. The loss of the circle shape with the pulse number was obtained in the arrays of isolated domains arising at the edges of the irradiated zone. The simulation of the domain growth under the action of the pyroelectric field allowed revealing two stages of domain shape evolution in the regular domain array. An important role of the neighboring domains was revealed.

Optical and Thermal Properties of 2-inch Nd3+, Mg2+ Co-doped Near Stoichiometric Lithium Niobate Crystal

The physical properties of near stoichiometric lithium niobate crystals (NSLN) can be improved by adding magnesium. For example, the light damage threshold of NSLN crystals mixed with a small amount of magnesium ions is higher than that of congruent lithium niobate (CLN) crystals mixed with high magnesium [1]. However, the current difficulties also lie in the deviation of SLN crystals from the congruent point, difficulty in growth, size limitation and uneven doping. SLN crystals are still not commercially available, and the size of the crystals is limited to two inches. We use the partial liquid phase method to synthesize polycrystalline materials of co-doped SLN crystals, and successfully solve the problem of uneven doping. The adjustment of process parameters and the use of continuous feeding technology greatly improved the success rate of SLN crystal growth. Then we characterized the grown NSLN crystal on optical and thermal properties.

Conductive domain walls in x-cut lithium niobate crystals

Conductive ferroelectric domain walls have been the focus of intensive studies in nanoelectronics due to their promising electronic properties. Here, both head-to-head and tail-to-tail domain walls with a large inclination angle (≈90°) were fabricated on the surface of x-cut nominally pure congruent lithium niobate crystals. Our results show that both types of domain walls are conductive, while the conductivity of the head-to-head domain walls is larger than that of the tail-to-tail domain walls by about two orders of magnitude at the same inclination angle of domain walls.

Anisotropic growth of domain rays in lithium niobate crystal induced by IR laser scanning

Formation of nanodomain structure after linear scanning by IR laser beam in congruent lithium niobate crystal was attributed to switching under the action of pyroelectric field. The complicated domain patterns consisted of strictly oriented narrow stripe domains (width about 200 nm and depth about 20 µm) arising in a wide switched band. Width of the switched band and domain length density at different distances from the center of the irradiated zone depend on the scanning velocity v s. Relevance of the used model was supported by similarity of the experimental and calculated dependences.

Degradation of the electrical conductivity of the charged domain wall in reduced lithium niobate

In the present work, we investigated the effect of prolonged exposure on the electrical conductivity of crystals with a charged domain wall (CDW) in congruent lithium niobate crystals (LiNbO3, LN) of a nonpolar x-cut. Bidomain ferroelectric structures containing charged head-to-head domain boundaries were formed in the samples using methods of diffusion annealing in the air near the Curie temperature and infrared annealing in an oxygen-free environment. Reduction annealing of crystals in a nitrogen atmosphere was carried out to form color centers and concomitant increase in conductivity. Using an atomic force microscope (AFM) we observed the effect of degradation of the current value recorded when measuring the I-V curve. The influence of storage conditions on the electrical conductivity of CDW was studied. It was found that this effect was not related to the influence of the surrounding atmosphere on the surface but was presumably related to the redistribution of charge carriers shielding the bound charge of the CDW.

Degradation of the electrical conductivity of charged domain walls in reduced lithium niobate crystals

In this work, the effect of long-term room temperature exposure on the electrical conductivity of the charged domain wall (CDWs) in nonpolar x-cut congruent lithium niobate (LiNbO3, LN) crystals has been studied. Bidomain ferroelectric structures containing head-to-head charged domain boundaries have been produced by diffusion annealing in air near the Curie temperature and by infrared annealing. The crystals have been reduction annealed in a nitrogen atmosphere for the formation of color centers and growth of the electrical conductivity. The current measured during the recording of the I-V curves of the specimens using scanning probe microscope after room temperature exposure for 91 days has been found to decrease. The effect of storage conditions on the electrical conductivity of the CDWs has been studied. Degradation of the electrical conductivity does not originate from the effect of environment on the crystal surface. It has been hypothesized that the degradation is caused by distribution of charge carriers shielding the bound charge of the CDWs.

Observation of Neutron-Induced Absorption and Phase-Mismatch for Quasi-Phase-Matched Second Harmonic Generation in Congruent Lithium Niobate

We report an experimental observation of radiation-induced optical absorption in undoped congruent lithium niobate (LiNbO3) crystals between the 190 nm and 3200 nm wavelength range. It was found that high-dose (up to 288 Gy) gamma rays did not significantly affect the optical absorption of LiNbO3 during that wavelength range. However, upon the order of 1016 cm−2 neutron irradiation, the change in the absorption coefficient was up to 0.6 cm−1 at a wavelength of 532 nm and remained after 13 days in the dark at room temperature. The nonlinear optical performance was characterized by conducting third-order quasi-phase-matched second harmonic generation in neutron-irradiated periodically-poled LiNbO3, showing that the phase-matching condition was altered, and the conversion efficiency was still comparable with a non-irradiated one.

Measurement of the internal electric field in periodically poled congruent lithium niobate crystals by far-field diffraction

A simple nondestructive diffraction method is introduced to estimate the internal electric field present in the congruent lithium niobate crystal. Due to the presence of an unrelaxed internal field, the as-poled sample acts as an index grating and diffracts the incident light beam. By analyzing the diffraction patterns, we calculated the refractive index difference ($\Delta n = 1.52 \times {{10}^{- 4}}$) between the poled and the unpoled domains, and the duty cycle error of 7.9%. Using the Pockels effect, the internal field ${E_i}$ present in the periodically poled lithium niobate crystal is estimated to be 2.56 kV/mm.

Efficient generation of a high-field terahertz pulse train in bulk lithium niobate crystals by optical rectification.

We demonstrate a highly efficient method for the generation of a high-field terahertz (THz) pulse train via optical rectification (OR) in congruent lithium niobate (LN) crystals driven by temporally shaped laser pulses. A narrowband THz pulse has been successfully achieved with sub-percent level conversion efficiency and multi MV/cm peak field at 0.26 THz. For the single-cycle THz generation, we achieved a THz pulse with 373-μJ energy in a LN crystal excited by a 100-mJ laser pulse at room temperature. The conversion efficiency is further improved to 0.77 % pumped by a 20-mJ laser pulse with a smaller pump beam size (6 mm in horizontal and 15 mm in vertical). This method holds great potential for generating mJ-level narrow-band THz pulse trains, which may have a major impact in mJ-scale applications like terahertz-based accelerators and light sources.

Parametrization of nonstoichiometric lithium niobate crystals with different states of defectivity

Abstract Quasi-elastic light scattering (QELS) and Raman scattering have been studied at 296 K in congruent lithium niobate (CLN) crystals with a different degree of defectiveness. The defectiveness degree has been detected due to acoustic Q-factor. A quantitative analysis of spectra has been carried out in the frequency range 0–70 cm−1 in samples with different Q-factor. The applied model considers a connection between low-frequency optical mode A1(TO) symmetry type with relaxing intrinsic energy. The model describes stoichiometry defects. Results of model calculations and experiments allows us to conclude that different types of defects play a substantial role in formation of dynamic central peak at a structure phase transition in CLN crystals.

Photoluminescence and Particular Features of the Defect Structure of Congruent and Near-Stoichiometric Lithium Niobate Crystals Obtained Using Different Technologies

Bulk photoluminescence of congruent and near-stoichiometric lithium niobate crystals that were obtained by different technologies is studied. It is found that the luminescence intensity in crystals whose composition is close to stoichiometric is lower than that in congruent crystals. It is shown that not only luminescence centers of the main types, which are caused by the occurrence of NbLi, VLi, etc., point defects, but also complex defects that are caused by the occurrence of OH groups in the structure can contribute to luminescence.

Photoluminescence of Nominally Pure Lithium Niobate Single Crystals Produced by Various Technologies

Volume and surface photoluminescence of congruent and stoichiometric lithium niobate crystals obtained using various technologies was studied. The luminescence intensity of the stoichiometric crystal was less than that of the congruent one. Volume luminescence of the crystals was mainly caused by NbLi defects while luminescence quenching of the near-surface layer was observed in the long-wavelength spectral region (>500 nm) because of energy scattering in crystal-lattice vibrations and increased luminescence intensity of the $$ {\mathrm{Nb}}_{\mathrm{Nb}}^{4+}-{\mathrm{O}}^{-} $$ pair. Luminescence bands with maxima at 426 and 446 nm were caused by complex defects in the form of electron–hole pairs $$ {\mathrm{Nb}}_{\mathrm{Nb}}^{4+}-{\mathrm{O}}^{-} $$ in which the Nb atom was bound to oxygen atoms by covalent and ionic bonds. An increase of the Li/Nb ratio led to a shift of the luminescence bands to shorter wavelengths and a change of the fundamental absorption edge of the studied crystals.

Self-assembled shape evolution of the domain wall and formation of nanodomain wall traces induced by multiple IR laser pulse irradiation in lithium niobate

The formation of the domain structure in congruent lithium niobate crystals by multiple pulse irradiation using an infrared laser was studied experimentally. The qualitative change of the domain wall shape with an increase of the pulse number was revealed. The chains of isolated nanodomains representing the domain wall traces appeared after each laser pulse irradiation in the samples with an irradiated polar surface covered by an indium–tin oxide layer. This effect allowed extraction of the detailed information about the evolution of the domain wall shape. The domain kinetics under the action of the pyroelectric field has been considered. The mechanism of the self-assembled shape evolution of the moving domain wall as a result of the local wall accelerations and retardations caused by the shape fluctuations (bumps and valleys) was proposed. The experimentally observed formation of the quasi-regular comb-like domain structures with randomly distributed bumps and valleys was confirmed by computer simulation. The discovered effects allowed us to create the stable random and quasi-regular domain structures and open the way for further development of the domain engineering methods.

Фотолюминесценция и особенности дефектной структуры конгруэнтных и близких к стехиометрическому составу кристаллов ниобата лития, полученных по разным технологиям

Photoluminescence was studied in the volume of congruent and near-stoichiometric lithium niobate crystals obtained by different technologies. Luminescence intensity in near-stoichiometric crystals was found to be lower than that in a congruent crystal. We have shown that not only basic centers (caused by point defects NbLi, VLi) contribute to luminescence, but also complex defects caused by existence of OH-groups.

Domain structure formation by electron beam irradiation in lithium niobate crystals at elevated temperatures

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.

Tailoring of stable induced domains near a charged domain wall in lithium niobate by probe microscopy

Ferroelectric lithium niobate (LiNbO3) crystals with an engineered domain structure have a number of applications in optical systems for generation of multiple laser radiation harmonics, acoustooptics, precision actuators, vibration and magnetic field sensors, including those for high-temperature applications, and prospectively, in non-volatile computer memory. We have studied the effect of charged domain boundary on the formation of induced domain structures in congruent lithium niobate (LiNbO3) crystals at the non-polar x-cut. Bi- and polydomain ferroelectric structures containing charged “head-to-head” and “tail-to-tail” type domain boundaries have been formed in the specimens using diffusion annealing in air ambient close to the Curie temperature and infrared annealing in an oxygen free environment. The surface potential near the charged domain wall has been studied using an atomic force microscope (AFM) in Kelvin mode. We have studied surface wedge-shaped induced microscopic domains formed at the charged domain boundary and far from that boundary by applying electric potential to the AFM cantilever which was in contact with the crystal surface. We have demonstrated that the morphology of the induced domain structure depends on the electrical conductivity of the crystals. The charged “head-to-head” domain boundary has a screening effect on the shape and size of the domain induced at the domain wall. Single wedge-shaped domains forming during local repolarization of reduced lithium niobate crystals at the AFM cantilever split into families of microscopic domains in the form of codirectional beams emerging from a common formation site. The charged domain wall affects the topography of the specimens by inducing the formation of an elongated trench, coincident with the charged boundary, during reduction annealing.

Congruent lithium niobate crystal with a stoichiometric structure

Congruent lithium niobate crystal with a stoichiometric structure

Квазиупругое и низкочастотное комбинационное рассеяние света в кристаллах ниобата лития с дефектами стехиометрии

The paper investigates quasielastic light scattering spectra at a temperature of 296 K in lithium niobate samples of various degrees of imperfection as measured by means of the acoustic quality factor. We performed a quantitative spectrum analysis in the 0--70 cm-1 frequency range for samples with different Q-factor values in a model accounting for the connection between a low-frequency optical mode of the A1(TO) symmetry type and the acoustic density of states observed in a spectrum as a result of violating the wavevector selection rule in a stoichiometrically defective crystal. The results of comparing these simulations to experimental data show that stoichiometrical defects significantly contribute to the quasielastic light scattering intensity in congruent lithium niobate crystals

Kinetic Dependencies of the Photorefractive Effect in Lithium Niobate Crystals

The kinetic dependence of the photorefractive effect in the congruent lithium niobate crystal is determined in the work. The adequacy of the model for calculating the photoelectric fields from the indicatrix of photoinduced light scattering is shown. Two methods for determining the induced birefringence by interferograms and on the parameters of photoinduced light scattering are compared.

Physicochemical and Optical Characteristics of LiNbO3 Single-Crystals Doped with Boron

The chemical interaction in the Li2CO3–Nb2O5 and Li2O–B2O3–Nb2O5 systems and crystallization features of LiNbO3 crystals from the melts containing boron impurity are considered. Raman spectra of LiNbO3:B crystals grown from a congruent melt containing ~0.55–1.24 mol % of B2O3 are studied. There are noticeable changes in the entire Raman spectrum upon doping with boron, which indicates a change in alternation of the main cations and vacancies along the polar axis of the LiNbO3:B crystal and “perturbation” of oxygen octahedra. Moreover, expansion of oxygen octahedra is anisotropic with an increase in boron concentration in the melt. Boron, hardly entering the structure of the lithium niobate crystal, significantly changes the structure of the melt and thus has a significant effect on structure and physical characteristics of the LiNbO3:B crystals. The results obtained for the LiNbO3:B crystals are compared with those obtained for nominally pure stoichiometric (LiNbO3stoich) and congruent (LiNbO3cong) lithium niobate crystals.