Congruent Lithium Niobate Single Crystals Research Articles

Theoretical studies and structural analysis of the Nd3+ centers with trigonal symmetry in congruent lithium niobate single crystals

In this paper, the superposition model (SM) and complete diagonalization procedure of energy matrix (CDM method) were used for a few selected Nd3+ (4f3) paramagnetic centers with trigonal symmetry in congruent lithium niobate (LN, LiNbO3) single crystals. Additionally, the g-shift method was applied to these systems. In general, the calculated g-factors (parallel and perpendicular) match the measured values very well. The displacements of the Nd3+ ions calculated by g-shift method are also fit very well to the experimental values. A detailed structural analysis of paramagnetic Nd3+ centers with trigonal symmetry in congruent LN single crystals was also performed. The charge compensation mechanism and its role in establishing Nd3+ centers in congruent LN are also debated.

High-temperature electrical conductivity in piezoelectric lithium niobate

Lithium niobate is a promising candidate for use in high-temperature piezoelectric devices due to its high Curie temperature (≈1483 K) and strong piezoelectric properties. However, the piezoelectric behavior has, in practice, been found to degrade at various temperatures as low as 573 K, with no satisfactory explanation available in the literature. We, therefore, studied the electrical conductivity of congruent lithium niobate single crystals in the temperature range of 293–1273 K with an 500 mV excitation at frequencies between 20 Hz and 20 MHz. An analytical model that generalizes the universal dielectric relaxation law with the Arrhenius equation was found to describe the experimental temperature and frequency dependence and helped discriminate between conduction mechanisms. Electronic conduction was found to dominate at low temperatures, leading to low overall electrical conductivity. However, at high temperatures, the overall electrical conductivity increases significantly due to ionic conduction, primarily with lithium ions (Li+) as charge carriers. This increase in electrical conductivity can, therefore, cause an internal short in the lithium niobate crystal, thereby reducing observable piezoelectricity. Interestingly, the temperature above which ionic conductivity dominates depends greatly on the excitation frequency: at a sufficiently high frequency, lithium niobate does not exhibit appreciable ionic conductivity at high temperature, helping explain the conflicting observations reported in the literature. These findings enable an appropriate implementation of lithium niobate to realize previously elusive high-temperature piezoelectric applications.

Investigations on Crystalline Perfection, Raman Spectra and Optical Characteristics of Transition Metal (Ru) Co-Doped Mg:LiNbO3 Single Crystals.

Congruent lithium niobate single crystals with a Ru:Mg co-dopant have been successfully grown using the Czochralski technique from the melt containing 0.02 mol % Ru with Mg of two varied concentrations (4.0 and 6.0 mol %). The effects of Ru and Mg co-doping on the crystalline quality were determined by high-resolution X-ray diffractometry, which confirmed that the crystalline quality is good and that the dopants are statistically distributed in the crystal. The Raman scattering analysis shows no change in the lattice vibration except a slight change in the peak width and intensity due to more asymmetry in the molecular charge, which leads to enhancement of the polarizability. The optical transmission spectra indicate that both the crystals have high optical transparency in the visible region, with a shift of the absorption edge toward shorter wavelengths, as compared to un-doped LN. The weak absorption band observed below 400 nm is attributed to Ru ions. The influence of co-doping in the electronic band gap energies is calculated by the Tauc relation. The refractive index is measured by using a prism coupler at two wavelengths (532 and 1064 nm). The calculated absorption coefficients and direct and indirect band gap energies for both the samples are found to be nearly the same within experimental error. A decrease in the birefringence is observed for the Ru:Mg(6 mol %) doped sample. The observed slight reduction in refractive indices with Ru:Mg co-doping is consistent with a rise in band gap energy, which is related to the change in absorption edge to the lower wavelength. The second harmonic generation (SHG) efficiency is measured by the Kurtz and Perry powder method, and a decrease in SHG efficiency for Ru:Mg(6 mol %) is observed.

Domain structure evolution under multiple pulse heating of lithium niobate by infrared laser

Domain structure evolution in congruent lithium niobate single crystals under multiple infrared laser pulse irradiation was studied. The structure of narrow domain rays appeared after the first pulse became denser with pulse number due to ray widening and intensive fingering, which led to increasing the domain walls density. The almost constant wall density in the central region of irradiated zone significantly increased near the zone edge. Increase in the initial crystal temperature led to decrease in the wall density. The obtained radial dependence of the wall density was attributed to computer simulated spatial distribution of the pyroelectric field.

Superfast domain wall motion in lithium niobate single crystals. Analogy with crystal growth

In this paper, we studied the superfast domain walls that appear after domain merging during polarization reversal in congruent lithium niobate single crystals by in situ optical visualization with a high temporal resolution. The superfast domain walls were observed during the fast transformation of the domain shape from concave polygonal that appears after merging to the convex polygonal one with slow domain walls only. Orientations and velocities of superfast domain walls were measured during transformation of an artificial polygonal X-shaped domain (with all concave angles appearing after merging) to a hexagonal one. The “convex” growth of isolated hexagonal domains was governed by the slowest domain walls, while the “concave” growth after domain merging—by the superfast walls with three orders of magnitude higher velocity. We reconstructed experimentally the v-plot (kinetic Wulff plot) for domain wall motion by analysis of both convex and concave domain growth. The slowest walls (Y-walls) correspond to the v-plot deep cusps having six-fold symmetry, whereas the superfast walls correspond to the v-plot maxima having three-fold symmetry with a difference between Y+ and Y− crystallographic directions. The field dependences of the velocities of the superfast domain walls followed the activation law, taking into account the existence of the bias field.

Photoelectric fields in doped lithium niobate crystals

Photoinduced light scattering (PILS) in nominally pure stoichiometric and congruent lithium niobate single crystals (LiNbO3), and ones doped with B3+, Cu2+, Zn2+, Mg2+, Gd3+, Y3+, Er3+ cations was studied. All crystals have a relatively low effect of photorefraction and are promising materials for frequency conversion, electro-optical modulators and shutters. It was found that the photovoltaic and diffusion fields for some crystals have a maximum at a wavelength of 514.5 nm. All the crystals studied are characterized by a maximum of the integral intensity of the speckle structure of the PILS at a wavelength of 514.5 nm.

Improvement of pyroelectric figures of merit in zirconia-doped congruent lithium niobate single crystals

Pyroelectric materials are widely investigated for thermal devices. As an efficient standard for evaluation, the pyroelectric figures of merit (FoM) for specific thermal-related applications can clearly represent the performance of voltage responsivity and detectivity. In this paper, six congruent lithium niobate (CLN) single crystals with different ZrO2 doping concentrations were grown by the top-seeded melt growth method including pure CLN. In order to analyze the dielectric and pyroelectric performances, the concentrations, and occupations were extensively studied by ultraviolet and infrared absorption spectra. Our results present an efficient way to improve the FoM of CLN by using zirconia doping. The working temperature of Zr:CLN thermal device can be widened obviously.

Self-Organized Nanodomain Structures Arising in Lithium Tantalate and Lithium Niobate after Pulse Heating by Infrared Laser

Formation of nanodomain structures during cooling after pulse heating by infrared laser has been studied in congruent lithium tantalate (CLT) and lithium niobate (CLN) single crystals. In situ study of the domain structure evolution allowed to reveal that in CLN the isolated domains appeared at the edges of the irradiated zone and grew to the center. In contrast, the quasi-regular stripe domain structure appeared in CLT near the edge of irradiated zone. The difference has been attributed to lower Curie temperature and larger pyroelectric coefficient of CLT. The self-organized domain structures can be used for periodical poling with submicron periods.

Electron Beam Domain Patterning of MgO-Doped Lithium Niobate Crystals Covered by Resist Layer

The electron beam domain patterning of MgO-doped congruent lithium niobate (LN) single crystals covered by electron beam resist layer were studied for various layer thicknesses and electron accelerating voltages. The obtained domain patterns were divided in four types. The increasing of the domain structure quality under presence of the resist layer on irradiated surface was confirmed. This effect was attributed to high concentration of the electron traps in resist, which localized incident electrons in limited volume over LN surface and formed effective electrode. The voltage dependence of space charge localization relative to the LN surface was studied by computer simulation.

Effect of Mg doping on the growth aspects, crystalline perfection, and optical and thermal properties of congruent LiNbO3single crystals

Mg-doped congruent lithium niobate single crystals were grown by the Czochralski technique. High-quality single crystals were grown using a novel seeding technique in a resistive heating furnace. Analysis of crystalline perfection carried out by a multi-crystal X-ray diffractometer revealed that the grown crystals do not contain any structural grain boundaries but do contain point defects. The transmission characteristics showed an enhancement of band gap with an increase in Mg concentration. Conoscopy patterns revealed that the grown crystals are homogeneous and the incorporation of Mg into the lattice does not affect the optical sign (negative uniaxial) of the crystal. The refractive index measurements carried out using a prism coupler showed an increase in the optical birefringence (Δn), while the refractive index was found to decrease with the increase in doping concentration. Further, thermal conductivity was found to decrease with Mg incorporation in the lattice owing to phonon scattering from the Mg ions and, as a consequence, at high concentrations (>4 mol%) crack formation occurred. However, optimization of growth conditions reveals that a slower pulling rate leads to crack-free lithium niobate crystals even at 6 mol% Mg doping.

Structural, optical and thermal properties of Zr–Fe co-doped congruent LiNbO3single crystals

Zr–Fe-doped congruent lithium niobate single crystals were grown by the Czochralski technique. The crystal structure and lattice parameter of the grown crystals were assessed by powder X-ray diffraction and the strain developed as a result of doping has been calculated (−1.19 × 10−3) by using the Williamson–Hall relation. The incorporated dopant concentration along with the dopant distribution in the specimen crystal was estimated by X-ray florescence spectrometry. A multi-crystal X-ray diffraction analysis was carried out to identify the crystalline perfection of the sample and revealed that the investigated crystal does not contain any structural grain boundaries but does contain point defects and micrometre size mosaic blocks. Birefringence measurements were carried out using a prism coupler spectrometer and found that the optical birefringence is 0.0822 for 532 nm and 0.705 for 1064 nm. A thermal conductivity (κ) study reveals that the doped sample has a lower κ value than the undoped equivalent.

Domain Kinetics in Lithium Niobate Single Crystals with Photoresist Dielectric Layer

The influence of the artificial surface layer (photoresist film) on the domain kinetics was studied in congruent lithium niobate single crystals. The switching current data was fitted by modified Kolmogorov-Avrami formula. The qualitative change of the domain structure evolution for photoresist film thickness above 2 μm was revealed by in situ optical visualization and analysis of the switching current data. The linear dependence of the threshold field increase on the film thickness was found. The quasi-regular nanodomain structures on Z+ polar surface were studied by scanning probe and Raman confocal microscopy and the domain evolution scenario was revealed.

The Domain Kinetics in Congruent Lithium Niobate Modified by Low and High Energy Ion Irradiation

Domain kinetics was studied during polarization reversal in congruent lithium niobate (CLN) single crystals irradiated by low energy Ar ions and high energy Br and F ions. The optical microscopy was used for in situ visualization of the domain structure evolution. In CLN irradiated by low energy ions the charged domain walls were formed in the fields below the threshold value. In fields above the threshold the jump-like walls motion was resulted in formation of the domain chains along the wall rest positions. The smooth motion of X-oriented domain walls was obtained in CLN irradiated by high energy ions.

Raman Study of Neutral and Charged Domain Walls in Lithium Niobate

In this work, we demonstrate the application of micro-Raman spectroscopy for the investigation of the neutral and charged domain walls in the congruent and MgO-doped lithium niobate single crystals. We present local changes of the intensity and frequency of three Raman lines in the vicinity of the domain walls and discuss the influence of the bulk screening of depolarization field on these parameters.

Recharging processes, radiation induced strain and changes of OH − bands under H + ion implantation in Ti doped lithium niobate

A systematic analysis of variations in structural and optical characteristics of Z-cut plates of titanium doped congruent lithium niobate single crystals implanted with 120 keV proton beam at various fluences of 10 15, 10 16 and 10 17 protons/cm 2 is presented. Through, high resolution X-ray diffraction, atomic force microscopy, Fourier transform infrared and UV–visible–NIR analysis of congruent lithium niobate, the correlation of properties before and after implantation are discussed. HRXRD (0 0 6) reflection by Triple Crystal Mode shows that both tensile and compressive strain peak are produced by the high fluence implantation. A distinct tensile peak was observed from implanted region for a fluence of 10 16 protons/cm 2. AFM micrographs indicate mountain ridges, bumps and protrusions on target surface on implantation. UV–visible–NIR spectra reveal an increase in charge transfer between Ti 3+/Ti 4+ and ligand oxygen for implantation with 10 15 protons/cm 2, while spectra for higher fluence implanted samples show complex absorption band in the region from 380–1100 nm. Variations of OH − stretching vibration mode were observed for cLN Pure, cLNT2% virgin, and implanted samples with FTIR spectra. The concentration of OH − ion before and after implantation was calculated from integral absorption intensity. The effect of 120 keV proton implantation induced structural, surface and optical studies were correlated.

Manifestation of the specific structural features of lithium niobate single crystals of different composition in their Raman spectra

The Raman spectra of nominally pure lithium niobate single crystals of congruent, close to stoichiometric, and stoichiometric compositions and congruent lithium niobate single crystals doped with Gd3+, Y3+, and Mg2+ ions have been investigated. Weak lines whose widths anomalously decrease with an increase in cation sublattice disordering at a change in the single crystal composition were found for the first time. These lines may be indicative of fine ordering processes involving structural units of the cation sublattice, as a result of which this sublattice is disordered as a whole.

Influence of Surface Layers Modified by Proton Exchange on Domain Kinetics of Lithium Niobate

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.

Self-similar surface nanodomain structures induced by laser irradiation in lithium niobate

Self-similar surface structures composed of nanodomain rays, which are formed in congruent lithium niobate single crystals under pulsed laser irradiation, are investigated. The computer simulation of the formation of the domain structure is performed using the experimentally revealed rules of ray growth. It is demonstrated that the domain structures formed are fractal objects with a limited range of scaling.

Precision nanoscale domain engineering of lithium niobate via UV laser induced inhibition of poling

Continuous wave ultraviolet laser irradiation at λ=244nm on the +z face of undoped and MgO doped congruent lithium niobate single crystals has been observed to inhibit ferroelectric domain inversion. The inhibition occurs directly beneath the illuminated regions, in a depth greater than 100nm during subsequent electric field poling of the crystal. Domain inhibition was confirmed by both differential domain etching and piezoresponse force microscopy. This effect allows the formation of arbitrarily shaped domains in lithium niobate and forms the basis of a high spatial resolution microstructuring approach when followed by chemical etching.

Direct-writing of inverted domains in lithium niobate using a continuous wave ultra violet laser

The inversion of ferroelectric domains in lithium niobate by a scanning focused ultra-violet laser beam (lambda = 244 nm) is demonstrated. The resulting domain patterns are interrogated using piezoresponse force microscopy and by chemical etching in hydrofluoric acid. Direct ultra-violet laser poling was observed in un-doped congruent, iron doped congruent and titanium in-diffused congruent lithium niobate single crystals. A model is proposed to explain the mechanism of domain inversion.