Stoichiometric Lithium Niobate Crystals Research Articles

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.

Near stoichiometric lithium niobate crystal with dramatically enhanced piezoelectric performance for high temperature acceleration sensing

Lithium niobate (LN) is a multifunctional crystal with excellent piezoelectric properties, making it a potential candidate for piezoelectric sensing applications. In this study, the mechanism of the discrepancies of piezoelectric...

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.

Temperature-dependent terahertz time-domain spectroscopy study of Mg-doped stoichiometric lithium niobate

Terahertz time-domain spectroscopy measurements were performed on 0.7 mol% Mg-doped stoichiometric lithium niobate crystal with ordinary and extraordinary polarization in the 4-460 K temperature range. The absorption coefficient and refractive index spectra were recorded in the terahertz frequency range from 0.5 to 1.8 THz. The data extracted from the measurements are given in simple and concise form in order to provide easily usable practical information for those who would like to use this material in the terahertz range. Through a practical example it was also pointed out, that the effect of the temperature must not be neglected during the design and adjustment of terahertz sources if the goal is to maximize the optical-to-THz conversion efficiency.

Stoichiometric Lithium Niobate Crystals: Towards Identifiable Wireless Surface Acoustic Wave Sensors Operable up to 600 °C

Wireless surface acoustic wave (SAW) sensors constitute a promising solution to some unsolved industrial sensing issues taking place at high temperatures. Currently, this technology enables wireless measurements up to 600-700$^\circ$C at best. However, the applicability of such sensors remains incomplete since they do not allow identification above 400$^\circ$C. The latter would require the use of a piezoelectric substrate providing a large electromechanical coupling coefficient K 2 , while being stable at high temperature. In this letter, we investigate the potentiality of stoichiometric lithium niobate (sLN) crystals for such purpose. Raman spectroscopy and X-ray diffraction attest that sLN crystals withstand high temperatures up to 800$^\circ$C, at least for several days. In situ measurements of sLN-based SAW resonators conducted up to 600$^\circ$C show that the K 2 of these crystals remains high and stable throughout the whole experiment, which is very promising for the future achievement of identifiable wireless high-temperature SAW sensors.

Defect Complexes in Stoichiometric Lithium Niobate Crystals Prepared by Different Processes

Using IR absorption spectroscopy in the stretching region of OH groups and Raman spectroscopy in the region of two-particle states of acoustic phonons, we have demonstrated that high-temperature top-seeded solution growth from a melt of congruent composition containing 6.0 wt % K2O flux in Kristall-2 commercially available growth units makes it possible to obtain nearly stoichiometric LiNbO3 crystals having approximately the same perfection of the proton sublattice as in stoichiometric crystals grown from a melt containing 58.6 mol % Li2O, but considerably surpassing them in general optical and structural homogeneity.

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.

Photoelectric Fields and Band Gap in Doped Lithium Niobate Crystals

Photoinduced light scattering and optical spectroscopy have been used to study the photorefractive effect and determine the band gap in nominally undoped congruent and stoichiometric lithium niobate crystals, as well as in a series of congruent LiNbO3 crystals doped with Mg, Zn, B, Gd, Y, and Er cations and LiNbO3 single crystals codoped with Mg:Gd, Mg:Fe, Mg:Y, and Mg:Ta.

Photoelectric Fields and Band Gap in Doped Lithium Niobate Crystals

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Optical properties of lithium niobate crystals

According to optical absorption spectra the width of the forbidden band of nominally pure congruent and stoichiometric lithium niobate crystals, as well as a series of congruent crystals doped with cations of Mg2+(0.35wt.%), Zn2+(2.05), B3+(0.12), Gd3+(0.26, 0.44, 0.51), Y3+(0.46), Gd3+(0.23):Mg2+(0.75), Mg2+(0.86):Fe3+(0.0036), Ta5+(1.13):Mg2+(0.01), Y3+(0.24):Mg2+(0.63), Er3+(3.1) is determined. The photorefractive effect in crystals, their structural and optical homogeneity was studied by photo induced and Raman scattering methods, laser conoscopy.

Optical homogeneity and photorefractive properties of stoichiometric and congruent lithium niobate crystals grown using charges of different origins

A stoichiometric lithium niobate crystal (LiNbO3 st) and congruent lithium niobate crystals grown from a charge prepared using cyclohexanone as an extractant (LiNbO3 cong(CHN)) and a charge prepared using cyclohexanone and carboxylic acid dimethylamides as extractants (LiNbO3 cong(CHN + A)) have been characterized by photoinduced light scattering and laser conoscopy. The results demonstrate that the LiNbO3 cong(CHN + A) crystals are rather homogeneous along their growth direction and possess good optical properties, similar to those of the LiNbO3 cong(CHN) crystal. At the same time, the LiNbO3 cong(CHN + A) crystal offers significantly better electro-optical properties (r e = 29.3 pm/V).

Manifestation of structural features of LiNbO3:Zn and LiNbO3:Mg crystals in their IR absorption spectra in the stretching region of OH– groups

Analysis of IR absorption spectra of LiNbO3:Zn (0.04–4.46 mol % ZnO) and LiNbO3:Mg (0.19–5.91 mol % MgO) single crystals in the stretching region of OH– groups has been used to gain insight into composition-dependent structural changes in the crystals. The results demonstrate that, characteristically, the OH– groups occupy different sites in the doped and congruent LiNbO3 crystals and have different quasielastic O–H bond constants in their structure. In stoichiometric lithium niobate crystals, all of the OH– sites and quasi-elastic O–H bond constants are identical. At threshold Zn and Mg dopant concentrations, the frequencies, widths, and intensities of the observed lines change sharply. The linewidths in the spectra of the LiNbO3:Zn crystals near their first concentration threshold (≈2.0 mol % ZnO) and the LiNbO3:Mg crystals near their first and second concentration thresholds (≈3.0 and 5.5 mol % MgO) decrease markedly, indicating ordering of the position of the OH– groups in the structure of the crystals.

Investigation of the cluster formation in lithium niobate crystals by computer modeling method

The processes occurring upon the formation of energetically equilibrium oxygen-octahedral clusters in the ferroelectric phase of a stoichiometric lithium niobate (LiNbO3) crystal have been investigated by the computer modeling method within the semiclassical atomistic model. An energetically favorable cluster size (at which a structure similar to that of a congruent crystal is organized) is shown to exist. A stoichiometric cluster cannot exist because of the electroneutrality loss. The most energetically favorable cluster is that with a Li/Nb ratio of about 0.945, a value close to the lithium-to-niobium ratio for a congruent crystal.

Substitution mechanisms and location of Co2+ ions in congruent and stoichiometric lithium niobate crystals derived from electron paramagnetic resonance data

Electron paramagnetic resonance (EPR) spectra and their angular dependencies were measured for Co2+ trace impurities in stoichiometric samples of lithium niobate doped with rhodium. It was found that Co2+ substitutes for Li+ in the dominant axial center (CoLi) and that the principal substitution mechanism in stoichiometric lithium niobate is 4Co2+ ↔ 3Li+ + Nb5+. The four Co2+ ions can occupy the nearest possible cation sites by occupying a Nb site and its three nearest-neighbor Li sites, creating a trigonal pyramid with C3 symmetry, as well as non-neighboring sites (e.g. a CoNb–CoLi pair at the nearest sites on the C3 axis with two nearby isolated single Co2+ ions substituted for Li+). In congruent crystals and samples with Li content enriched by vapor transport equilibrium treatment the excess charge of the Co2+ centers is compensated by lithium vacancies located rather far from the Co2+ ions for the dominant axial center or in the nearest neighborhood for low-symmetry satellite centers (the Co2+ ↔ 2Li+ substitution mechanism). The use of exact numerical diagonalization of the spin-Hamiltonian matrices explains all the details of the EPR spectra and gives a value for hyperfine interaction A|| that is several times smaller than that obtained using perturbation formulae. The refined values of A and g-tensor components can be used as reliable cornerstones for ab initio and cluster calculations.

Longitudinally polarized single-cycle terahertz pulses generated with high electric field strengths

We demonstrate the generation of single-cycle longitudinally polarized terahertz pulses with field amplitudes in excess of 11 kV/cm using the interferometric recombination of two linearly polarized terahertz beams. High field strength transversely polarized pulses were generated by optical rectification in a matched pair of magnesium-oxide doped stoichiometric lithium niobate (MgO:SLN) crystals with a reversal in the χ333(2) orientation. The discontinuity in χ333(2) produces a polarity flip in the transverse field; the longitudinal field produced as a consequence of the transverse field discontinuity was measured in the far-field. Both the spatial and temporal profiles of the measured longitudinally polarized terahertz radiation were consistent with the propagation of the transverse discontinuity.

Site-selective measurement of relaxation properties at 980 nm in Er $$^{3+}$$ 3 + -doped congruent and stoichiometric lithium niobate crystals

A pump-probe-type saturation spectroscopic experiment has been performed at 980 nm to measure the homogeneous linewidth of the $$^4$$ I $$_{11/2}$$ – $$^4$$ I $$_{15/2}$$ transition and the lifetime of the $$^4$$ I $$_{11/2}$$ state of erbium embedded into lithium niobate single crystals. There are several non-equivalent Er $$^{3+}_{\mathrm{Li}^+}$$ –V $$_{\mathrm{Li}^+}$$ sites with differently oriented defect structure for charge compensation. These non-equivalent centers differ in their transition energies. Hence, our measurements have been done at two nearby frequencies, addressing groups of erbium ions located at non-equivalent sites. The results are compared with lifetime calculations and measurements found in the literature and with the predictions of our simple model calculation.

Determination of g-tensors of low-symmetry Nd3+ centers in LiNbO3 by rectification of angular dependence of electron paramagnetic resonance spectra

Two procedures for facilitation of line tracing and deciphering of complicated spectra of electron paramagnetic resonance (EPR) were developed: a correction of microwave frequencies for every orientation of external magnetic field on the base of known values of g-tensor components for a reference paramagnetic center and followed rectification of measured angular dependences using plots of effective deviation of g2-factors of observed lines from effective g2-factors of the reference center versus angles or squared cosines of angles describing magnetic field orientations. Their application to EPR spectra of nearly stoichiometric lithium niobate crystals doped with neodymium allowed identifying two axial and six different low-symmetry Nd3+ centers, to determine all components of their g-tensors, and to propose common divacancy models for a whole family of Nd3+ centers.

Measurement of Refractive Index and Absorption Coefficient of Congruent and Stoichiometric Lithium Niobate in the Terahertz Range

Time domain THz spectroscopy measurements were performed on a series of undoped and Mg-doped congruent lithium niobate crystals with 1.2, 6.1, and 8.4 mol% Mg concentrations and on undoped and Mg-doped stoichiometric lithium niobate crystals with 0.7, 1.5, and 4.2 mol% Mg concentrations with polarization parallel (extraordinary) and perpendicular (ordinary) to the z axis of the crystal at 300 K. The absorption coefficient and refractive index spectra were determined in the THz frequency range from 0.25 to ~2.5 THz. In the case of congruent samples for both polarizations, both the refractive index and the absorption coefficient have minimal values for compositions close to the photorefractive threshold. In the case of stoichiometric samples, similar tendencies close to the photorefractive threshold at lower Mg concentration were observed but only for extraordinary polarization, while for ordinary polarization the measured values, especially for the absorption coefficient, were only weakly dependent on the Mg content.

Complex investigations of structural and optical homogeneities of low-photorefractivity lithium niobate crystals by the conoscopy and photoinduced and Raman light scattering methods

Using photoinduced light scattering, conoscopy, and Raman spectroscopy methods, we have studied stoichiometric lithium niobate crystals and congruent crystals that were doped with Mg(0.078, 0.89 mas %), Zn(0.03, 0.52, 0.62), Cu(0.015), B(0.12), Gd(0.51), Y(0.46), Gd(0.23):Mg(0.75), Mg(0.86):Fe(0.0036), Ta(1.13):Mg(0.011), and Y(0.24):Mg(0.63) cations. It has been found that, depending on the kind of the pattern of photoinduced light scattering, investigated specimens can be divided into three groups. We have shown that the asymmetry of the indicatrix of photoinduced light scattering of LiNbO3 crystals is caused by birefringence of exciting laser radiation as it propagates perpendicularly to the polar axis of the crystal, whereas the asymmetry of the Raman spectrum arises due to the occurrence of spontaneous polarization, the vector of which is directed along the polar axis, and by birefringence. The pattern of the photoinduced light scattering depends on the difference of the refractive indices Δn = no − ne of the ordinary (no) and extraordinary (ne) rays and their energies E. If Eno {ie259-1} Ene, the proportion of the photoinduced light scattering has the shape of a three-layer round spot. For equal energies, the pattern has the shape of a symmetric figure-eight. At Eno < Ene, the figure-eight is asymmetric. In this case, its large “lobe” is directed in the positive direction of the polar axis of the crystal.

Homogeneous linewidth measurements of Yb3+ ions in congruent and stoichiometric lithium niobate crystals

We report the result of a series of pump–probe spectral hole burning measurements on Yb3+ doped stoichiometric and congruent lithium niobate single crystal. The measurements were performed with a stabilized diode laser tuned to the ≈980nm transition of the dopant ions. The crystal samples were placed in a closed cycle helium cryostat, the experiments have been done in the temperature range 9–20K. The observed spectral holes have complex structure, at least two Lorentzian curves are needed to reproduce their shape (double spectral hole). A simple theoretical model has been developed to describe the line-shape resulting from the interaction between the light pulses and the dopant ions. Using the predictions of the theory, the homogeneous linewidths and the population relaxation rates have been estimated for both crystal samples. Using these results, the temperature dependence of the spectral hole widths and the spectral diffusion rates have also been determined. Some considerations are presented to explain the origin of the double spectral hole structure.