Growth and optical homogeneity investigations of Fe and Fe:Mn codoped LiNbO3 crystals

Lithium niobate (LiNbO₃) crystals doped with Fe and Fe:Mn were grown by Czochralski technique. The doping concentrations of Fe and Mn were optimized. Transmission studies reveal broad absorption band centered at 488 nm. The UV cutoff observed for Fe doped LiNbO₃ is 358 nm whereas for Fe:Mn codoped LiNbO₃ is 352 nm. This decrease in UV cutoff for Fe and Mn codoped LiNbO₃ compared to only Fe doped LiNbO₃ is due to the increase in Li/Nb ratio. Optical homogeneity was assessed using conoscopy and birefringence interferometry. Dark and photo conductivity measurements prove that LiNbO₃ is a negative photo conducting material. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of thermal and thermoelectric treatments on structure and electric properties of B2O3–Li2O–Nb2O5 glasses

A transparent glass with the composition 60B₂O₃–30Li₂O–10Nb₂O₅ (mol%) was prepared by the melt quenching technique. The glass was heat-treated with and without the application of an external electric field. The as-prepared sample was heat-treated (HT) at 450, 500 and 550 °C and thermoelectric treated (TET) at 500 °C. The following electric fields were used: 50 kV/m and 100 kV/m. Differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman, dc and ac conductivity, as a function of temperature, were used to investigate the glass and glass-ceramics properties. LiNbO₃ crystals were detected, by XRD, in the 500 °C HT, 550 °C HT and 500 °C TET samples. The presence of an external electric field, during the heat-treatment process, improves the formation of LiNbO₃ nanocrystals at lower temperatures. However, in the 550 °C HT and in the TET samples, Li₂B₄O₇ was also detected. The value of the σ_dc decreases with the rise of the applied field, during the heat-treatment. This behavior can indicate an increase in the fraction of the LiNbO₃ crystallites present in these glass samples. The dc and ac conduction processes show dependence on the number of the ions inserted in the glass as network modifiers.The Raman analysis suggests that the niobium ions are, probably, inserted in the glass matrix as network formers.These results reflect the decisive effect of temperature and electric field applied during the thermoelectric treatment in the structure and electric properties of glass-ceramics.     

Double‐frequency properties of In:LiNbO3 crystals

1 mol%, 2 mol%, 3 mol%, 4 mol% and 5 mol% In³⁺ doped LiNbO₃ crystals were grown by the Czochralski method, respectively. Oxidized treatment of some crystals was carried out. The infrared transmission spectra and photo‐damage resistance of the samples were measured. The results showed that the OH^‐ absorption peaks of In(3mol%):LiNbO₃, In(4mol%):LiNbO₃ and In(5mol%):LiNbO₃ crystals were located at about 3508 cm^‐1, while those of In(1mol%):LiNbO₃ and In(2mol%):LiNbO₃ crystals were located at about 3484cm^‐1. When the doped In³⁺ concentration reached its threshold in LiNbO₃ crystal, photo‐damage resistance of In:LiNbO₃ crystals was two orders of magnitude higher than that of pure LiNbO₃ crystal. The experimental results of the second harmonic generation (SHG) showed that the phase matching temperatures of In:LiNbO₃ crystals were lower than those of Zn:LiNbO₃ and Mg:LiNbO₃ crystals and the SHG efficiency reached 38%. Oxidization treatment was also found to make the dark trace resistance of crystals increase. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The dielectric behavior of a thermoelectric treated B2O3-Li2O-Nb2O5 glass

A transparent glass with the composition 60B₂O₃-30Li₂O-10Nb₂O₅ (mol%) was prepared by the melt-quenching technique. Glass-ceramics, containing LiNbO₃ ferroelectric crystallites, were obtained by heat-treatment (HT) above 500 °C, with and without the presence of an external electric field. The dielectric properties of the glass and glass-ceramic were investigated, as a function of temperature (270-315 K), in the 10 mHz-32 MHz frequency range. The presence of an external electric field, during the heating process, improves the formation of LiNbO₃ crystallites. The rise of the treatment temperature and the applied field, during the heat-treatment, leads to a decrease in the dc electric conductivity (σ_dc), indicating a decrease of the charge carriers number. The dielectric permittivity (ε′) values (300 K;1 kHz) are between 16.25 and 18.83, with the exception of the 550 °C HT sample that presents a ε′ value of 11.25. An electric equivalent circuit composed by an R in parallel with a CPE element was used to adjust the dielectric data. The results reflect the important role carried out by the heat-treatment and the electric field during the HT in the electric properties of glass-ceramics.     

Optimization of the calculation of a local electric field in crystals by the transient-region method

The specific features of using the method of transient-region for calculating local electric fields and the corresponding potentials are considered on examples of the calculations for NaF, CsCl, and LiNbO₃ crystals. It is shown that for crystals with a primitive cubic lattice, this method can provide any given accuracy of the calculated Madelung constant; for displacive-type ferroelectrics (LiNbO₃), the method allows the calculation of the local electric field with a very high accuracy. It is emphasized that for each specific object, one has to carefully select the parameters of the convergence function.     

Propagation of elastic waves near acoustic axes in crystals under the action of extreme external forces: I. Piezoelectric crystal in an electric field

The directions of an external electric field at which extreme changes occur in the difference in the velocities of elastic waves propagating along the initial acoustic axis in a piezoelectric medium of arbitrary symmetry are theoretically determined. The problem of degeneracy in an external electric field is considered for elastic waves propagating in a given direction from the vicinity of an initial acoustic axis. The extreme electric fields and corresponding changes in the characteristics of transverse waves are calculated by the example of the behavior of acoustic axes in Bi₁₂GeO₂₀, Bi₁₂SiO₂₀, La₃Ga₅SiO₁₄, and LiNbO₃ crystals.     

Complex study of the structural and optical homogeneity of lithium niobate crystals

Methods of Raman spectroscopy, laser conoscopy, optical microscopy, and electron spin resonance have been used to study the photorefractive properties and structural and optical homogeneity of the following lithium niobate (LiNbO₃) crystals: nominally pure crystals of congruent composition (LiNbO_3con); LiNbO₃:Cu[0.015 wt %] crystals grown from a melt of congruent composition and nominally pure crystals of stoichiometric composition grown from a melt with 58.6 mol % Li₂O (LiNbO_3st). A small deformation of optical indicatrix and regular microdomain structures of fractal type are revealed for the LiNbO₃:Cu[0.015 wt %]; the microdomain structures may be due to the nonuniform impurity incorporation into the structure. It is shown that oxygen octahedra in the LiNbO₃:Cu[0.015 wt %] crystal are deformed in comparison with the octahedra in LiNbO_3st and LiNbO_3con crystals and that the main and impurity cations are clusterized along the polar axis. It is established that the LiNbO₃:Cu[0.015 wt %] crystal exhibits photorefractive properties not only due to the presence of intrinsic defects with localized electrons, as in the case of LiNbO_3st, but also due to the charge exchange in copper cations (Cu²⁺ → Cu⁺) under illumination.     

Growth and optical damage properties of In:Zn:LiNbO3 waveguide substrate

In:Zn:LiNbO₃ crystals doped with different indium concentrations were grown by Czochralski technique. The optical damage threshold value and ultraviolet‐visible absorption spectra of the In:Zn:LiNbO₃ crystals were measured. The In:Zn:LiNbO₃ crystals were made into optical waveguide substrates using hexanedioic acid as proton exchange agent. The optical damage resistant ability of those optical waveguide substrates was investigated by the m‐line method. The optical damage threshold values of In(2mol.%):Zn(3mol.%):LiNbO₃ crystal and optical waveguide substrate are two orders of magnitude higher than those of pure LiNbO₃. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure defects and optical damage resistance in doubly doped In:Nd:LiNbO3 crystal waveguide substrates

A series of In:Nd:LiNbO₃ crystals were grown by Czochralski technique and were made into waveguide substrates. The optical damage resistance of the In:Nd:LiNbO₃ waveguide substrates was characterized by measurement of the holographic method. The optical damage resistance of In (3.0 mol%):Nd:LiNbO₃ was much higher than that of other In:Nd:LiNbO₃. The ultraviolet‐visible (UV‐Vis) absorption spectra the In:Nd:LiNbO₃ crystals were measured and investigated. The structure defects were discussed in this paper to explain the enhance of the optical damage resistance in the In:Nd:LiNbO₃ crystals. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ionic impurity transport and partitioning at the solid–liquid interface during growth of lithium niobate under an external electric field by electric current injection

Transport of ionic species in the melt and their partitioning at the solid–liquid interface during growth of lithium niobate was studied under the influence of intrinsic and external electric fields. A Mn-doped lithium niobate (Mn:LiNbO₃) single crystal was grown via the micro-pulling-down (μ-PD) method with electric current injection at the interface. Mn ions were accumulated or depleted at the interface, depending on the sign of the injected current. The electric current injection induced an interface electric field as well as a Coulomb force between the interface and Mn ions. The electric field modified the transportation of Mn ions and their partitioning into the crystal, while the Coulomb force led to adsorption or rejection of Mn ions at the interface in addition to Mn concentration change due to the electric field. Effect of the Coulomb force was often observed to be larger on Mn concentration at the interface than that of the induced electric field, and dominated the redistribution of Mn in the solid. It has been experimentally and analytically shown that Mn concentration partitioned into the crystal can be obtained by multiplying Mn concentration at the interface by a field-modified partition coefficient, k_E0, instead of the conventional equilibrium partition coefficient, k₀.     

Growth of LiNbO<Subscript>3</Subscript>:Er Crystals and concentration dependences of their properties

A series of lithium niobate (LiNbO₃) crystals of congruent and stoichiometric compositions, doped with erbium, have been grown under non-steady-state thermal conditions. A series of LiNbO₃:Zn crystals, nominally pure LiNbO₃ crystals of congruent and stoichiometric compositions, and a LiNbO₃:B crystal have also been grown. Both growth conditions and concentration dependences of physicochemical, ferroelectric, and structural characteristics of LiNbO₃:Er crystals are investigated. The growth regular domain microstructures and periodic nanostructures in LiNbO₃:Er crystals are analyzed by optical microscopy and atomic force microscopy (AFM). A comparative study of the optical homogeneity and photorefractive properties of LiNbO₃:Er crystals of congruent and stoichiometric compositions and the Raman spectra of LiNbO₃ crystals of different compositions is performed.     

Conversion of non-stoichiometry of LiNbO3 to constitutional stoichiometry by impurity doping

Several chemical components exist in the oxide melt, including ionic species. During crystal growth these are partitioned into the solid phase with their own equilibrium partitioning coefficients. Therefore, they are usually segregated, even at the congruent composition. However, when a compound has a stoichiometric structure at the congruent composition, the equilibrium partitioning coefficients of all chemical components become unity and segregation does not occur. Segregation of ionic species was observed at the binary congruent composition of LiNbO₃ because of its non-stoichiometric structure. On the other hand, the segregation of ionic species did not occur at the ternary congruent composition of MgO-doped LiNbO₃ (Li₂O:Nb₂O₅:MgO=45.5:50:4.5). This demonstrates that non-stoichiometry of congruent LiNbO₃ was converted to the constitutional stoichiometry at the ternary congruent composition by MgO doping.     

Second-order optical non-linearity initiated in Li2O–Nb2O5–SiO2 and Li2O–ZnO–Nb2O5–SiO2 glasses by formation of polar and centrosymmetric nanostructures

Amorphous nanoheterogeneities of the size less than 100 Å have been formed in glasses of the Li₂O–Nb₂O₅–SiO₂ (LNS) and Li₂O–ZnO–Nb₂O₅–SiO₂ (LZNS) systems at the initial stage of phase separation and examined by transmission electron microscopy, small-angle X-ray and neutron scattering. Both LNS and LZNS nanoheterogeneous glasses exhibit second harmonic generation (SHG) even when they are characterized by fully amorphous X-ray diffraction (XRD) patterns. Chemical differentiation and ordering of glass structure during heat treatments at appropriate temperatures higher T_g lead to drastic increase of SHG efficiency of LNS glasses contrary to LZNS ones in the frame of amorphous state of samples. Following heat treatments of nanostructured glasses result in crystallization of ferroelectric LiNbO₃ and non-polar LiZnNbO₄ in the LNS and LZNS glasses, respectively. Taking into account similar polarizability of atoms in LNS and LZNS glasses, the origin of the principal difference in the second-order optical non-linearity of amorphous LNS and LZNS samples is proposed to connect predominantly with the internal structure of formed nanoheterogeneities and with their polarity. Most probably, amorphous nanoheterogeneities in glasses may be characterized with crystal-like structure of polar (LiNbO₃) phase initiating remarkable SHG efficiency or non-polar (LiZnNbO₄) phase, which do not initiate SHG activity. It gives an opportunity to vary SHG efficiency of glasses in a wide rage without remarkable change of their transparency by chemical differentiation process at the initial stage of phase separation when growth of nanoheterogeneities is ‘frozen’. At higher temperatures, LiNbO₃ crystals identified by XRD precipitate in LNS glasses initiating even more increase of SHG efficiency but visually observable transparency is impaired.     

LiNbO3 layer growth in a LiVO3 high temperature solution

Homoepitaxy of LiNbO₃ takes a successful course in high temperature solutions (HTS) with LiVO₃ as a solvent. In order to optimize the deposition process the phase correlations in the pseudobinary system LiNbO₃-LiVO₃ were specified by means of DTA. At the liquidus temperatures the HTS with concentrations of 15 to 40 mole % have an approximately constant shear viscosity of η ≈ 13.5 mPa · s, similary to solvents containing lead compounds. Always smooth LPE-layers grow on singular {11 02}-substrates. Surfaces of all other orientations were found to be rough because rhomboedric faces of LiNbO₃ form in the stable state. On the negative polarized (0001 )-face epitaxy is still possible, however, on the opposite (0001)-face only a hexagonal cellular structure is observed because the singular faces develop, exclusively.     

Structural Characteristics of Er Doped LiNbO3 Thin Films Grown by the Liquid Phase Epitaxy Method

Erbium (Er³⁺) doped LiNbO₃ single crystal thin films have been grown LiNbO₃ (001) substrate by the liquid phase epitaxy method. The crystallinity was determined by high‐resolution X‐ray diffraction. The lattice mismatch between Er³⁺ doped LiNbO₃ films and LiNbO₃ (001) substrate was investigated by X‐ray rocking curve analysis. Also we studied the structural characteristics of Er³⁺ doped LiNbO₃ films and surface morphology dependent on the film thickness.     

Growth and ionic conductivity of Li3 + x P1 − x GexO4 (x = 0.34) single crystals

Li_{3 + x} P_{1 ? x} Ge_xO₄ crystals (x = 0.34) with dimensions of about 3 × 3 × 5 mm³ were grown for the first time from flux. The conductivities of the crystals measured along three directions have close values and are equal to σ ≈ 1.8 × 10^?6 and 3.7 × 10^?2 Sm/cm at the temperatures 40 and 400°C, respectively (similar to the case of pure lithium phosphate, somewhat lower values of electric conductivity were measure along the b axis). The activation energy of conductivity is equal to 0.54 eV. A considerable increase in the conductivity of the solid solution in comparison with the conductivity of pure lithium phosphate is explained by the specific features of the lithium sublattice in the crystal structure of the λ-Li₃PO₄ type.     

Growth and characterization of near stoichiometric LiNbO3 single crystal

A near stoichiometric LiNbO₃ single crystal has been grown by the Czochralski method from a 58.5% Li melt hold in a large platinum crucible. High resolution X‐ray rocking curves of 30 0 and 0006 reflections indicated that the near stoichiometric LiNbO₃ crystal possesses the high structural quality. Compared with the congruent LiNbO₃, the near stoichiometric LiNbO₃ possesses shorter ultraviolet absorption edge, thus higher Li concentration. The OH^– infrared absorption band analyses showed that the Li concentration in the near stoichiometric LiNbO₃ crystal is higher than that in the congruent LiNbO₃ crystal. This result is in good agreement with that of the ultraviolet absorption edge. The electro‐optic (EO) coefficient γ₂₂ of the near stoichiometric LiNbO₃ crystal was measured to be 6.75 pm/V higher than that of congruent LiNbO₃ crystal. It also proves the near stoichiometric LiNbO₃ electro‐optic Q‐switched requires a low driving voltage and it is advantageous for the device performance. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Increased optical damage resistance of Zr:LiNbO3 crystals

Zr: LiNbO₃ crystals has been grown. The crystal composition and phase are analyzed by X‐ray diffration. The optical damage resistance ability of Zr: LiNbO₃ crystals is studied by the Sénarmont compensation method and the transmitted beam pattern distortion method. The saturated value of the birefringence change of 6 mol% Zr: LiNbO₃ crystal is 1.01×10^‐4, which is seven times smaller than that of congruent pure LiNbO₃ crystal. The results of UV‐Visible and IR absorption spectra of Zr: LiNbO₃ crystals powerfully confirm that the optical damage resistance threshold concentration of the Zr⁴⁺ ions doped in LiNbO₃ crystals is about 6 mol% in the melt. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bulk crystal growth of congruent MgO-doped LiNbO3 crystal with stoichiometric structure and its second-harmonic-generation properties

A new LiNbO₃ bulk crystal has been grown by doping with MgO (cs-MgO:LN; Li₂O:Nb₂O₅:MgO=45.30:50.00:4.70, (Li_0.906Mg_0.047V^Li_0.047)NbO₃), which successfully has the congruent point coinciding with the stoichiometric point. Its second-harmonic-generation (SHG) properties were evaluated. It was found that cs-MgO:LN has a much more homogeneous composition leading to uniform in-plane distribution of the non-critical phase-matching wavelength than the conventional LiNbO₃ crystals such as congruent LiNbO₃ (c-LN), stoichiometric LiNbO₃ (s-LN), and MgO-doped congruent LiNbO₃ (5MgO:LN). This homogeneity arose from the observation that none of the solute components including ionic species were segregated at the interface during growth. The SHG conversion efficiency of cs-MgO:LN is comparable to those of s-LN and 5MgO:LN.     

Growth and characterisation of Zn:LiNbO3/Mg:LiNbO3 multilayer thin films grown by liquid phase epitaxy

1, 3 and 5 mol% ZnO doped LiNbO₃ film and 2 mol% MgO doped LiNbO₃ multilayer films were grown on the LiNbO₃ (001) substrate by liquid phase epitaxy (LPE) method with a Li₂O‐V₂O₅ system. We examined the optical transmission spectra of the Zn:LiNbO₃ by Fourier Transform‐Infrared Spectrophotometer (FT‐IR). The crystallinity and the lattice mismatch between the Zn:LiNbO₃ film and Mg:LiNbO₃ film was confirmed by x‐ray rocking curve (XRC) and observed the ZnO and MgO distribution in the cross‐section of the multilayer thin films by electron probe micro analyzer (EPMA). Furthermore, the surface morphology of the films was observed using atomic force microscopy (AFM). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)