Influence of Mg/Er co‐doping on the principal laser parameters of LiNbO3 crystals

Mg: Er: LiNbO₃ crystals were grown by the Czochralski technique with various concentrations of MgO = 2 mol%, 4 mol%, 6 mol% and the fixed concentration of Er₂O₃= 1 mol% in the melt, and the 8 mol%Mg: 1 mol%Er: LiNbO₃ crystal was fabricated by the Czochralski technique with special technology process. The crystals were treated by polarization, reduction and oxidation. The segregation coefficients of Mg²⁺ and Er³⁺ in Mg: Er: LiNbO₃ crystals were measured by X‐ray fluorescence spectrograph, as well as the crystal's defect structure and optical properties were analyzed by the UV‐Vis, IR and fluorescent spectroscopy. The pump wavelength and the surge wavelength were determined. Using m‐line method tested optical damage resistance of those crystals, the results show that photodamage threshold of Mg: Er: LiNbO₃ crystals are higher than that of Er: LiNbO₃ crystal, and the oxidation treat could enhance the photodamage resistant ability of crystals while the reduction treat could depress the ability. The optical damage resistance of 8 mol%Mg: 1 mol%Er: LiNbO₃ crystal was the strongest among the samples, which was two orders magnitude higher than that of 1 mol%Er: LiNbO₃ crystal. The dependence of the optical properties on defect structure of Mg: Er: LiNbO₃ crystals was discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental Study of Interface Inversion of Tb3ScxAl5‐xO12 Single Crystals Grown by the Czochralski Method

Thermal conditions and rotation rate were examined experimentally for obtaining a flat interface growth of high melting‐point oxide (Tb₃Sc_xAl_5‐xO₁₂ ‐ TSAG) by the Czochralski method. The critical crystal rotation rate can be significantly reduced, of about twice at low and very low temperature gradients comparing to medium temperature gradients in the melt and surroundings of the crystal. The interface shape of TSAG crystals is not very sensitive on crystal rotation rate at small rotations and becomes very sensitive at higher rotations, when the interface transition takes place. The range of crystal rotation rates during the interface transition from convex to concave decreases with a decrease of temperature gradients. At low temperature gradients interface inversion crystals takes place in very narrow range of rotation rates, which does not allow one to growth such crystals with the flat interface. Even changing crystal rotation rate during the growth process in a suitable manner did not prevent the interface inversion from convex to concave and thus did not allow to obtain and maintain the flat interface.     

Structure and Photo‐Damage Resistance of Li‐Rich LiNbO3 Crystals Co‐Doped with Zn2+/Er3+

The pure congruent LiNbO₃, Er:LiNbO₃ and Zn,Er co‐doped Li‐rich LiNbO₃ crystals were grown by Czochralski method. The X‐ray diffraction method and ultraviolet‐visible absorption spectra of the crystals were used to analyze the structure of the crystals. The photo‐damage ability resistance of the crystals was measured. The Zn,Er co‐doped Li‐rich LiNbO₃ crystals show a decrease in lattice constant values, a shift in absorption edge of ultraviolet‐visible absorption spectra towards shorter wavelength, and three orders of magnitude increase in photo‐damage resistance compared to congruent LiNbO₃ crystal. The intrinsic and extrinsic defects are discussed to explain the enhance of the photo‐damage ability resistance     

Growth and characterization of ferroelectric‐ferroelastic Tb2(MoO4)3 crystals

Transparent and nearly colorless ferroelectric‐ferroelastic β′‐Tb₂(MoO₄)₃ (TMO) single crystals have been grown by the Czochralski (CZ) method. The single crystal structure was investigated by X‐ray powder diffraction and was shown to be a single phase with the structure similar as the β′‐Gd₂(MoO₄)₃ crystal. The optical transparency of the TMO crystal has been measured and the crystal is almost transparent in the visible and near infrared regions. The defects of TMO crystal were evaluated by etching technique and the ferroelectric domain structures were observed by an optical microscope. A high‐resolution X‐ray diffraction analysis demonstrates that the as‐grown TMO crystal possesses relatively high optical quality. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bridgman and Czochralski growth of Ge?Si alloy crystals

Ge_1–xSi_x crystals were grown with the Bridgman and the Czochralski method over a wide concentration range. With the Bridgman process single crystals up to 40 at.% Si were possible. The reasons for polycrystalline growth were the permanent contact of the interface with the crucible wall in combination with curvature towards the crystal and inclusions of high Si concentration. Analysis of striations in Czochralski grown material showed that in this process the crystal does not continuously grow in one direction but consists of piece-wise grown crystals in which the composition permanently changes. According to that fact the main reason for polycrystalline growth in the Czochralski process is common due to lattice mismatch between the seed and equilibrium concentration transients in the crystal corresponding to the microscopic growth rates.     

Structural and optical investigations on Mn doped Li2B4O7 crystals

Lithium tetraborate crystals have great demand due to its non‐critical phase matching at 90°. Transparent and good optical quality single crystals of undoped and Mn doped Li₂B₄O₇ were grown by Czochralski technique in air atmosphere. The crystalline phase was determined by X‐ray diffraction. Doping of Mn in the grown crystals was confirmed by energy dispersive analysis of X‐rays (EDAX) technique. Transmission spectra show that the crystal is transparent in the visible region. Birefringence interferograms were recorded to qualitatively analyse the optical homogeneity of the grown crystals and to quantify the birefringence of the samples in desired crystal direction. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of Bi<Subscript>12</Subscript>GeO<Subscript>20</Subscript> and Bi<Subscript>12</Subscript>SiO<Subscript>20</Subscript> crystals by the low-thermal gradient Czochralski technique

Bi₁₂SiO₂₀ crystals have been grown for the first time by the low-thermal gradient Czochralski technique in the 〈111〉 and 〈110〉 directions. The conditions for reproducible crystal growth with a high-quality polyhedral faceted front are found. The systematic features of shaping Bi₁₂SiO₂₀ and Bi₁₂GeO₂₀ crystals, grown by the low-thermal gradient Czochralski technique, are compared. The defect formation in these crystals is studied and their optical homogeneity is analyzed by interferometry.     

Influence of internal radiation on the heat transfer during growth of YAG single crystals by the Czochralski method

Heat and mass transfer taking place during growth of Y₃Al₅O₁₂ (YAG) crystals by the Czochralski method, including inner radiation, is analyzed numerically using a Finite Element Method. For inner radiative heat transfer through the crystal the band approximation model and real transmission characteristics, measured from obtained crystals, are used. The results reveal significant differences in temperature and melt flow for YAG crystals doped with different dopands influencing the optical properties of the crystals. When radiative heat transport through the crystal is taken into account the melt‐crystal interface shape is different from that when the radiative transport is not included. Its deflection remains constant over a wide range of crystal rotation rates until it finally rapidly changes in a narrow range of rotation rates. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation on photorefractive properties of doped lithium niobate

Cu:LiNbO₃ crystal and Fe:Cu:LiNbO₃ crystals were grown by the Czochralski method from congruent melt. The OH^‐ absorption spectrum of doped lithium niobate crystals was measured. The photorefractive properties of doped crystals were studied by the two‐wave coupling method. The results of the two‐wave coupling experiments showed that as the concentration of doping ions increased, the diffraction efficiency and the dynamic range enhanced, the holographic response time shortened. The recording time of Fe(0.10wt%): Cu(0.10wt%): LiNbO₃ crystal is only a tenth of that of Cu(0.05wt%): LiNbO₃ crystal. Among all samples, the dynamic range of the Fe(0.10wt%): Cu(0.10wt%): LiNbO₃ crystal was the most largest (up to 40.78). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Czochralski growth of Lu1‐xScxBO3:Ce solid solution crystals

Single crystals of Lu_1‐xSc_xBO₃:Ce (x=0.2, 0.3, 0.5, 0.7) were grown by Czochralski method. Continuous solid solution with calcite structure and a linear compositional dependency of crystal lattice parameter in the system Lu_1‐xSc_xBO₃:Ce are formed and their symmetry belong to hexagonal system with R3c space group checked by X‐ray powder diffraction. The electron probe micro‐analysis measurements show that the main inclusions in Lu_1‐xSc_xBO₃:Ce crystals are in the form of Sc rich oxide and Ce rich oxide. The ICP‐AES tests show that the more Sc ion content in Lu_1‐xSc_xBO₃:Ce, the smaller effective segregation coefficient of Ce in crystal will be. The X‐ray excited luminescence spectra of Lu_1‐xSc_xBO₃:Ce crystals all present a double peaked emission band with maxima round 370 and 400 nm corresponding to Ce³⁺ emission and a self trapped excitons (STE) band peaking at 269 nm. In addition, due to high density, high relative light yield, fast decay time and no‐hygroscopic property, Lu_0.8Sc_0.2BO₃:1 at%Ce crystal could be a good candidate material for scintillation application by improving the crystal quality and cerium concentration. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High photorefractive sensitivity and fast response time of near‐stoichiometric and MgO doped LiNbO3:Fe crystals

Congruent LiNbO₃:Fe and LiNbO₃:Mg,Fe crystals were grown by Czochralski method, and vapor transport equilibration technique was employed to improve the [Li]/[Nb] ratios of these crystals. The influence of stoichiometry and MgO dopant on the photorefractive sensitivity and response time of LiNbO₃:Fe crystals was investigated. Both stoichiometry and MgO dopant can effectively reduce the amount of intrinsic defects, but MgO can also decrease the concentration of Fe²⁺ ions in Li‐sites. Near‐stoichiometric and MgO doped LiNbO₃:Fe crystal has high photorefractive sensitivity and fast response time. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and some properties of TeO<Subscript>2</Subscript> single crystals with a large diameter

Large-diameter single crystals of TeO₂ are grown by the Czochralski method in specially designed setups with automatic monitoring of the crystal growth. The degree of perfection of the grown crystals is examined using selective etching and X-ray topography (the Shultz method). The temperature dependence of the microhardness of TeO₂ single crystals is investigated for different crystallographic planes, namely, (001), (100), and (110).     

Etching Figures in Neodymium Gallate and Yttrium Aluminate Crystals

The dislocations existing in single crystals of neodymium gallate and yttrium aluminate grown by the Czochralski technique have been studied by means of etch pits. The data concerning their solubility in cases of different directions of a face orientation, various treatment temperatures and several enchant types are reported. The investigation of etch pits in the twinned YAlO₃ and NdGaO₃ crystals showed that twins are formed during a growth process. In the [110]‐pulled NdGaO₃ crystals the discrepancy between the twin and matrix parts of a crystal is accommodated by the dislocation congestion and the dislocation low‐angle boundaries whereas in [010]‐pulled YAlO₃ crystals the microcracks perform this function.     

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)     

SQUID magnetometry,EPR and XPS characterization of Y3Al5O12: Co,Si single crystals

Single crystals of garnet Y₃Al₅O₁₂ (YAG): Co and Co, Si were grown by Czochralski method as materials for optoelectronic applications. Silicon doping is responsible for the change of the Co³⁺ ions into Co²⁺ in order to keep charge neutrality of the material and for a coloration of materials. Magnetization and Electron Paramagnetic Resonance measurements show that the most cobalt ions in the silicon co‐doped crystals were Co²⁺ in octahedral positions. Estimated concentration of the Co²⁺ ions were close to Si⁴⁺ nominal concentration. A little excess of Co²⁺ may be ascribed to defects present in the crystals. For the crystal doped only with Co, the concentration of Co²⁺ was about 13% of the nominal Co amount and might be caused by crystal defects. X‐ray Photoelectron Spectroscopy measurements exhibit aluminium deficiency. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modeling analysis of liquid encapsulated Czochralski growth of GaAs and InP crystals

The results of three‐dimensional unsteady modeling of melt turbulent convection with prediction of the crystallization front geometry in liquid encapsulated Czochralski growth of InP bulk crystals and vapor pressure controlled Czochralski growth of GaAs bulk crystals are presented. The three‐dimensional model is combined with axisymmetric calculations of heat and mass transfer in the entire furnace. A comprehensive numerical analysis using various two‐dimensional steady and three‐dimensional unsteady models is also performed to explore their possibilities in predicting the melt/crystal interface geometry. The results obtained with different numerical approaches are analyzed and compared with available experimental data. It has been found that three‐dimensional unsteady consideration of heat and mass transfer in the crystallization zone provides a good reproduction of the solidification front geometry for both GaAs and InP crystal growth.     

Microstructure and optical properties of Zn:Mn:Fe:LiNbO3 crystals

Zn:Mn:Fe:LiNbO₃ crystals were prepared by Czochralski technique. Its microstructure was measured and analyzed by UV‐Vis absorption spectra. The optical damage resistance of Zn:Mn:Fe:LiNbO₃ crystals was characterized by the transmitted beam pattern distortion method. It increases remarkably when the concentration of ZnO is over a threshold concentration. Its value in Zn(7.0 mol%):Mn:Fe:LiNbO₃ crystal is about three orders of magnitude higher that in the Mn:Fe:LiNbO₃ crystal. The dependence of the defects on the optical damage resistance was discussed. The non‐volatile holographic storage was realized in all crystals, and the sensitivity of the Zn(7.0 mol%):Mn:Fe:LiNbO₃ crystal is much higher than that of others. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Evolution of real structure in Ge‐Si mosaic crystals

The structure and properties of Czochralski (Cz)‐grown Ge_1‐xSi_x mosaic crystals were investigated using optical microscopy, atomic force microscopy, X‐ray diffraction analysis, microprobe analysis, FTIR and transmission electron microscopy. The role of segregation, form of solid‐liquid interface and dislocation generation in the development of mosaic structure were analyzed and used for optimization of growth parameters such as Si concentration and growth rate. The dislocation density estimated experimentally was compared with the calculated data. Composition fluctuations caused by formation of cellular structure at the interface lead to a local lattice misorientation that is one of the reasons for crystal mosaicity. Model of mosaic structure generation in terms of dislocation density and composition variations is presented.     

Growth of doped-halide single crystals for infrared windows

Single crystals of KCl doped with RbCl have been grown by the Czochralski and Bridgman techniques. Czochralski crystals were grown in air, using platinum crucibles and Bridgman crystals were grown in a Trans-Temp glass furnace in air and also under a reactive atmosphere of CCl₄ in argon. This reactive atmosphere process (RAP) is known to greatly reduce impurities, such as hydroxyl ions, which degrade optical quality. A series of KCl-RbCl-SrCl₂ single crystals have been grown to stabilize microstructure. Some optical absorption and mechanical strength data is presented. An alternative approach to increase halide material strength is to hot-forge a single crystal under temperature and pressure sufficient to cause pressure-induced recrystallization (PIR). Hot-forging techniques to produce high-strength IR windows without degrading optical transmission are given.