Effect of reduction/oxidation treatment on blue photorefraction in In:Fe:Cu:LiNbO3 crystals

In:Fe:Cu:LiNbO₃ crystals with reduced/oxidized treatments were prepared by the Czochralski method. The defect structure was analyzed by the UV‐Visible absorption spectra. The blue photorefractive properties, such as the refractive index change, response time, recording sensitivity, dynamic range as well as two‐wave coupling gain coefficient, were also investigated at 488 nm wavelength using the two‐wave coupling experiment. Comparing the as‐grown and oxidized In:Fe:Cu:LiNbO₃ crystals, the reduced sample has the highest recording sensitivity and largest dynamic range. Meanwhile, the high diffraction efficiency is still maintained. Experimental results definitely show that reduction treatment is an effective method to improve the blue photorefractive performance of In:Fe:Cu:LiNbO₃ crystals. (© 2010 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)     

Effect of post treatment on the photorefractive properties of Ru‐doped lithium niobate

The effect of post treatment on the photorefractive properties of Ru‐doped lithium niobate was studied. The absorption spectra examination of Ru‐doped LiNbO₃ crystals with different post treatments showed that the absorption coefficient at 530 nm increased after the reduced treatment was employed and the absorption edge of the reduced crystal shifted towards the infrared band. On the contrary, the trend reversed after the oxidized treatment was employed. In addition, the photorefractive properties were investigated with the two‐beam coupling method conducted via a 532 nm solid state laser. It was found that the oxidized Ru:LiNbO₃ had smaller exponential gain coefficient and diffraction efficiency because the charges in the shallow level were exchanged to the deep level. On the other hand, the reduced Ru:LiNbO₃ crystals had larger exponential gain coefficient and diffraction efficiency due to the increase of the Ru³⁺ which existed in the shallow level. The response times of both oxidized and reduced Ru:LiNbO₃ were longer than those of the as‐grown ones. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Defect structure and optical fixing holographic storage of Mg:Mn:Fe:LiNbO3 crystals

Mg:Mn:Fe:LiNbO₃ crystals were grown by the Czochralski method. The defect structure was analyzed by UV‐vis spectra and IR spectra. The holographic storage of Mg:Mn:Fe:LiNbO₃ crystals was measured by the two color fixed method. The results show that with the increase of MgO doping concentration, the writing time becomes shorter, the dynamic range decreases, photorefractive sensitivity increases and fixing diffraction efficiency decreases. When the MgO doping concentration exceeds 4.5 mol%, the fixing diffraction efficiency approaches zero. The effect of doping Mg ions on the holographic storage properties of Mn:Fe:LiNbO₃ crystals is discussed. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Improved blue photorefractive properties of near‐stoichiometric LiNbO3:Mn:Fe:Zr crystal

Near‐stoichiometric LiNbO₃ single crystal tri‐doped with ZrO₂, MnO and Fe₂O₃ was grown from Li‐riched melt by Czochralski method. The defect structures and composition of these crystals were analyzed by means of ultraviolet‐visible and infrared transmittance spectra. The appearance of 3466 cm^‐1 peak in infrared spectra showed that the crystal grown from Li‐riched melt was near stoichiometric. The photorefractive properties at the wavelength of 488 nm and 633 nm were investigated with two‐beam coupling experiment, respectively. The experimental results showed that the response speed and sensitivity were enhanced significantly and the high diffraction efficiency was obtained at 488 nm wavelength. This manifested that near‐stoichiometric LiNbO₃:Mn:Fe:Zr crystal was an excellent candidate for holographic storage. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and photorefractive properties of Mg:Ce:Cu:LiNbO3 crystals grown by Czochralski method

In this paper, photorefractive properties of Mg:Ce:Cu:LiNbO₃ crystals were studied. The crystals doped with different concentration of Mg ions have been grown by the Czochralski method. Mg concentrations in grown crystals were analyzed by an inductively coupled plasma optical emission spectrometry (ICP‐OE/MS). The crystal structures were analyzed by the X‐ray powder diffraction (XRD), ultraviolet‐visible (UV‐Vis) absorption spectra and infrared (IR) transmitatance spectra. The photorefractive properties of crystals were experimentally studied by using two‐beam coupling. In this experiment we determined the writing time, maximum diffraction efficiency and the erasure time of crystals samples with He‐Ne laser. The results showed that the dynamic range (M/#), sensitivity (S) and diffraction efficiency (η) were dependent on the Mg doping concentration, and the Mg(4.58mol%):Ce:Cu:LiNbO₃ crystal was the most proper holographic recording media material among the six crystals studied in the paper. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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)     

Photorefractive features of non‐stoichiometry codoped Hf:Fe:LiNbO3 single crystals

Hf(2mol%):Fe(0.05wt%):LiNbO₃ crystals with various [Li]/[Nb] ratios of 0.94, 1.05, 1.2 and 1.38 have been grown. The photorefractive resistant ability increases with the accretion of [Li]/[Nb] ratio. When the ratio of [Li]/[Nb] is 1.20 or 1.38, the OH^‐ absorption band shifts to about 3477cm^‐1. The mechanisms of the photorefractive resistant ability increase and the absorption band shift have been discussed. The exponential gain coefficient (Γ) of the crystals was measured with two‐beam coupling method and the effective charge carrier concentration (N_eff) was calculated. The results show that Γ and N_eff increase with the accretion of [Li]/[Nb] ratio. The temperature effect of codoped Hf:Fe:LiNbO₃ crystals was also studied, it was found that the exponential gain coefficient increase dramatically at about 55°C, 70°C and 110°C, this is due to the inner electric field which is resulted from structure phase change. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical damage resistance of Mg:Ce:Fe:LiNbO3 crystals

Mg:Ce:Fe:LiNbO₃ crystals were prepared with fixed concentrations of Fe₂O₃ and CeO₂, and differing concentrations of MgO by the Czochralski technique. Their infrared transmission spectra were measured in order to investigate their defect structures and their optical damage resistance was characterized by the photoinduced birefringence change and transmission facula distortion method. The optical damage resistance of Mg:Ce:Fe:LiNbO₃ crystals increases remarkably when the concentration of MgO exceeds a threshold concentration. The dependence of the optical damage resistance on the defect structure of Mg:Ce:Fe:LiNbO₃ crystals is discussed in detail. (© 2010 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)     

In doping effect on optical properties in Zn:In:Fe:LiNbO3 crystals

A series of Zn:In:Fe:LiNbO₃ crystals were prepared by Czochralski method. The crystal composition and defect structure were analyzed by ICP‐OE/MS, UV–vis and IR spectroscopy. The results show that with increasing In³⁺ doping concentration in melt, the segregation coefficients of both Zn and In ions decrease. The optical damage resistance of Zn:In:Fe:LiNbO₃ crystals was studied by the transmitted beam pattern distortion method. It is found that the optical damage resistance of Zn:In(3mol%):Fe LiNbO₃ crystals is two orders of magnitude higher than that of Zn:Fe:LiNbO₃. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Defect structure and optical damage resistance of Hf:Fe:LiNbO3 crystals with various [Li]/[Nb] ratios

Hf:Fe:LiNbO₃ crystals were grown in air by the Czochralski technique with various ratios of [Li]/[Nb]=0.94, 1.05, 1.20 in melt. The defect structure and location of doped ions were analyzed by the UV‐visible and infrared spectroscopy. The optical damage resistance ability of Hf:Fe:LiNbO₃ crystals was measured by the photo‐induced birefringence change and the transmitted light spot distortion method. The results show that the optical damage resistance ability of Hf:Fe:LiNbO₃ crystals is enhanced with the increase of the [Li]/[Nb] ratio. The dependence of the optical damage resistance of Hf:Fe:LiNbO₃ crystals on the defect structure is discussed in detail. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Defect structure and optical damage resistance of In:Fe:Cu:LiNbO3 crystals with various [Li]/[Nb] ratios

In:Fe:Cu:LiNbO₃ crystals were grown in air by the Czochralski technique with various [Li]/[Nb] ratios of 0.946, 1.050, 1.200, and 1.380 in melt. Based on the ICP‐AES (inductively coupled plasma atomic emission spectrometry) analyzed results, the chemical formula of the triple‐doped In:Fe:Cu:LiNbO₃ crystals were obtained. It can be seen that the near‐stoichiometric ratio value is between 1.050 and 1.200 for our samples. The optical damage resistance of In:Fe:Cu:LiNbO₃ crystals was characterized by changes in light‐induced birefringence and it increases with the increasing of [Li]/[Nb] ratios. The dependence of the optical damage resistance on the defect structure of In:Fe:Cu:LiNbO₃ crystals is discussed in detail based on the obtained chemical formulas. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical damage resistance of Hf:Fe:LiNbO3 crystals with various [Li]/[Nb] ratios

Hf:Fe:LiNbO₃ crystals were grown in air by the Czochralski technique with various [Li]/[Nb] ratios ([Li]/[Nb]=0.94, 1.05, 1.20) in melt. The defect structure and location of doped ions were analyzed by the UV‐visible absorption spectra. The optical damage resistance of Hf:Fe:LiNbO₃ crystals was investigated by the photoinduced birefringence change and the transmitted light spot distortion method. The results show that the optical damage resistance ability of Hf:Fe:LiNbO₃ crystals decreases with the increase of the [Li]/[Nb] ratio. The dependence of the optical damage resistance of Hf:Fe:LiNbO₃ crystals on the defect structure is discussed in detail. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

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)     

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)     

Influence of microscopic defects on optical damage resistance of Zn:Fe:LiNbO3 crystals

Influence of defect structure on the infrared transmission spectra of OH^‐ in Zn:Fe:LiNbO₃ crystals with various ZnO concentration and different Li/Nb ratios was investigated. It indicates that above the Zn concentration threshold the OH^‐ absorptions bands successively shift from 3482cm^‐1 to 3504cm^‐1 and 3529cm^‐1. The intensity of the 3504cm^‐1 band increases with ZnO concentration increasing. The optical damage resistance of the Zn:Fe:LiNbO₃ crystals increases rapidly when the ZnO concentration exceeds a threshold value. This result contributed to the site alteration from the Li sites to Nb sites due to Zn‐doping in crystal. © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim     

Defect structure and light‐induced scattering of Zr‐doped near‐stoichiometric Mn:Fe:LiNbO3 crystals grown by TSSG method

Near‐stoichiometric Mn:Fe:LiNbO₃ crystals doped with various concentration of ZrO₂ were grown by top seed solution growth (TSSG) method in the air atmosphere. The Zr concentration in the crystal was determined by inductively coupled plasma optical emission spectrometer. The defect structures were analyzed by means of ultraviolet‐visible and infrared transmittance spectra. The appearance of vibration peak at 3466 cm^‐1 in infrared spectra manifested that Li/Nb ratio in crystals approached to stoichiometric proportion. The fundamental absorption edge represented continuous red‐shift which was discrepancy with congruent doped LiNbO₃ crystals showed that doping ions possessed different location mechanism. The light‐induced scattering of the doped stoichiometric LiNbO₃crystals were quantitatively scaled via incident exposure energy. The results demonstrated that Zr(2 mol%):Mn:Fe:LiNbO₃ crystal had the weakest light‐induced scattering and the mechanism related to their defect structures was discussed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Micro‐Pulling Down Growth of Co‐doped Lithium Niobate Single Crystal Fibers According Er and Mg Contents and Photolumenescence Properties

The Er³⁺doped Mg:LiNbO₃single crystal fibers employed in our experiment were grown in air by a micro‐pulling down (μ‐PD) method from host materials of a congruent Li/Nb (0.945) ratio which were melt‐doped with a nominal molar concentration of 1, 3, 5% MgO and 0.6% Er₂O₃. The X‐ray diffraction analysis results indicated that the co‐doped crystals main tained the same structural characteristics as the undoped LiNbO₃, however the lattice parameters with Mg differed; c (Å) value decreased, and a (Å) increased than of pure LiNbO₃. The influence of dopants on the photoluminescence (PL) properties of the Er:Mg:LiNbO₃ single crystal fibers excited by laser lines of 514 nm was reported. Also, the PL properties according to temperature and the excitation power of Er:Mg:LiNbO₃ crystal fibers were analyzed.