Defect structure and optical damage resistance of Zn:Fe:LiNbO<Subscript>3</Subscript> crystals

A series of Zn:Fe:LiNbO₃ crystals were prepared by the Czochralski technique with 0.015 wt. % Fe₂O₃ content and various concentrations of ZnO. The ultraviolet-visible absorption spectra and the infrared absorption spectra of the Zn:Fe:LiNbO₃ crystals were detected in order to investigate their defect structure. Their optical damage resistance was characterized by the photoinduced birefringence change and transmission facula distortion method. The optical damage resistance of the Zn:Fe:LiNbO₃ crystals increases remarkably when the concentration of ZnO is over its threshold concentration (more than 6.0 mol. %). The effects of defects on the optical damage resistance of the Zn:Fe:LiNbO₃ crystals are discussed in detail. Received: 25 October 2002 / Revised version: 6 January 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +86-451/2300-926, E-mail: zzxxhhdoctor@sina.com     

Structure and optical damage resistance of Zn:Mn:Fe:LiNbO3 crystals

The new non-volatile holographic storage materials, Zn:Mn:Fe:LiNbO₃ crystals, were prepared by Czochralski technique. Their microstructure was measured and analyzed by infrared (IR) transmission 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 Mn:Fe:LiNbO₃ crystal. The photoinduced birefringence change was measured by the Sénarmont's method. It decreased with ZnO concentration increasing. The dependence of the defects on the optical damage resistance was discussed.     

Optical damage resistance of Ce:Fe:LiNbO3 crystals with various Li/Nb ratios

Ce:Fe:LiNbO₃ crystals were grown by the Czochralski technique with various ratios of Li/Nb = 0.94, 1.05, 1.20 and 1.38 in the melt. Their UV–Vis absorption spectra were measured in order to investigate their defect structures and their optical damage resistance was characterized by the photoinduced birefringence change and transmission facual distortion method. The optical damage resistance of Ce:Fe:LiNbO₃ crystals improves with the Li/Nb ratio increases. The dependence of the optical damage resistance on the defect structure of Ce:Fe:LiNbO₃ crystals is discussed in detail.     

Defect structure and optical damage resistance of Hf:Fe:LiNbO3 crystals

A series of Hf:Fe:LiNbO₃ crystals were grown by the Czochralski technique with various doping concentrations of HfO₂. Their defect structures were analyzed by the UV-visible absorption spectra and infrared absorption spectra. The optical damage resistance 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 enhances remarkably with the HfO₂ concentration increasing when the HfO₂ concentration is lower than its threshold concentration (4 mol%). However, when the HfO₂ concentration exceeds its threshold concentration, the optical damage resistance ability of the crystals returns to decrease. This unusual behavior is explained by using the photovoltaic field produced in the crystals.     

Investigation of optical photorefractive properties of Zr:Fe:LiNbO3 crystals

A series of Zr:Fe:LiNbO₃ crystals with various levels of ZrO₂ doping were grown by Czochraski technique. The optical damage resistance and photorefractive properties were deeply explored. The results showed that the ability optical damage resistance increased remarkably when the concentration of ZrO₂ is over threshold concentration, but which is lower than that of traditional damage resistant additive MgO. While, the holographic storage properties can be greatly enhanced by proper level of ZrO₂ doping in Fe:LiNbO₃. In terms of ions' site occupation model, the photo-damage resistant ability enhancement and the change of the photorefractive properties were discussed.     

OH-absorption properties of the optical damage region in codoped In/Mg:LiNbO3 crystals with various Li/Nb ratios

OH-absorption properties of the optical damage region in a series of codoped In/Mg:LiNbO₃ crystals with various Li/Nb ratios have been investigated. The OH⁻-associated vibrational peak at 3507 cm⁻¹ is confirmed to occur in crystals with Li/Nb ratio of 0.94. For codoped In/Mg:LiNbO₃ crystals with Li/Nb ratio of 1.05 and 1.20, the OH⁻-associated vibrational peaks are detected at 3536 and 3507 cm⁻¹ as well. A new peak at 3518 cm⁻¹ attributed to a (In_Nb)²⁻-OH⁻-(Mg_Nb)³⁻ defect center is revealed in crystals with Li/Nb ratio 1.38. When the “In-Mg threshold” concentration is reached, the optical damage resistance ability of codoped In/Mg:LiNbO₃ crystals is greatly improved.     

Defect structure and nonvolatile hologram storage properties in Hf:Fe:Mn:LiNbO3 crystals

A series of Hf:Fe:Mn:LiNbO₃ crystals with various levels of HfO₂ doping were grown by Czochralski technique. The infrared spectra and ultraviolet spectra were measured and discussed to investigate their structure and defects. The optical damage resistance was characterized by the transmitted beam pattern distortion method. The nonvolatile two-color holographic recording experimental results showed that the recording speed was faster with the increase of HfO₂ doping concentration and at the same time little loss of nonvolatile diffraction efficiencies could be achieved.     

The effect of In doping on optical properties of Fe:LiNBO3 crystals

Fe:LiNbO₃ and In:Fe:LiNbO₃ crystals were grown by Czochralski method. The absorption spectra were measured to investigate their defect structure. The photo damage resistance and photorefractive properties were measured. The photo damage resistance of the In:Fe:LiNbO₃ crystal in which the In concentration is above the threshold value is one order of magnitude higher than that of the Fe:LiNbO₃ crystal. The mechanisms of the violet shift of the absorption edge and the enhancement of the photorefractive effect of In:Fe:LiNbO₃ crystals were investigated.     

Structure and nonvolatile holographic recording of Mg:Ce:Cu:LiNbO3 crystals

A series of Mg:Ce:Cu:LiNbO₃ crystals has been grown by Czochralski method. Their infrared transmittance spectra and ultraviolet-visible absorption spectra were measured and discussed to investigate their defect structure. The nonvolatile holographic recording of Mg:Ce:Cu:LiNbO₃ crystals was characterized by the two-photon fixed method. We found that the recording time of Mg:Ce:Cu:LiNbO₃ crystals became shorter and nonvolatile diffraction efficiency decreases with the increase of Mg doping concentration, especially doping with Mg approaches and exceeds the so-called threshold. And the nonvolatility vanishes when the concentration of MgO exceeds 4 mol%. The intrinsic and extrinsic defects were discussed to explain the nonvolatile holographic properties in the Mg:Ce:Cu:LiNbO₃ crystals.     

高掺镁铌酸锂晶体的生长和倍频性能

我们通过测定MgO在同成分LiNbO₃中的有效分凝系数、相位匹配温度与MgO浓度之间的关系,找到了使Mg:LiNbO₃晶体的相位匹配温度达到最高的掺MgO配方,并克服了Mg:LiNbO₃晶体在高掺杂生长时易出现生长条纹和脱溶等问题,从而生长出了抗光折变能力强,光学均匀性良好的Mg:LiNbO₃晶体。用于连续泵浦Nd:YAG声-光调Q腔内倍频时,获得了平均功率最高达2瓦的二次谐波输出。 关键词：     

Nonvolatile hologram storage properties of tri-doped Zn:Ce:Cu:LiNbO3 crystals

Congruent Zn(7 mol%):Ce:Cu:LiNbO₃ single crystal was grown by the Czochralski method in air. The occupation mechanism of the Zn²⁺ was discussed by an infrared transmittance spectrum. The nonvolatile holographic recording in Zn(7 mol%):Ce:Cu:LiNbO₃ single crystal was measured by two-photon fixed method. Zn(7 mol%):Ce:Cu:LiNbO₃ single crystals present the faster recording time and higher light-induced scattering resistance ability comparing with Ce:Cu:LiNbO₃ single crystals.     

LiNbO3晶体中缺陷的X射线形貌研究

用X射线透射扫描形貌方法观察了直拉法生长LiNbO₃晶体中各种类型点阵缺陷,诸如铁电畴壁、生长层、位错、亚晶界和胞状组织等;用不同衍射矢量对[001]和[210]方向生长的晶体的形貌消象规律,结合X射线铁电异常散射效应和光学显微观察,讨论了晶体中180°铁电畴和生长层的衬度及其分布,并研究了LiNbO₃晶体中180°畴壁形成及其相互关系。 关键词：     

Growth and optical properties of Hf,Ce co-doped lithium niobate crystals

In this paper, a series of Hf,Ce co-doped lithium niobate crystals with various HfO₂ concentrations were grown by Czochralski method. The ultraviolet-visible absorption spectra and the infrared transmittance spectra were measured to study defect structure of the crystals. The optical damage resistance was measured by the transmitted facula distortion method. The results showed that the optical damage resistance of Hf:Ce:LiNbO₃ was greatly improved when the Hf-doping concentration was above 4 mol%.     

Influence of post-growth treatment on the optical properties of In:Ce:Cu:LiNbO3 crystals

Congruent In (3 mol%):Ce:Cu:LiNbO₃ crystals have been grown by the Czochralski method in air. Some crystal samples were reduced in Li₂CO₃ power, and others were oxidized in Nb₂O₅ power. The structure of crystals was studied by an infrared transmittance spectrum. The resistance ability to optical damage and the photorefractive properties were measured by light-induced scattering experiments and two-beam coupling, respectively. It has been found that the reduction treatment increased the photoconductivity , which resulted in decreased erasure time and diffraction efficiency, but higher light-induced scattering resistance ability. The oxidation treatment caused the inverse affect. Finally, the nonvolatile holographic recording in In:Ce:Cu:LiNbO3 crystals is realized.     

Investigation of optical homogeneity and photorefractive properties of lithium-niobate single crystals by the Raman-spectroscopy and laser-conoscopy methods

Photorefractive properties and structural and optical homogeneity of (1) LiNbO₃:Cu crystals ([Cu] = 0.015 mas %) grown from a congruent melt, (2) nominally pure stoichiometric crystals grown from a melt with 58.6 mol % of Li₂O (LiNbO_3stoich), and (3) nominally pure congruent crystals (LiNbO_3congr) have been studied using the Raman-spectroscopy method with excitation in the UV, visible, and near-IR ranges; the laser-conoscopy method; and the electron paramagnetic resonance-spectroscopy method. In optically uniaxial LiNbO₃ crystals, a weak optical biaxiality has been revealed, which is attributed to an insignificant deformation of the optical indicatrix. This deformation can be caused both by the initial structural inhomogeneity of crystals and by the photorefractive effect. It has been shown that, under the action of light, charge exchange of copper cations Cu²⁺ → Cu⁺ takes place in the crystal LiNbO₃:Cu ([Cu] = 0.015 mas %). The LiNbO₃:Cu crystal exhibits photorefractive properties not only because of the occurrence of intrinsic defects with electrons localized at them, as is the case with the LiNbO_3stoich and LiNbO_3congr crystals, but also due to the charge exchange of copper cations under the action of the laser radiation.     

Low-loss, damage-resistant optical waveguides in Zn-diffused LiNbO3 by a two-step procedure

In the present work the fabrication of high-quality LiNbO₃ waveguides, based on Zn diffusion from vapor phase using a two-step diffusion process, has been demonstrated. The optical waveguides fabricated using this method have low propagation losses (less than 0.5 dB/cm measured at 633 nm), as a consequence of the high surface quality obtained. High photorefractive damage resistance is reported, and it has been related to the Zn incorporation into the LiNbO₃ crystals. Additional advantages of this waveguide fabrication method due to the low temperature of the process (<900 °C) are to prevent Li out-diffusion and to preserve ferroelectric domain structures. Received: 2 March 2000 / Accepted: 21 June 2000 / Published online: 13 September 2000     

Polarons in magnesium-doped lithium niobate crystals induced by femtosecond light pulses

Strong light-induced absorption has been observed in lithium niobate crystals doped with magnesium after application of femtosecond illumination. In this material there are no Nb-on-Li-site defects and hence no antisite polarons occur, but small free polarons close to the conduction band can be generated. The light-induced absorption observed is attributed to these polarons. For LiNbO₃:Mg, their decay times are about two orders of magnitude smaller than those of the Nb-on-Li-site polarons in undoped material. The results are relevant for a better understanding of the suppression of the so-called optical damage in these crystals and for their use in femtosecond applications.     

Dispersion of the dielectric function real part for Mg:LiNbO3 crystals at terahertz frequencies

We study dispersion of the dielectric function real part ε′ in the terahertz-frequency range for bulk and periodically poled congruent LiNbO₃ and Mg:LiNbO₃ crystals. The concentration of Mg in Mg:LiNbO₃ samples was close to 5 mol%, which is the photorefractive threshold. Approximate expressions for extraordinary polariton dispersion dependence were obtained in the range 0.5–6.5 THz. The influence of Mg-dopant on the optical properties of crystals in the terahertz range is revealed. Changes of the defect structure of lithium niobate crystals are discussed.     

Spectral properties and quasi-phase-matched second-harmonic generation in a new active medium: optical superlattice Nd:MgO:LiNbO3

3 crystal – was grown in our Laboratory. The absorption and fluorescence spectra were measured and show some differences from the results of common Nd:MgO:LiNbO₃ with monodomain structure. Third-order quasi-phase-matched violet second-harmonic generation in the sample was demonstrated to characterize the quality of the crystal. The properties indicate that the periodically poled optical superlattice Nd:MgO:LiNbO₃ has great potential for constructing a blue or green light source through self-frequency-doubling operation using the maximum nonlinear optical coefficient d₃₃ of LiNbO₃ and the high-gain π polarization at the same time. Received: 22 August 1997/Revised version: 10 January 1998     

Temperature dependence of the Cu(2) NQR line width in YBa2Cu3O7−y

Precision X-ray structural studies were carried out for LiNbO₃:Zn_x single crystals with x=0.0, 2.87, 5.20, and 7.60 at. %. It was found that the insertion of the Zn atoms into the Li position was accompanied by a decrease in the concentration of intrinsic Nb_Li defects. At x=7.6%, the Zn atoms change their locations in the lattice and partially occupy the Nb positions. This clarifies the structural nature of the “threshold” Zn concentration, which manifests itself as singularities in the concentration dependences of various optical properties. The structural origin of the threshold concentration is likely a common feature of all nonphotorefractive impurities (Mg, Zn, In, and Sc) in LiNbO₃. A change in the intrinsic defect structure of the LiNbO₃ crystals with different Zn concentrations is discussed.