Stoichiometry dependence of the OH- absorption band in LiNbO3 crystals

Hydrogen in the form of OH^- ions can be incorporated into LiNbO₃ single crystals during the growth process. The infrared absorption band due to the OH^- defects has been measured at room temperature and a considerable change of the band shape has been observed depending on the crystal composition. The structure of the band found in nearly stoichiometric crystals indicates three slightly different proton sites which can be explained on the basis of the LiNbO₃ crystal structure.     

An EPR study of different Gd3+ centers in LiNbO3

Electron Paramagnetic Resonance (EPR) and Zero-(magnetic)-Field (paramagnetic) Resonance (ZFR) spectra have been observed for LiNbO₃:Gd. EPR spectra measured at 9 and 35 GHz consisted of at least seven independent patterns; all of predominantly axial symmetry and all ascribed to Gd³⁺. The two most intense patterns have been analyzed in detail, and while one was found to be axially symmetric the other, though predominantly axial, had a small rhombic crystal field term. The ZFR spectrum, measured between 1 and 18 GHz, consisted of at least two patterns of zero field transitions. Both EPR and ZFR spectra were found to be insensitive to variations in LiNbO₃ stoichiometry, c- of a-axis crystal growth direction, and various sample treatments.     

Radiation effects in crystalline SiO2: Infrared absorption from OH--related defects

Infrared absorption bands due to OH^- molecules have been investigated in unswept and hydrogen-swept crystals of synthetic quartz. The study included one series of bands introduced by electron irradiation, another series of bands present in the as-grown material, and two bands associated with the Al-OH^- center. They were studied first as a function of irradiation temperature and then as a function of radiation dose at 80 K. After irradiation at 80 K, the bands were monitored as a function of anneal temperature from 80 to 760 K.     

OH-1 absorption bands of LiNbO3 with varying composition

This paper reports that OHabsorption bands of lithium niobate crystal have been measured at room temperature, and the band shape depending on the crystal composition has been observed. The OHabsorption bands are fitted with three Lorentzian peaks by varying position, halfwidth, and height. Nearly constant peak positions (3468, 3481 and 3490 cm-1) are obtained for all samples. It shows that the height and area of the decomposed peaks vary with the Li composition in a complex way. However, the combinations of these fitting parameters show a linear dependence on the composition up to nearly 50 mol%, which is very useful for the composition determination in a wide range. The linear relationships between the parameter combinations and Li composition are also presented quantitatively. In addition, the explanations were given for the excellent composition linearity of parameter combinations.     

单晶LiNbO3:Mn2+的ESR谱研究

用室温电子自旋共振（ESR）实验研究单晶LiNbO3中Mn2+的精细结构和超精细结构.对ESR谱的分析得出,零磁场的能级分裂数值：自旋角动量能级±12〉与±32〉之间的间隔为Δε1=-587×10-4cm-1,而±32〉与±52〉之间的能级间隔为Δε2=-2633×10-4cm-1；其各向异性朗德因子g∥=21810，g⊥=20937；精细结构常数D=-536×10-4cm-1；超精细结构常数A∥=8836×10-4cm-1，A⊥=8120×10-4cm-1，即精细结构相互作用要比超精细结构相互作用大得多.另外，特别值得提到的是实验中还发现两组明显的禁戒跃迁，-32〉12〉和-12〉52〉. 关键词： 单晶 精细结构 超精细结构 禁戒跃迁     

Ni^2＋：LiNbO3的光学吸收谱和EPR的研究

应用三角晶场中d^2(d^8)电子组态的包括静电相互作用和自旋－轨道耦合相综合利用物强场能量矩阵,采用完全对角化方法,精确地计算了具有C3V对称的Ni^2 :LiNbO3的光学吸收谱和EPR谱。理论结果与实验值符合得很好。     

Luminescence kinetics of LiNbO3:Yb3+-Er3+, LiNbO3:Er3+, and LiNbO3:Yb3+ crystals under selective excitations in the impurity subsystem

The results of investigations of luminescent radiations’ kinetic characteristics for LiNbO₃:Yb³⁺-Er³⁺, LiNbO₃:Er³⁺, and LiNbO₃:Yb³⁺ crystals under optical excitations at 532 nm and 1064 nm wavelengths are presented. The shapes and times of rise and damping of luminescent signals at 550 nm, 980 nm and 1555 nm wavelengths under selective excitations in the impurity subsystem of the investigated materials are determined. Comparison of the temporal characteristics of luminescent responses of LiNbO₃ crystals doped separately with Yb³⁺ and Er³⁺ ions with those of the LiNbO₃:Yb³⁺-Er³⁺ crystal allows identifying the contributions from different energy transfer processes of optical excitation taking place in the impurity subsystem of the material.     

First-principles study on the formation energies of intrinsic defects in LiNbO3

First-principles plane-wave ultrasoft pseudopotential method within local density approach (LDA) has been used to study three possible vacancy-defect models for non-stoichiometric lithium niobate (LiNbO₃): (1) the oxygen-vacancy model , (2) the niobium-vacancy model , and (3) the lithium-vacancy model . The corresponding formation energies are obtained via energy minimization of a supercell. In Nb-rich environment, the calculated defect formation energies, both under oxidation and reduction conditions, show little effect on the intrinsic defect structures. We find that the lithium vacancy model has the most stable configuration in the non-stoichiometric lithium niobate crystals. Our calculations also show that the formation of any type of neutral defects and Frenkel pairs in a Nb-rich environment is difficult.     

Photorefractive properties of MnO-doped near stoichiometric LiNbO3 crystals

In this paper, photorefractive properties of MnO-doped near stoichiometric LiNbO₃ (SLN) were studied. The SLN crystals doped with different concentration of Mn ions have been grown by the top-seeded-solution-growth method by adding K₂O flux to Li₂O-Nb₂O₅ melt. The crystal structures were analyzed by the UV-vis spectra and IR spectra. The photorefractive properties of SLN were experimentally studied by using two-beam coupling. The results of the two-wave coupling experiments showed that, with a He-Ne laser of 632.8 nm wavelength, the MnO-doped SLN crystals had long writing time and long erasure time constants, and the dynamic range (M/#), sensitivity (S), and diffraction efficiency were dependent on the doping concentration.     

Fast electrically switchable holographic optical elements in LiNbO3

We present here the realization of fast electrically switchable holographic optical elements based on electric field multiplexing of volume holograms in lithium niobate crystals. We demonstrate the electrical control of holographic lenses and holographic mirrors for fast switching of the focal length and the direction of the reconstructed light beam, respectively. The switching time in the range of few hundred microseconds has been demonstrated using this technique.     

Reflectance and absorption spectra near the band gap in CuInSe2

The reflectance and absorption spectra of CuInSe₂ near the band gap were obtained at 7 K. From the analysis of the data, an estimate of the exciton binding energy, Bohr radius, and low frequency dielectric constant has been made, which are 15 meV, 5.5 nm, and 8.5 respectively.     

Optical behaviour of VTE treated near stoichiometric LiNbO3 crystals

Near stoichiometric LiNbO₃ crystal wafers of thickness up to 2 mm were prepared by vapour transport equilibration technique (VTE) at various process temperatures. Crystals were characterised by measurement of the UV absorption edge, refractive index, second harmonic generation (SHG) efficiency, and conoscopy pattern analysis. The comparison of VTE treated crystals show that the blue shift in cut off wavelength occurred with the increasing process temperature (i.e. increasing Li/Nb ratio). The refractive indices were found decreasing with increasing process temperature of VTE samples. The SHG efficiency increases in the range of 1.98-2.3 times for the VTE processed samples with respected to congruent crystals. Conoscopy pattern reveals the optical homogeneity of the VTE treated crystal.     

Correlation of the photorefractive sensitivity in doped LiNbO3 with chemically induced changes in the optical absorption spectra

Single crystals of LiNbO₃ doped with Mn, Fe, or Cu have been reduced or oxidized at 800°C. Changes in the optical absorption spectra and in the photorefractive sensitivity have been determined, and a close correlation between the latter and a particular optical absorption band is demonstrated.     

Stabilization of Fe2+ centers in LiNbO3: Ti waveguides

Holographic measurements in LiNbO₃: Ti waveguides are reported indicating a mode dependent enhancement of photoconductivity. This enhancement can be explained by a stabilization of Fe²⁺ centers in the waveguide which are responsible for optical damage effects.     

LiNbO3和LiTaO3晶体Er3+/Yb3+掺杂水热外延层室温吸收光谱分析

本文引入与浓度和厚度有关的k_NL待定参数, 在J-O理论基础上, 对Er³⁺/Yb³⁺掺杂的LiNbO₃和LiTaO₃单晶衬底上 的多晶水热外延样品进行了基于吸收光谱的拟合计算. LiNbO₃:Ω₂=2.34× 10^-20 cm², Ω₄=0.77× 10^-20 cm², Ω₆=0.31×10^-20 cm², k_NL=4.32× 10^-2 mol·m^-2. LiTaO₃:Ω₂=1.68×10^-20 cm², Ω₄=0.84×10^-20 cm², Ω₆=0.45×10^-20 cm², k_NL=9.17×10^-3 mol· m^-2. 该方法可尝试推广到粉体或胶体等难以直接获得浓度和厚度数据的体系. 经上转换发光测试及光谱参数计分析认为Er³⁺/Yb³⁺离子的掺杂浓度比为1:1的情况下, 样品呈现绿色上转换发光光谱; 可尝试以降低基质声子能量的方法提高⁴I_13/2能级 对²H_11/2和⁴S_3/2能级的量子剪裁效率.     

Evaluation of losses and group effective refractive index of Er3+:Ti:LiNbO3 optical waveguides

In this paper, we report some experimental results concerning several types of loss measurements of Er³⁺:Ti:LiNbO₃ optical waveguides by using optical methods. Using the Fabry-Pérot interferometry method, we evaluated the attenuation coefficient of an optical waveguide resonator for a laser radiation having λ = 1.55 μm. We also evaluated the insertion losses, polarization dependent losses, and coupling with external losses. Additionally, from the transmitted spectra of the symmetrical monomode Fabry-Pérot optical waveguide resonator, we were able to evaluate the value of the group effective refractive index.     

Anomalous homogeneous optical absorption line broadening of Eu3+ in superionic PbF2

Homogeneous broadening of one of the ⁵D₁←⁷F₀ absorption lines of Eu³⁺ in PbF₂ was measured in the temperature range 300–800 K. Below 625 K the linewidth depends linearly on T; above 625 K it increases rapidly followed by a plateau beginning near 700 K. This behavior is quite distinct from that expected for the normal Raman process in crystalline materials and appears to be correlated with F^- ion motion responsible for the superionic conductivity of PbF₂.     

Degenarate four-wave mixing in LiNbO3 and LiTaO3

Continuous wave generation of complex conjugate wavefront via four-wave mixing in LiNbO₃ and LiTaO₃ with diffusion non-linearity is studied. Strong anisotropy of the degenarate four-wave mixing is observed and explained.     

Time-resolved optical absorption in YAlO3 crystals

The present work is devoted to the investigation of transient absorption (TA) induced by a pulsed electron beam (E=250 keV) in pure and doped YAlO₃ (YAP) single crystals. The nature of centers responsible for TA is discussed.