双掺杂LiNbO3:Fe:Mn全息存储动力学

建立了包括扩散、漂移和光伏打效应三种输运机制,小信号光强、小调制度近似下描述双掺杂LiNbO3:Fe:Mn晶体用双色光进行全息存储的动力学的耦合微分方程组,数值求解并解释了晶体光存储的时间动态发展过程.在此基础上,分析了晶体的氧化还原程度对全息存储过程的影响,只有在晶体总的受主数密度Na(即Fe3+和Mn3+的数密度之和)大于铁离子数密度N2的条件下,双掺杂LiNbO3:Fe:Mn晶体全息存储才能达到非破坏性存储的目的.经过光固定的光栅的衍射效率随氧化增大,光折变灵敏度随氧化而减小,要获得高衍射效率就必须以降低光折变灵敏度为代价.在掺铁浓度一定的情况下,掺锰浓度越高,能实现信息长期存储对应的氧化还原状态的有效动态范围越大.     

连续光条件下对LiNbO3:Fe:Mn晶体全息存储性能的理论研究

以双中心模型为基础,在低光强连续光条件下研究了LiNbO3:Fe:Mn晶体在稳态情况下的非挥发双光双步全息存储性能.采用数值方法.通过比较双中心模型中深(Mn2 /Mn3 )、浅(Fe2 /Fe2 )能级之间所有可能的电子交换过程,发现由隧穿效应引起的深浅能级之间直接电子交换过程对LiNbO3:Fe:Mn晶体总的空间电荷场大小起着决定性的作用.同时.这一电子交换过程对晶体非挥发全息存储性能也起着至关重要的作用.此外.通过相同实验条件下LiNbO3:Fe:Mn晶体与近化学比LiNbO3:Fe晶体总的空间电荷场的比较,显示LiNbO3:Fe:Mn晶体在低抽运光和高记录光光强条件下有着比近化学比LiNbO3:Fe晶体更佳的伞息存储性能.     

第一原理研究Mn掺杂LiNbO3晶体的磁性和光吸收性质

采用基于密度泛函理论和局域密度近似的第一性原理分析了Mn掺杂LiNbO₃晶体的结构, 磁性, 电子特性和光吸收特性. 文中计算了Mn占据Li位和Nb位体系的形成焓, 对应的形成焓分别为-8.340 eV/atom和-8.0062 eV/atom, 也就意味着Mn 原子优先占据Li位. 这也就意味着Mn原子占据Li位的掺杂LiNbO₃晶体结构更稳定. 磁性分析的结果显示, 其对应磁矩也比占据Nb位的高. 进一步分析磁性的来源, 自旋态密度结果显示: Mn掺杂LiNbO₃晶体的磁性主要源于掺杂原子Mn, Mn原子携带的磁矩高达 4.3 μ_B, 显示出高自旋结构. 由于Mn-3d与近邻O-2p及次近邻Nb-4d 轨道的杂化作用, 计算表明: 诱导近邻O原子及次近邻Nb原子产生的磁矩对总磁矩的贡献较小. 通过光学吸收谱的分析, 得出在可见光区Li位被Mn原子替代以后显示出更好的光吸收响应相比于Nb位. 本文还分析了O空位对于LiNbO₃晶体磁性与电子性质的影响, 结果显示O空位的存在可以增加Mn掺杂LiNbO₃体系的磁性.     

Fe:Mg:LiNbO3晶体电子结构和吸收光谱的第一性原理研究

基于密度泛函理论的第一性原理, 研究了LiNbO₃晶体以及不同Mg浓度的Fe:Mg:LiNbO₃晶体的电子结构和吸收光谱. 研究结果显示: 掺铁铌酸锂晶体的杂质能级由Fe 的3d轨道和O的2p轨道贡献, 禁带宽度为2.845 eV; 对于Mg, Fe共掺样品, Mg的浓度小于或等于阈值时, 禁带宽度分别为2.901 和2.805 eV; 掺铁铌酸锂晶体的吸收谱在2.3和2.6 eV处分别存在一个吸收峰, 其强度因Mg的浓度不同而发生变化. 研究结果还表明, 不同浓度的Mg对晶体内Fe²⁺和Fe³⁺的浓度以及占位产生了不同的影响. 还提出了光电子的形成不应单独考虑铁的轨道电子态, 而应同时考虑与铁成键的氧的轨道电子态的观点.     

LiNbO3:Fe晶体二波耦合弱光放大的物理机理

对LiNbO₃: Fe晶体中二波耦合过程进行了动态观测.进一步探讨了LiNbO ₃:Fe晶 体中弱光放大的物理机理.发现LiNbO₃ : Fe晶体中二波耦合过程的弱光放大 对c轴指 向有明显的依赖关系，虽然光生伏打效应对光生载流子的迁移有主要贡献，但扩散机理的贡 献仍不可忽略.弱光最终得到放大是瞬态能量转移与扩散机理引起的能量转移的共同贡献.弱 光放大达到准稳态之后的下降过程为瞬态能量转移的时间指数衰减过程与光散射引起的能量 耗散的共同贡献. 关键词： 光折变效应 光放大 掺杂铌酸锂     

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：     

连续光条件下对LiNbO3∶Fe∶Mn晶体全息存储性能的理论研究

以双中心模型为基础,在低光强连续光条件下研究了LiNbO3∶Fe∶Mn晶体在稳态情况下的非挥发双光双步全息存储性能。采用数值方法,通过比较双中心模型中深(Mn2+/Mn3+)、浅(Fe2+/Fe3+)能级之间所有可能的电子交换过程,发现由隧穿效应引起的深浅能级之间直接电子交换过程对LiNbO3∶Fe∶Mn晶体总的空间电荷场大小起着决定性的作用。同时,这一电子交换过程对晶体非挥发全息存储性能也起着至关重要的作用。此外,通过相同实验条件下LiNbO3∶Fe∶Mn晶体与近化学比LiNbO3∶Fe晶体总的空间电荷场的比较,显示LiNbO3∶Fe∶Mn晶体在低抽运光和高记录光光强条件下有着比近化学比LiNbO3∶Fe晶体更佳的全息存储性能。     

基于单颗粒NaYF4微米棒的上转换白光发射特性

上转换白光材料在固态照明、液晶显示器和生物成像等领域展现出其他光源无法比拟的优势,倍受研究者们广泛关注.为此,本文采用水热法合成了一系列掺杂不同离子浓度的NaYF₄∶Yb³⁺/Re³⁺/Tm³⁺(Re³⁺=Ho³⁺,Er³⁺)微米晶体.在980 nm近红外光激发下,通过调控掺杂离子的浓度,研究Ho³⁺/Tm³⁺及Er³⁺/Tm³⁺共掺杂单颗粒NaYF₄微米晶体的白光发射特性.研究表明:在NaYF₄∶Yb³⁺/Ho³⁺/Tm³⁺微米晶体中,通过调控Yb³⁺离子的掺杂浓度易于实现其白光发射;而在NaYF₄∶Yb³⁺/Er³⁺/Tm³⁺微...     

近化学计量组分掺铒铌酸锂晶体光学特性研究

用气相输运平衡技术成功地制备出了近化学计量组分的Er∶LiNbO₃晶体,系统 地研究了该 晶体的吸收边移动、可见—红外透射谱、荧光光谱的物理特性.与同成分Er∶LiNbO_{3晶体 相比, 近化学计量组分Er∶LiNbO3晶体的吸收边出现了“蓝移”;OH^{-吸收带明显减弱 ;在可见—近红外波段呈现出更高的透过率;同时Er3+离子的发光强度也明显 提高. 这些结果表明近化学组分的Er∶LiNbO关键词： 近化学组分掺铒铌酸锂晶体 -}吸收带')" href="#">OH^-吸收带 透射谱 荧光光谱}     

Er3+/Yb3+共掺NaYF4/LiYF4微米晶体的上转换荧光特性

采用水热法成功制备了Er³⁺/Yb³⁺共掺杂的NaYF₄和LiYF₄微米晶体. 通过X射线衍射仪和环境扫描电子显微镜对样品的晶体结构及形貌进行表征. 实验结果表明: 六方相NaYF₄微米晶体为棒状结构, 而四方相LiYF₄微米晶体则为八面体结构. 在近红外光980 nm激发下, NaYF₄:Yb³⁺/Er³⁺和LiYF₄:Yb³⁺/Er³⁺ 微米晶体均展现出很强上转换荧光发射. 且NaYF₄:Yb³⁺/Er³⁺微米晶体的荧光发射强度大约是LiYF₄:Yb³⁺/Er³⁺微米晶体的2倍, 但红绿比明显较低. 根据荧光光谱, 并借助激光光谱学及发光动力学深入探讨基质变化及表面修饰剂乙二胺四乙二酸(EDTA)对荧光特性的影响. 实验结果发现: 影响荧光强度的主要因素是基质环境的局域对称性, 而导致不同红绿比则是由于样品表面较多的EDTA分子所引起. Er³⁺掺杂的NaYF₄和LiYF₄ 微米晶体呈现出很强的绿光发射可被应用于全色显示, 荧光粉和微光电子器件中.     

Investigation of nonvolatile holographic storage in LiNbO3:Mn:Ce:Fe crystal

We investigate the persistent holographic recording in triply doped LiNbO₃:Mn:Ce:Fe crystals at different oxidation/reduction states. The experimental results show that there is an optimum oxidation/reduction state, which results in the best dynamic range M/#. Compared with doubly doped LiNbO₃:Ce:Fe, we found that the nonvolatile diffraction efficiency and the best dynamic range M/# obtained in triply doped samples are larger than that obtained in doubly doped samples. The reason for the increase of the crystal about the nonvolatile diffraction efficiency and the dynamic range M/# was also explained.     

Control of sensitivity to hologram storage in LiNbO3 using an accessory photovoltaic crystal

Enhancement of sensitivity to hologram storage in a LiNbO₃ crystal is reported by using an accessory photovoltaic LiNbO₃ crystal, highly iron doped and reduced. With an illumination of 600 mW/cm² at λ=0.48 μm of the photovoltaic crystal the photogenerated electric field is about 10 kV/cm. This field is applied to a storage crystal having a higher resistivity and improves substantially the photorefractive sensitivity and saturation of the diffraction efficiency.     

Photorefractive properties of iron-doped lithium tantalate crystals

Iron-doped lithium tantalate crystals are grown by the Czochralski method and their photorefractive properties are examined with holographic methods. Dynamic range, holographic sensitivity, photoconductivity, and dark storage time are measured in dependence on the iron concentration and light intensity. The largest refractive-index change for ordinarily polarized light is 3.5×10^-4, in comparison with 6.2×10^-4 for iron-doped lithium niobate. Due to a small mobility of protons the dark storage time of holograms in lithium tantalate is larger than that in lithium niobate. PACS 42.40.Pa; 42.70.Ln     

Enhanced nonvolatile holographic properties in Zn,Ru and Fe co-doped LiNbO3 crystals

A series of LiNbO₃ crystals doped with various concentrations of ZnO and fixed concentrations of RuO₂ and Fe₂O₃ have been grown by the Czochralski method from the congruent melts. The type of charge carriers was determined by Kr⁺ laser (476 nm) and He–Ne laser (633 nm). The results revealed that the holes were the dominant charge carriers at blue light irradiation. Dual-wavelength and two-color techniques were employed to investigate the nonvolatile holographic storage properties of Ru:Fe:LiNbO₃ and Zn doped Ru:Fe:LiNbO₃ crystals. The essential parameters of blue nonvolatile holographic storage in Zn:Ru:Fe:LiNbO₃ crystals were enhanced greatly with the increase of Zn concentration. This indicates that the damage resistant dopants Zn²⁺ ions enhance the photorefractive properties at 476 nm wavelength instead of suppressing the photorefraction. The different mechanisms of blue photorefractive and nonvolatile holographic storage properties by dual wavelength recording in Zn:Ru:Fe:LiNbO₃ crystals were discussed.     

Formation dynamics of nonvolatile crossed-beam photorefractive gratings in doubly doped LiNbO3 crystals: Theoretical investigation

The formation dynamics of crossed-beam photorefractive gratings formed by the method of two-center holographic recording in doubly doped LiNbO₃ crystals is investigated in this paper based on the theoretical model combining the two-center band transport model with the two-dimensional coupled-wave theory. The numerical simulations are presented for two-center holographic recording crossed-beam photorefractive gratings in LiNbO₃:Fe:Mn crystals. The temporal and spatial evolutions of the refractive index modulation and the diffraction efficiency are shown. The spatial variation of the wave intensity is also presented.     

Nonvolatile photorefractive properties in triply doped stoichiometric Mg：Fe：Mn：LiTaO3 crystals

We have grown triply doped Mg:Fe:Mn:LiTaO3 crystals with near stoichiometry using the top seeded solution growth technique. The defect structure was investigated by infrared absorption spectra and Curie temperature. Using a blue laser as the source, excellent photorefractive properties were obtained. Nonvolatile holographic storage properties were investigated using the dual wavelength technique. We got a very high fixed diffraction efficiency and nonvolatile holographic storage sensitivity. The blue light has more than enough energy to excite holes of deep(Mn) and shallow(Fe) trap centers with the same phase, which enhance dramatically the blue photorefractive properties and the nonvolatile holographic storage. Mg2+ ion is no longer damage resistant at blue laser, but enhances photorefractive characteristics.     

Photorefractive properties for holographic application of Ce:Fe:LiNbO3 crystals with various [Li]/[Nb] ratios

Ce:Fe:LiNbO₃ crystals with various [Li]/[Nb] ratios were grown by the Czochralski method from melts having compositions varying between 48.6 and 58 mol% Li₂O. The Ce, Li and Nb concentrations in the grown Ce:Fe:LiNbO₃ crystals were analyzed by the inductively coupled plasma atomic emission spectrometer (ICP-AES). It was found that as the [Li]/[Nb] ratio increases in the melt, the [Li]/[Nb] ratio in the crystal and the distribution coefficients of Ce ions increase also. The photorefractive properties of the Ce:Fe:LiNbO₃ crystals were experimentally studied by the two-wave coupling method. The results show that as the [Li]/[Nb] ratio increases, the dynamic range decreases, but the photorefractive sensitivity and the signal-to-noise ratio improve. In a coherent volume 0.192 cm³ of a Ce:Fe:LiNbO₃ crystal with [Li]/[Nb] ratio of 1.2, 3800 holograms with 800×600 pixels have been successfully multiplexed in a compact volume holographic data storage system.     

Photorefractive centers in LiNbO3, studied by optical-, Mössbauer- and EPR-methods

The photorefractive properties of LiNbO₃∶Fe and LiNbO₃∶Cu have been studied in combination with optical absorption-, Mössbauer- and EPR-measurements. The charge states of Fe in successively reduced LiNbO₃∶Fe have been investigated with respect to the influence on the photorefractive sensitivity and saturation value of the refractive index change. The results of this experiment demonstrate clearly the close correlation between the concentration of Fe²⁺ impurities and the optical absorption band around 2.6 eV in LiNbO₃∶Fe, which is known to give rise to an anisotropic charge transport upon optical excitation. The resulting photocurrents determine the photorefractive sensitivity mainly in the initial state of halographic exposure. With increasing conversion from Fe³⁺ to Fe²⁺ the photorefractive sensitivity saturates and the saturation value of the refractive index change decreases remarkably. In the case of LiNbO₃∶Cu a similar behaviour of the photorefractive storage parameters after successive reduction treatments has been observed qualitatively. However, in contradiction to LiNbO₃∶Fe the Cu²⁺ centers cannot be related to the photorefractive sensitivity of LiNbO₃∶Cu. These results are discussed with respect to the predictions of two models concerning the microscopic nature of the photorefractive process in doped LiNbO₃.     

Growth and photorefractive properties of near-stoichiometric In:Fe:Cu:LiNbO3 crystals

The near-stoichiometric LiNbO₃ crystal co-doped with In₂O₃, Fe₂O₃, and CuO has been grown from a Li-rich melt (Li/Nb = 1.38, atomic ratio) by the Czochralski method in air atmosphere for the first time. The OH⁻ absorption spectra were characterized to investigate the structure defects of the crystals. The appearance of the 3506 cm⁻¹ absorption peak manifests that the composition of the grown crystal is close to the stoichiometric ratio. The photorefractive properties were also measured by the two-wave coupling experiments. The results show that the near-stoichiometric In:Fe:Cu:LiNbO₃ crystal has a larger refractive index change, higher recording sensitivity and larger two-wave coupling gain coefficient than those obtained in the congruent In:Fe:Cu:LiNbO₃ crystal under the same experimental conditions. The material of near-stoichiometric In:Fe:Cu:LiNbO₃ crystal is a promising candidate for blue photorefractive holographic recording.