LiNbO_3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LiNbO~3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LiNbO~3晶体中180°畴壁形成及其相互关系。     

铌酸锂晶体中多畴层带区倍频光的研究

本文研究用声光调Q-YAG激光器作为基波光源,测量了LiNbO₃及BNN晶体中倍频光强-温度曲线。在生长层区,倍频曲线上往往出现双峰,同时有一一对应的NPM双光环出现。LiNbO₃的双峰之一在极化处理后消失,本文对此峰的产生进行了分析。LiNbO₃多畴层带区倍频光强比单畴区大10倍左右,实验表明此乃层带区180°畴的形成导致应力松弛的结果。 关键词：     

磷酸铝(AlPO4)晶体中缺陷的X射线形貌研究

用X射线衍射形貌法研究了AlPO₄晶体中的微观缺陷。在所研究的晶体中,主要晶体缺陷是生长层,沉淀物和位错。位错密度在晶体表面附近最大,晶体中部较低。位错主要起源于热应力和由沉淀物或生长层所造成的晶格畸变。多数位错的柏氏矢量是b=(a+c)<1123>型,部分的是b=a[2110]。分析了晶体缺陷与生长条件之间的关系。控制生长过程中的温度波动,特别是晶体出炉时的冷却速度,对提高晶体完美性是重要的。 关键词：     

测量单轴晶体光学不均匀性和生长层的一种简便方法

报道一种测量LiNbO₃∶Fe单轴晶体光学不均匀性及其生长层的简便方法，由o— e光点间的距离随入射光束在晶体中位置的变化可以计算出晶体不同位置的Δn_e的变化， 而且可以测得晶体的不均匀性并确定出生长层的位置及其厚度. 关键词： 不均匀性 生长层 折射率     

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：     

周期极化LiNbO3晶体中电极布置对反转畴扩张的影响

利用外加电场极化的方法,分别研究了三种不同方式布置的极化电极对C向切割LiNbO_{3< /sub>晶体极化反转畴扩张的影响.实验结果表明在相同极化条件下，当极化电极长度方向与六边 形反转畴一对边呈90°和0°角时，极化扩张速度分别为0.089μm/ms和0.011μm/ms.实验结果揭示，为了有效地控制极化占空比，在制备一维准相位匹配LiNbO3晶体时应 避免极化电极与六边形反转畴壁中一对边呈90°的情况. 关键词： 周期极化 极化扩张 准相位匹配}     

掺铥钨酸镱钾激光晶体结构模拟及振动光谱

采用泡生法生长了掺铥钨酸镱钾（KTm_0.1Yb_0.9（WO₄）₂）激光晶体。利用X射线单晶衍射法和X射线粉末衍射法研究了KTm_0.1Yb_0.9（WO₄）₂晶体的物相与结构。采用X射线单晶衍射法并结合Diamond晶体结构绘图软件,获得了晶体内部结构模型图：该晶体是由WO₆、TmO₈/YbO₈和KO₁₂基团组成。W₂O₁₀二聚体通过WOW单氧桥相连,在平行于c轴方向上形成（W₂O₈）_n多重带。ReO₈和KO₁₂多面体共顶相连,沿[101]和[110]方向形成了具有二维层结构的延长带。X射线粉末衍射分析表明：该晶体具有低温β相双钨酸盐结构,属于单斜晶系,空间群C2/c;计算了晶胞参数、晶粒尺寸和结晶度。测试了晶体的振动光谱,对各个峰值的红外和拉曼活性及分子振动情况进行了归属,进一步验证了晶体中WO₆原子基团及单氧桥（WOW）和双氧桥（WOOW）的存在。     

静电场作用下α一LiIO3单晶中子衍射增强现象的理论解释

本文根据(1)在偏振锥光下,用显微镜观察到α-LiIO₃单晶中层状缺陷在静电场作用下的变化;(2)静电场对a-LiIO₃单晶的X射线形貌象的影响;(3)用X射线双晶衍射测得α-LiIO₃单晶晶格参数的不均匀性,指出α-LiIO₃单晶在静电场作用下中子衍射增强现象是由于晶体中的空间电荷(载流子、杂质离子和空位)在宏观尺度缺陷处富集,造成晶格参数有一定梯度。我们对通常计算中子布喇格散射截面的玻恩近似,引入消光修正,得到畸变晶格中子衍射强度公式,可以解释文献[4—6]中观察到的各种现象。 关键词：     

高分辨X射线衍射研究杂质对晶体结构完整性的影响

用水溶液生长晶体的方法生长出了不同掺杂的Sr(NO₃)₂晶体.用电子探针研究了杂质在晶体中的分布情况.结果表明,杂质在晶体中存在扇形分凝,其中Ba在{100}扇形区的含量大于{111}扇形区,而Pb的分凝情况相反,在{111}扇形区的含量大于{100}扇形区.用高分辨X射线衍射摇摆曲线技术研究了纯的、掺Ba的和掺Pb的Sr(NO₃)₂晶体的完整性情况,并用X射线衍射动力学理论计算了完整Sr(NO₃ 关键词： 高分辨X射线衍射 杂质 水溶液晶体生长     

周期极化掺镁不同组分LiNbO3晶体的研究

利用气相平衡扩散法研制出掺镁不同组分的LiNbO₃晶体，并对其极化特性进行了研究.研究表明晶体的开关电场和自发极化不仅与晶体组分［Li］/［Nb］比有关而且与掺镁量有关，［Li］/［Nb］比为0.973掺入2mol% MgO的近化学比LiNbO₃晶体的开关电场仅为1.8kV/mm，是同成分晶体的1/12，且其极化结构的质量要远好于同成分LiNbO₃晶体和近化学比LiNbO₃晶体. 关键词： 气相平衡扩散 3晶体')" href="#">掺镁LiNbO₃晶体 周期极化     

Growth of ultrathin layers of Au on LiNbO3(0 0 0 1) measured with atomic force microscopy

Atomic force microscopy (AFM) has been used to characterize the growth of Au deposited via evaporation onto the positive face of single crystalline, lithium niobate, LiNbO₃(0 0 0 1) surface. In order to study the mechanisms for the ordering and aggregation of a noble metal on this ferroelectric surface, topographic and phase contrast imaging of the fractional surface coverage of Au were performed. Atomically flat, uniformly poled LiNbO₃ surfaces were prepared via an ambient high temperature anneal and served as a support for the thin gold films. These gold atomic layers were grown using electron bombardment evaporation sources under ultra-high vacuum (UHV) conditions and subsequently characterized under both vacuum and ambient environments. Using AFM it was found that gold preferentially nucleates at the top of LiNbO₃ substrate step edges. With increased coverage, island formation proceeds due to local aggregation of adsorbed gold on each substrate terrace. Based on local imaging of the growth morphology, the data is discussed in terms of thin film growth mechanisms as well as the influence of native surface features such as defects and charge distribution. Understanding growth mechanisms for gold layers on ferroelectric surfaces allows for a fuller appreciation of how atomic deposition of metal atoms on patterned poled LiNbO₃ surfaces would occur as well as yielding greater insight on the atomic characteristics of metals on ferroelectric interfaces.     

Role of stoichiometric point defect in electric-field-poling lithium niobate

Non-congruent LiNbO₃ crystals has been used to determine the influence of the intrinsic defect density in the ferroelectric domain inversion mechanism. The poling processes have been carried out either at high temperature or at RT throughout the electric-field-poling technique. It is shown that the coercive field of LiNbO₃ crystals is strongly influenced by the intrinsic defect density in the crystals. Moreover it is shown that it is possible to realign the ferroelectric domain structure of near-stoichiometric LiNbO₃ crystals to prepare periodic poled structures which allows second harmonic generation at shorter wavelengths than with congruent crystals at a fixed wavelength. Received: 6 July 1999 / Accepted: 24 November 1999 / Published online: 24 March 2000     

Ferroelectric domain inversion in near-stoichiometric lithium niobate for high efficiency blue light generation

LiNbO₃ single crystals with a composition close to stoichiometry ([Li]/[Li+Nb]=0.496), 16 mm in diameter and 40 mm in length were grown by the Czochralski method using K₂O flux. The domain reversal characteristics of near-stoichiometric LiNbO₃ single crystals were investigated. The switching field required for 180° ferroelectric domain reversal in the near-stoichiometric crystal at room temperature was 7.5 KV/mm. This is about one third of the switching field required for conventional LiNbO₃ crystals. Domain reversal (180°) in near-stoichiometric LiNbO₃ samples of 1.0 mm thickness has been achieved. Samples have been evaluated by second harmonic generation and conversion efficiencies of up to 32% have been obtained. Received: 8 November 2000 / Accepted: 29 January 2001 / Published online: 20 June 2001     

Radiation-damage-assisted ferroelectric domain structuring in magnesium-doped lithium niobate

Irradiation of 5% magnesium-doped lithium niobate crystals (LiNbO₃:Mg) with high-energy, low-mass ³He ions, which are transmitted through the crystal, changes the domain reversal properties of the material. This enables easier domain engineering compared to non-irradiated material and assists the formation of small-sized periodically poled domains in LiNbO₃:Mg. Periodic domain structures exhibiting a width of ≈520 nm are obtained in radiation-damaged sections of the crystals. The ferroelectric poling behavior between irradiated and non-treated material is compared.     

Periodic domain structures obtained by growth of LiNbO<Subscript>3</Subscript> crystals doped with gadolinium

The growth of a periodic domain structure in LiNbO₃ crystals doped with gadolinium (Gd, 0.44 wt %) was investigated by special electron beam surface charging in a scanning electron microscope and on a atomic force microscope. Ferroelectric domain boundaries with “tail-to-tail” and “head-to-head” P _s orientations were comparatively analyzed. The domain boundaries had different forms and differed by their physical properties. Micron-size surface crystal regions close to the “tail-to-tail” boundaries were charged slower by electron irradiation and had modified elastic properties. This region was detected by using the lateral force mode of atomic force microscopy. The observed morphology and property features were supposed to be due to different concentration gradients of the Gd impurity and different width of its distribution close to the domain boundaries of different types.     

Electro‐optical interferometric microscopy of periodic and aperiodic ferroelectric structures

The ferroelectric domain structures of periodically poled KTiOPO₄ and two‐dimensional short range ordered poled LiNbO₃ crystals are determined non‐invasively by interferometric measurements of the electro‐optically induced phase retardation. Owing to the sign reversal of the electro‐optical coefficients upon domain inversion, a π phase shift is observed for the inverted domains. The microscopic setup provides diffraction‐limited spatial resolution allowing us to reveal the nonlinear and electro‐optical modulation patterns in ferroelectric crystals in a non‐destructive manner and to determine the poling period, duty cycle and short‐range order as well as detect local defects in the domain structure. Conversely, knowing the ferroelectric domain structure, one can use electro‐optical microscopy so as to infer the distribution of the electric field therein.

     

聚片多畴LiNbO3晶体的倍频效应

通过对聚片多畴LiNbO₃晶体倍频效应的研究,从实验上对Bloembergen等人所提出的准位相匹配理论进行了细致的检验。首先运用Maker条纹法在实验上直接测定了单畴LiNbO₃晶体在e^ω·e^ω→e^2ω非位相匹配下的倍频光强,并采用角度扫描和温度扫描方法,以获得满足准位相匹配条件时的不同片数的聚片多畴晶体的最大倍频强度,从而验证了聚片多畴晶体倍频相对光强与片畴数N成平方关系。在N<200时,取得 关键词：     

Rearrangement of Rare Earth Defects Under Domain Inversion in LiNbO3

Using high resolution excitation-emission spectroscopy we investigated the changes occurring in the optical transition of Er^3? ions in LiNbO₃ during inversion of the ferroelectric axis. In stoichiometric LiNbO₃ we find that a drastic reconfiguration among the different defect sites takes places favoring those centers which have already been dominant in the as-grown sample. The reconfiguration is attributed to changes in the arrangement of the local charge compensators. Furthermore, we find a small shift of the emission transition energy, which is consistent with an increase of the intrinsic electric field. These findings make the Er^3? ions very suitable probes for 3D imaging of domain structures and for in-situ studies of the dynamics of the domain inversion processes and the defect/domain wall interaction.     

Dielectric relaxation in magnetoelectric composite 0.85BiFeO<Subscript>3</Subscript>–0.15MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript>

Dielectric properties of 0.85BiFeO₃–0.15MgFe₂O₄ composite fabricated with conventional ceramic technology are studied in the temperature interval of 20–250°C at frequencies of 25 Hz to 1 MHz. Dielectric relaxation is observed, the nature of which is discussed in the context of a mechanism of interaction between ferroelectric domain boundaries and point defects.     

Domain inversion in LiNbO3 and Zn-doped LiNbO3 crystals by the electron-beam irradiation of the nonpolar Y-surface

Individual domains and domain gratings were fabricated on nonpolar Y-cuts of LiNbO₃ and LiNbO₃-Zn crystals by electron beam irradiation. The domains which nucleated in the irradiation points are frontally growing along the direction +Z within a thin (of about several microns) surface layer. The regularities of this motion are discussed in the framework of the approach to formation of space-charge fields under e-beam charging of insulators. The obtained dependency of the domain length on the exposure time permits us to propose the viscous-friction mechanism for the observed frontal domain growth. The velocity of the frontal growth in LiNbO₃-Zn is higher than in LiNbO₃ obviously due to a decreased number of pinning centers at the Nb-antisites. In LiNbO₃-4 %Zn crystals planar domain gratings were fabricated by means of point-to-point irradiations along the X- and Z-directions with specified distances between the irradiation points. It is shown that the domain gratings are generated by a total field of point charges $ \vec{E} = \sum\nolimits_{i = 1}^{n} {\vec{E}_{i} } $ , where E _i is the space-charge field induced in any irradiation point, and n is the number of points. Some preliminary estimates indicate that the frontal growth of domains under e-beam irradiation occurs at fields E < E _c.