LiNbO3表面势抗屏蔽措施研究

铁电单晶体自发极化引起的强周期性表面电场可能用来代替Wiggler磁铁产生自由电子激光。但落在铁电晶体表面上的自由电子将逐渐把表面势屏蔽掉。本文提出一个抗屏蔽措施:在LiNbO₃极化晶面上每一个正极化畴区铺上一层绝缘膜,在上面再覆盖一层接地导体膜,可以防止电子对极化晶面的屏蔽。本文证明了在导体膜每部分两表面应电荷之和为零,落在上面的自由电子将立刻逃逸入地。 关键词：     

基态非简并聚合物中的极化子和双极化子动力学

研究了极化子和双极化子在基态非简并聚合物中的动力学.弱电场下，发现载流子为带电极 化子或双极化子，它们的晶格态与电荷态始终耦合在一起同步运动.极化子比双极化子的运 动速度快，存在饱和速度，且它们的饱和速度随聚合物非简并度的增加而降低；强电场下， 元激发的电荷态将脱离晶格态的束缚而快速运动，参与导电的不再是极化子或双极化子，而 是电子直接导电. 关键词： 极化子 双极化子 动力学模拟     

低电场下LiNbO3薄膜在非晶衬底上的取向生长

报道了外加低电场和衬底温度对脉冲激光淀积法制备的LiNbO₃薄膜取向的影响.在衬底温度为600℃和外加电场为7V/cm的条件下，在石英玻璃等非晶态衬底上获得了完全(001)取向的LiNbO₃薄膜.在对具有自发极化的LiNbO₃在电场作用下成核生长机制和温度对LiNbO₃自发极化强度的影响进行分析的基础上，给出了低电场诱导铁电薄膜取向生长的物理依据. 关键词：     

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     

铁电屏蔽理论

利用电极化的边界屏蔽近似,可计算出一些典型铁电单晶的自发极化强度．铁电屏蔽电荷处于表面束缚极化子偶状态;其产生和湮没要跨越位垒．由统计方法给出的一些常见晶体和陶瓷的四种典型电滞回线形状,和实验观察到的结果一致 关键词：     

电子关联对C60激子极化子的影响

本文采用扩展的Ｈｕｂｂａｒｄ模型，在电子－晶格耦合的紧束缚近似下，研究了光激发下的Ｃ₆₀，分子的激子态和极化子态。我们发现，电子关联效应相当敏感地影响到单态和三态激子的束缚能。同时，Ｃ₆₀分子动力学晶格弛豫使激子极化子得以形成。光激发下的Ｃ₆₀分子，在电子相互作用Ｕ～５．０ｅＶ，Ｖ～２．０ｅＶ时，能够定性地解释荧光发射谱。 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}     

混合卤化物中的极化子与双极化子

应用紧束缚双带模型对混合卤化物的电荷掺杂性质进行了计算,发现单电荷掺杂在混合卤化物中形成极化子,双电荷掺杂则形成稳定的双极化子;单体的长度和界面耦合都对极化子和双极化子的产生和稳定性有影响.混合卤化物的单体对掺杂电荷具有选择性;电子-电子相互作用并不能使形成双极化子的两个同号电荷相互排斥而形成两个单极化子.在具有电荷自发转移的材料中,混合卤化物单体对掺杂电荷没有像其他混合卤化物一样的选择性,掺杂电荷的定域位置取决于两种单体对该电荷的束缚能力. 关键词： 混合卤化物 极化子 电荷约束     

溅射氧化铁薄膜的Hall效应

在磁场垂直于薄膜表面,磁感应强度B由0—1.4T变化的条件下,测量了溅射氧化铁薄膜的Hall效应。从实验数据得到了Fe₃O₄薄膜和γ-Fe₂O₃薄膜的寻常Hall系数R₀和非常Hall系数R_s,计算出这两种薄膜的Hall迁移率μ_H分别是2.35和1.56(cm)²/V·s。这个结果适合于大极化子导电机制的条件(ν≥1(cm)^关键词：     

外加电场法制备LiNbO3周期畴的反转电流特性研究

采用外加电场法制备了LiNbO₃单晶周期畴结构. 在对不同尺寸周期畴的反转电流进行比较研究的基础上,提出了一种确定反转畴成核时间和纵向贯穿速率的方法. 根据这一方法,得到在电场强度为25.1kV/mm,脉冲宽度为50 ms的脉冲方波作用下,LiNbO₃单晶反转畴的成核时间约为80ns,纵向贯穿速率约为0.1667m/s. 关键词： 3单晶周期极化')" href="#">LiNbO₃单晶周期极化 反转电流 反转畴成核时间 反转畴纵向贯穿速率     

Photoconductivity and photovoltaic behaviour of LiNbO3 and LiNbO3 waveguides at high optical intensities

The photoconductivity and photovoltaic currents of pure LiNbO₃ and proton exchanged waveguides in LiNbO₃ have been measured as a function of the optical intensity up to about several kW/cm² by the use of surface electrodes. For pure LiNbO₃ the observed dependences are a simple extrapolation of the well known low intensity behaviour. The photoconductivity of proton exchanged waveguides is considerably increased compared with pure LiNbO₃ and the curves are strongly nonlinear in the high intensity region. These results can explain, at least qualitatively, the previously observed characteristic time and intensity dependence of light-induced refractive index changes in this type of waveguides. Both the time and temperature behaviour of the dark conductivity of all proton exchanged waveguides give strong evidence of ionic charge transport in the proton exchanged region.     

极化子荧光及其断层扫描对Ti:LiNbO3光波导表征研究

小极化子(Nb⁴⁺_Li)荧光处于700—950 nm谱段，对不同化学还原程度同组分的纯铌酸锂晶体的研究表明荧光的强度反映了化学还原程度的强弱.从小极化子浅层能级出发，提出以“1中心3级态跃迁”来描述小极化子的受激荧光过程.钛扩散铌酸锂光波导(Ti：LiNbO₃)表面还原程度的均匀性可能在波导制造的热扩散过程中被破坏.在对铌酸锂波导表面、深部的扫描结果中，荧光的强度呈现出表面与深部、波导内与外的差异，表面的强度是深部的6—8倍.研究表明，利用非损伤性的极化子荧光谱，对加工过程热处理进行检控是非常有效的.同时，波导内与外的荧光强度差异间接反映了波导的横切面的外形.     

LiNbO3晶体在室温—130℃的锥光干涉及折射率的异常

研究了纯及掺杂LiNbO₃(下简写为LN)晶体z向通光的锥光干涉图。纯LN,在45℃和75℃附近,双折射率均有异常变化,晶体由单轴变为双轴晶。掺杂(Fe,Cr,Cu)LN也有类似的变化,但变化的温度范围被加宽。分别测量了折射率n₀和n_e,在这两个温度附近有异常跳变。通过与x方向加电场的室温锥光图相比较,可以认为在75℃附近的双折射异常变化是相当于离子位移在x方向产生10⁵V/m量级的内电场所引起的。 关键词：     

Reduction-induced polarons and optical response of Mg-doped LiNbO<Subscript>3</Subscript> crystals

We studied the visible and IR dispersion of absorption coefficient and refractive index for congruent LiNbO₃ and Mg:LiNbO₃ crystals before and after chemical reduction at different annealing temperatures. The concentration of Mg in Mg:LiNbO₃ samples was just below or above the photorefractive threshold. The reduction-induced changes in the absorption coefficient reveal the formation of polarons typical for doped LiNbO₃ crystals. It was shown that the polaron concentration is maximal when the Mg concentration is just below the photorefractive threshold and the annealing temperature is near 500 °C. This temperature is optimal for the most efficient polaron formation at all considered concentrations of Mg. The fitting of the experimental absorption dispersion curves indicates that intermediate polarons are formed in LiNbO₃:Mg crystals preferably. The spectral dependence of transmission for samples of lithium niobate of various thicknesses was studied. The results indicate that there are spatial regions with much greater absorption than that of bulk crystals. We assume that, in general, polarons are localized in thin near-surface regions. The spectral dependence of the refractive index in the vicinity of the phonon absorption edge indicates some essential changes of the phonon subsystem taking place after reduction. The infrared contribution into the dispersion of the dielectric function real part increases considerably after reduction. PACS 71.38.Ht; 71.38.-k; 78.20.Ci     

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.     

Observation of the Photoinduced Conductivity in a Regular Domain Structure with Tilted Walls in MgO:LiNbO3 at a Wavelength of 632.8 nm at Bragg Diffraction

A 632.8-nm radiation-induced change in the conductivity of a regular domain structure (RDS) formed in a 5% MgO:LiNbO₃ crystal has been detected for the first time. As a result, the relaxation rate for the Bragg diffraction efficiency on the RDS, which is observed after the application of an external electric field, increases with the intensity of a probe beam. This dependence is linear in the initial stage of relaxation caused by the screening of the external field because of the redistribution of charges over tilted conductive domain walls of the RDS. For the probe beam with an intensity of 49 mW/mm², the induced effective conductivity of the RDS, which is estimated as σ_eff = 3.5×10⁻⁹Ω⁻¹m⁻¹, is more than four orders of magnitude higher than the dark conductivity of the single-domain MgO:LiNbO₃ sample.

     

Dynamic process of self-trapped polaron in photoexcited SrTiO3

Relaxation process of self-trapped polaron is investigated by a nonadiabatic molecular dynamic method. We show localized disorder due to lattice fluctuations can give rise to a tightly-bound electronic state in ultraviolet illuminated SrTiO₃ crystal. This bound state is actually a self-trapped polaron in accordance with the experimentally observed large Stokes-shift. The formation of the self-trapped polaron is shown to be an ultrafast process.     

飞秒激光烧蚀LiNbO3晶体的形貌特征与机理研究

采用了不同能量的单脉冲和多脉冲飞秒激光对LiNbO₃晶体进行烧蚀，并刻蚀了表面衍射型光栅.通过扫描电镜和原子力显微镜观察了烧蚀点的形貌特征，首次发现利用单束飞秒激光脉冲对LiNbO₃晶体烧蚀，可以得到超衍射极限的烧蚀点，当聚焦光斑直径约为2μm、能量为170nJ的单脉冲飞秒激光作用时，烧蚀点的直径约为400nm，100nJ，17个脉冲作用时烧蚀点的直径约为800nm.同时可以观察到在能量较低的多脉冲飞秒激光作用下， LiNbO₃晶体呈现出大约200nm周期性分布的波纹状结构.实验结果表明，选择合适参数的飞秒激光脉冲可以对LiNbO₃晶体进行超衍射极限加工，这对于利用飞秒激光制作LiNbO₃基质的微纳光电子器件有十分重要的意义.     

Double-layer PVDF transducer and V(z) measurement system for measuring leaky Lamb waves in a piezoelectric plate

This paper presents a new experimental measurement method for leaky Lamb waves propagating in a piezoelectric plate immersed in a conductive fluid. The measurement system is a low-frequency version of lens-less acoustic microscopy which has been developed based on a line-focus double-layer PVDF transducer. The transducer and its defocusing measurement system can perform V(z) measurements on a sample plate immersed in a fluid, and therefore can obtain the leaky Lamb wave velocities with high accuracy. An X-cut LiNbO₃ plate is investigated with this experimental measurement system to find out its fluid-loading effects, especially the conductive loading effects by water of various conductivities. Angular dependence of this conductive loading effect along different propagating directions on the X-cut LiNbO₃ plate is measured. It is found out the conductive loading effects are strongly dependent on the piezoelectric coupling factor. Theoretical calculations based on partial wave theory have also been carried out and compared with experimental data. Good agreements have been observed.     

Electrical Conductivity and Barrier Properties of Lithium Niobate Thin Films

The thin-film structures made of LiNbO₃ and obtained via laser ablation and magnetron sputtering are studied with volt-farad and volt-ampere characteristics. A potential barrier on the Si–LiNbO₃ interface was found for both types of the films with the capacitance-voltage characteristics. The current-voltage characteristics showed that there are several conduction mechanisms in the structures studied. The Poole–Frenkel effect and the currents limited by a space charge mainly contribute to the electrical conductivity in the LiNbO₃ film produced with the laser ablation method. The currents limited by a space charge contribute to the main mechanism in the film heterostructure obtained with the magnetron sputtering method.