【作者】 高成勇；

【导师】 夏海瑞；

【作者基本信息】 山东大学 ， 光学， 2008， 博士

【摘要】 具有钨青铜结构的铌酸盐晶体许多是优良的光折变材料,由于其内部存在大量的结构空位,因此可以通过分了设计和离子掺杂进一步提高此类晶体材料的质量或改变其各种性能（如晶体的光折变特性等）。近年来,这方面的研究异常迅速。CSBN((Ca_0.28Ba_0.72)_x(Sr_0.60Ba_0.40)_1-xNb₂O₆)晶体是由山东大学晶体材料研究所新近生长的一种具有钨青铜结构铌酸盐。经X-ray粉末衍射数据表明:CSBN系列晶体属于四方晶系,空间群为P4bm;具有非中心对称性,类似SBN结构。CSBN晶体内部含有A1,A2,B1,B2和C等结构空位,其中A1位和A2位被Ca²⁺,Sr²⁺,Ba²⁺部分填充,B1和B2位被Nb⁵⁺完全充满,而C位未被填充,属于非充满型结构。本文主要对(Ca_0.28Ba_0.72)_x(Sr_0.60Ba_0.401-xNb₂O₆晶体的光学特性进行了系统研究。其中包括该晶体的晶格振动光谱特性（拉曼谱,红外吸收谱）及与之相关的热学特性,晶体双折射及偏振光干涉特性,光折变中心及光折变非线性,晶体的体光栅衍射特性和光学自相位共轭等。论文的主要工作包括如下几个方面:1、根据空间群理论计算分析了CSBN系列晶体的晶格振动模式。全面报道了该系列晶体在不同几何配置下的晶格振动谱,并对不同Ca²⁺掺杂的SBN晶体的晶格振动谱进行了分析比较。CSBN晶体属于钨青铜型晶体,其原胞中含有5个分子式的分子,理论上有135个基本晶格振动模式。本研究中利用激光拉曼散射技术测量了该晶体的室温拉曼光谱,采用傅里叶变换红外分光光度计测量了晶体的红外吸收谱。通过谱图分析,对CSBN各晶体与其他同构晶体类似的特征峰值分别予以确认。在拉曼光谱试验中采用六种配置状态,共观测到了55支简正模;红外吸收光谱试验中共观察到了5条红外谱线。实验中虽未能观察到三重简并的反对称伸缩振动模v₃,但对八面体离子的其他四个简正振动模v₁,v₂,v₄及v₅进行了确认。对于谱线少的原因,文中分析认为除了样品制备因素外还应与晶体的形成过程有关:由Nb-O八面体络合成晶体时,晶体保持相对稳定,其它金属离子的作用很难改变其整体运动的特点,只产生局部影响;同时,空位的随机性造成晶体的无序,使谱线减弱,造成大波数范围频谱的本底隆起,这直接了影响到许多谱线的观测。2、系统地研究了CSBN各晶体的热学特性。CSBN各晶体表现出了特有的热学性质:沿c轴方向,在居里点下,CSBN各晶体随着温度的增加,表现出负的膨胀特性;高于居里点呈现正膨胀。沿a轴方向,随着温度的升高,该晶体产生热膨胀,其热膨胀系数为正值。CSBN各晶体a向和c向的热导率在居里点附近趋于相同,但偏离居里点表现出较强的各向异性:居里点以上c向的热导率随温度的升高大于a向的热导率;居里点以下c向的热导率小于a向热导率。此种特性的成因被认为是主要由于铌原子的热运动垂直于O—Nb—O链,从而降低了O—O链分离的可能所导致的。3、系统的研究了CSBN晶体的双折射及其偏振光干涉特性。在讨论晶体的双折射与结构的关系的基础上,测量了晶体在偏振光入射下的消光角。通过光心提取法测量了晶体的折射率及双折射率,测量精度达到10^-4,并且确定出了激光入射波长为532nm的消光角中k值的大小,即:k_CSBN75≈270;k_CSBN50≈259;k_CSBN25≈255。在未采用传统的偏光显微镜技术前提下,通过自行设计光路,研究了双折射晶体CSBN的锥光干涉特性。结合锥光干涉图,理论分析计算了平行光轴入射的晶体锥光干涉图中干涉级次变化的成因,干涉图由内向外疏密分布的规律以及垂直光轴入射的干涉图的形成原因等;探讨了不同掺杂对晶体的双折射特性的影响;讨论了晶体锥光干涉图随光锥入射角度的变化的规律,定性了小角度旋转晶体后黑色十字劈裂的成因。基于理论分析,利用计算机模拟实现了负单轴晶体的锥光干涉图。根据实验结果判定了文中所使用的CSBN系列晶体均具有良好的光学均匀性,其双折射率梯度均达到10^-5。4、根据CSBN晶体的透射谱,研究讨论了CSBN晶体的光折变中心问题。文中通过CSBN晶体的透射谱,计算了该类晶体的在能带吸收边缘附近的衰减指数,分析了晶体的透射特性:CSBN75晶体的衰减指数比较大,代表晶体具有较大无序程度;晶体从396nm至768 nm,由于Ca²⁺掺杂加强了对该波段的吸收,并且在红光波段（675nm附近）出现了一个新的小吸收峰,相应深陷能级为1.84eV,与未掺杂的SBN透过曲线相比较,CSBN晶体的波长响应范围向红光和红外区扩展。由于在近红外波段,激发光子能量较小,此时的吸收光谱反映的是晶体内缺陷离子的振动特性,或晶体中浅陷阱能级的性质;同时,非计量化学比掺杂可引起Nb-O₆八面体团簇周围电势分布的变化,这些因素已足以影响晶体的某些光学特性如晶体吸收特性。基于以上因素,通过分析认为:Ca²⁺掺杂虽然能够在一定的波长范围内加强了晶体的吸收,可以增强晶体的光折变效应,但Ca²⁺不可能充当光折变效应的中心。5、从不同的角度系统地研究了CSBN晶体的光致折射率变化的非线性特性。一方面,基于传统的Z-扫描技术进行了装置改进,实施开孔探测和去基模的方法同时进行测量,减少了接受光能的叠加效应,提高了测量灵敏度;利用此改进的技术,试验测定了CSBN25晶体的Z扫描曲线,并且计算了CSBN25晶体的非线性折射系数。另一方面,通过非偏振光写入的方法结合Michelson干涉仪研究了晶体的光致折射率变化特性,并且得到了有关晶体光折变特性的一个重要结论:在激光波长为632.8nm的情况下,CSBN25的折射率呈现正的增长趋势,即Δn＞0,晶体表现为自聚焦的特性。CSBN75的折射率呈现负的增长趋势,即Δn＜0,晶体表现为自散焦的特性;而CSBN50的光致折射率变化呈现特殊的变化趋势,在1min前CSBN50的折射率呈现正的增长趋势,即Δn＞0,1min后CSBN50的折射率呈现负的增长趋势,即Δn＜0;在CSBN50晶体中存在着自聚焦向自散焦的动态转化过程。偏振光写入的方法和非偏振光写入的方法的研究得到了同样的结论,文中对以上结论利用带输运模型理论及热自聚焦诱导光折变非对称自散焦理论进行了比较分析,并且通过分析指出:Feinberg的热自聚焦诱导光折变非对称自散焦理论对CSBN50晶体中观测到的光致自聚焦到自散焦现象的解释是适用的。另外,在此基础上首次研究了CSBN晶体的双折射率动态变化特性。在不同功率的激光入射条件下,定量的测试了CSBN晶体的双折射率变化,根据试验结果描绘了CSBN50中双折射率随入射功率及时间的变化特性曲线。6、以全新的角度研究了CSBN晶体的体光栅衍射特性。文中首先测试了不同偏振态写入情况下的晶体体光栅衍射效率,测试结果表明:不同偏振态入射光的体光栅衍射效率变化规律基本一致,e光的最大衍射效率远大于o光写入时的最大衍射效率,测得的衍射效率变化规律与Kukhtarev及Hong等人的理论符合较好。体光栅衍射效率的测试为该晶体在全息存储和光学信息处理领域的应用奠定了基础。其次,探讨了光折变晶体CSBN75和CSBN50在平面单色偏振光束入射条件下的自衍射特性。试验中发现当入射光偏振为异常偏振时,在远场屏上可观察到明显的自衍射现象并且衍射图形和晶体光轴成一定的夹角,不同掺杂的晶体其形成的夹角大小不同。自衍射起因于晶体噪音栅,文中根据平面光栅衍射理论系统地研究了CSBN75和CSBN50晶体的噪音栅特性,由试验结果确定出了噪音栅的波矢方向,并建立了上述两种晶体的噪音栅模型。显然,噪音栅模型的建构将对合理的限制和利用噪音栅具有较大的现实意义。在自衍射的前提下,实验观察并讨论了CBN28晶体的光感应光散射（光扇效应（前向散射光放大）及散射光锥）,并且分析了该光感应光散射的成因,即:晶体内的入射光,反射光及各向同性的散射光满足了一定的相位匹配条件,各向异性散射光与入射光及反射光分别写入噪音栅,光感应光散射现象是噪音栅波面叠加的结果。更换CSBN系列晶体进行试验,未发现如此强烈的光感应光散射,这说明不同化学计量比的掺杂会直接影响到光折变晶体的光折变特性。另外,还试验分析了汇聚光束入射条件下的晶体的远场衍射特性及其成因,确定了F.S.Chen等人在光折变晶体LiNO3中观察到的现象（即:光致类透镜效应）在CSBN晶体中同样存在。7、研究了CSBN晶体的光学自相位共轭及其应用。在试验中首次观察到了CSBN晶体中“奇异”的自泵浦相位共轭现象,并用“猫式互作用区”理论成功的解释了此种现象的产生机制:入射光束投射到晶体上时,会受到晶体内部的杂质,缺陷,甚至畴结构的散射作用形成多个方向的散射光,从而为晶体内部的四波混频及光扇效应创造条件。散射光和信号光的耦合以及扇形散射光之间的能量竞争,在晶体内部通过折射率的调制形成实时体相位光栅。当入射到晶体表面的光斑尺寸小于0.85mm时,反射光束被相位光栅所调制,从晶体的XOZ平面出射的光束为高频增强了的相位共轭光;当入射到晶体表面的光斑尺寸大于0.9mm时,两互作用区的相位共轭条件被破坏,此时出射的光斑仅为经YOZ平面出射部分和没有经过晶体的部分组成,实验结果表明CSBN75晶体是一种可被用作光学信息处理（如本文中利用此现象实现的光学图像边缘增强处理）的良好材料。另外,本工作中还测试了晶体的自泵浦共轭反射率及响应时间,实验发现CSBN75晶体器件具有较宽的角度响应范围,在很宽的角度范围内都可实现自泵浦相位共轭,掺杂浓度对晶体的光折变效应有较强的影响。更多还原

【Abstract】 Doped and un-doped Strontium barium niobate crystals have been studied extensively for their important electro-optic,piezoelectric,pyroelectric,and photorefractive applications.For several vacancies lying in the inner structure of tungsten bronze niobites,crystal quality and other proprieties can be improved by means of ions dopant or further molecular design.Recently,works with these ideas have been developed rapidly.Calcium doped Strontium barium niobate crystal were grown by the Institute of Crystal Materials of Shandong University of China are ferroelectric and photoelectric crystals having the structure of ferroelectric tetragonal tungsten bronze-type （TTB）.The elementary 45 atoms cell of CSBN((Ca_0.28Ba_0.72)_x(Sr_0.60Ba_0.40)_1-xNb₂O₆)has tetragonal system with the space group P4bm(C_4v²)and the Ca²⁺partly replaces the Ba²⁺ at A2 site of the un-filled TB structure.Optical proprieties of CSBN crystals,which including lattice vibration spectra （Raman and infrared）and thermal proprieties;birefringence and polarized light interference;photorefractive nonlinear characteristic and photorefractive effect center; crystal diffraction and optical phase conjugation etc,were mainly performed in this dissertation.Main contents are listed as following:1)The space group theoretical analyses and assignment of the lattice vibration modes of the CSBN crystals with various geometries have been performed.The lattice vibration spectra of this crystal are studied by Raman scattering and infrared absorption measurements.At the same time,the lattice vibration spectra with various geometries were compared and analyzed with different Ca²⁺-doped crystals.Although,in theory,120 or 119 Raman peaks and 53 Infrared activity bands can be observed in experiments,only 55 normal modes and 5 Infrared spectrums were acquired.Characteristic peaks arising mainly from internal vibrations of the Nb-O anionic cluster were identified from vibration spectra except three-fold degeneracy anti-symmetric stretching vibration modesv₃.The reasons why fewer modes were observed attribute to the preparation of samples,the relative stability of complex crystal of Nb-O octahedron and randomicity of crystal vacancies. 2)Thermal properties of CSBN single crystals were systemically studied.CSBN crystals have its own brand of thermal properties:Experimental results suggest that negative thermal expand property along c axis of CSBN crystals below the Curie temperature while positive thermal expand above the Curie temperature;When the measurement was along a axis of crystals,crystals tend to positive thermal expand with the temperature increasing.Thermal conductivity of CSBN crystals tend to be the similar near the Curie point whether the measurement is along c axis or a axis and its appear biggish aeolotropism deflecting the Curie point.The thermal properties of CSBN crystals were considered of thermal motion of Nb atom,which is vertical to the O-Nb-O lines to decrease its separation,in such a way that negative thermal expand result.3)Birefringence as well as its polarized lights interference were systemically studied in this dissertation.The extinction angles of CSBN crystals under the incident with polarized lights were affirmed with the formula asφ=kπ+θand the refraction indexes, birefringence of crystals were measured by the method of light center distill,which the measurement accuracy is up to 10^-4.In addition,the value k of extinction angles at 532nm were calculated as k_CSBN75≈270;k_CSBN50≈259;k_CSBN25≈255.The study on polarized lights interference of crystals were executed by self-designed optical systems instead of the traditional polarizing microscope technique.The following parts were discussed based on conoscopic interference patterns:the origin of order of interference change,influence on Ca²⁺dopen to crystals,the change regularity of conoscopic intereference patterns with the incident angle and the optical homogeneous of CSBN crystals,which are affirmed to be good optical homogeneous and the birefringence grads is up to 10^-5.The conoscopic intereference patterns were simulated accoding to the theoretic analysis and calculating,which valadated the theoretic analysis and calculating in this paper.4)The photo-refraction center of CSBN crystals were discussed based on the transmission spectrums:The influence on Ca²⁺dopen to the photo-refraction properties of crystals were analyzed through transmissions spectrums（including:absorption edge of CSBN crystals,attenuation index and transmission curves along a and c axes of crystals）.It comes to the conclusion that photo-refraction effects of CSBN crystals were enhanced in specific wavelength scopes with the Ca²⁺dopen instead of Ca²⁺acting as the centre of the photo-refraction crystals.5)Photo-refractive nonlinear characteristic of CSBN crystals were studied in different ways.Z-Scan curves and nonlinear refractive index were acquired based on improved Z-Scan technique.Photo-refractive characteristic were measured under Michelson interferometer with the incident of un-polarized lights and an important characteristic about photo-refractive characteristic was drawn:Refractive index change of CSBN25 represent self-focusing and CSBN75 is self-defocusing,while CSBN50 represents dynamic change properties from self-focusing to self-defocusing.When the experiment was executed under polarized lights,the same conclusion could be acquired. The change from self-focusing to self-defocusing could be explained based on the Feinberg’s theory-Thermal self-focusing induced photo-refractive asymmetry self-defocusing.Furthermore,dynamic change properties of birefringence in CSBN crystals were firstly studied and its temporal change curves with input power were plotted in this investigation.6)Body-grating diffraction charicreristic was discussed with some novel ways in this disquisition.Measurements of diffraction efficiencises of body-grating showed that the diffraction efficiency of extraordinary rays are much more than that of ordinary rays,but they shares the samilar change regularity coinciding with theories of Kukhtarev and Hong. Investigation on self-diffraction witnesses the fact that the direction of the Light-induced self-diffraction spot changes with different Ca²⁺-doped CSBN crystals,indicating that the formed noise-grating wave vector direction changes with different ion-doped.Based on the wave optics theory and light-induced self-diffraction under the incident of plane monochromatic light wave in CSBN crystals,the theoretical model of noise-grating is analyzed,constructed and validated in this work firstly.Obviously,it can provide a theoretical foundation for retraining noise to study the noise grating.Going on this premise,the photo induction and scattering of CBN28 crystal was discussed in this paper: The photo induction and scattering of CBN28 was the result of the nestification of noisegrating wave plane.Although,the photo induction and scattering could not be found in CSBN crystals,which suggested that stoichiometric proportion dope can directly influence the photo-refractive characteristic of crystals.The far-field diffraction of CSBN was distinctly observed in experiments,which is considered as the result of lens-like effect being same like as of LiNO3 crystals.7)The self-pumped phase conjugation in CSBN75 was firstly studied based on an anti-traditional method,which using part laser spot to irradiate crystal.In the case,when the site of the laser spot on the crystal is less than0.85mm,the deformed laser spot and its complementary pattern are distinctly displayed on the screen,also the deformed laser spot was observed along the inverse direction of the incidence beam,which affirmed the existence of the conjugation mechanism in this investigation.Theories analysis basing self-pumped conjugation succeeded in explaining these phenomena.The experimental results demonstrate that the CSBN75 crystals will be widely applied in optical information processing,such as the edge enhanced of optical correlation pattern effect realized in this paper.Furthermore,the self pump phase conjugation reflectance ratio and response time of CSBN crystals had been measured in this paper,experiments showed that wide response field with these crystals to realize self pump phase conjugation and doping content can affect photo refractive effect of crystals deeply.更多还原