【作者】 刘晓静；

【导师】 许东； 张光辉；

【作者基本信息】 山东大学 ， 材料物理与化学， 2009， 博士

【摘要】 非线性光学晶体材料由于在高速光通信、光信息处理、光调制、光开关、频率转换和高密度光存储等领域的重要应用,成为科学研究的前沿领域。有机非线性光学晶体因其非线性系数大、响应速度快、光学损伤阈值高和易加工性而日益成为非线性光学晶体材料的热点。L-精氨酸磷酸盐（LAP）晶体是山东大学在国际上首创发明与研制,并受到了国内外高度重视和认同的一种很有希望的紫外有机非线性光学晶体。由于LAP晶体在强激光下具有其它晶体所无法比拟的高激光损伤阈值及高达90%以上的高转换效率,使得它在不断升级发展的强激光频率转换和相位共轭镜等方面表现出特别重要的潜在价值,引起了世界各国非线性光学研究者的广泛关注,相关研究得到了迅速发展和广泛开展。L-精氨酸三氟乙酸盐（LATF）晶体是本课题组继LAP晶体之后,在国际上首创的又一种具有高损伤阈值的非线性光学新晶体,它由天然碱性氨基酸之一的L-精氨酸（L-Arginine）分子和三氟乙酸（CF₃COOH）分子按等摩尔比合成。初步测试结果发现:LATF晶体属于单斜晶系,P2₁空间群,粉末倍频效应是KDP晶体的2.5倍,与LAP晶体接近,紫外截止波长为232nm,其比热是LAP晶体的2倍,在低重复频率的强激光作用下,LATF晶体具有比LAP（DLAP）晶体更高的激光损伤阈值。因此,LATF晶体可能是一种性能更优异、更富有潜在价值的新型非线性光学晶体材料。鉴于此,本论文针对LATF晶体的大尺寸单晶生长、结构、缺陷、表面形貌、生长机制及性能测试表征和品质鉴定工作进行了较深入的研究,主要内容包括:1.大尺寸LATF单晶生长的研究由于LATF晶体沿不同方向生长速度存在较大各向异性,c轴生长速度极其缓慢而b轴、a轴生长速度极其迅速,呈现出片状生长的特点,给晶体测试工作带来了较大困难。通过控制溶液组分,找到了阻止晶体片状生长的规律,并采用了一套连续籽晶优化技术,解决了大尺寸LATF优质单晶生长中的关键技术问题。在此基础上,对过饱和溶液成核过程进行了研究,包括测定亚稳区宽度和成核诱导期,计算固液界面张力、临界成核半径和临界成核自由能等成核特征参数,从而加强了生长条件的完善和控制,生长出了最大尺寸可达50×27×7mm³、外形规则、高光学质量的LATF体块单晶,并对晶体形貌进行了分析。晶体沿[010]方向拉长,（（?）01）、（（?）00）和（001）晶面是晶体易显露的三个大面。2.LATF晶体结构与性能的研究利用X-射线衍射、红外拉曼光谱和核磁共振谱,确定并表征了晶体的结构。手性分子L-Arginine分子存在电荷转移结构,CF₃COOH分子的引入优化了L-Arginine分子的排列取向,即L-Arginine分子和CF₃COOH分子沿z轴方向有序排列,形成了晶体较强的非线性光学效应。通过核磁共振谱法发现晶体中L-Arginine分子与CF₃COOH分子之间存在氢键和静电作用,这种相互作用有可能会导致L-Arginine分子的构象发生转变。通过对晶体光学、热学和电学性能的系统研究,表明了LATF晶体是一种性能优异的新型非线性光学晶体材料。LATF晶体粉末倍频效应是KDP晶体的2.5倍,与LAP晶体接近;透光波段是232～2000nm,紫外截止波长是232nm,且随着溶剂极性的增强,晶体的吸收光谱将发生蓝移;采用V棱镜法测量了晶体的主轴折射率,拟合了折射率色散方程,绘制出了折射率色散曲线;从理论上数值模拟计算了晶体的位相匹配曲线,给出了有效非线性系数的计算公式,为下一步实验研究和测试工作提供理论指导;测定了晶体在激光波长1053nm,脉宽20ns和激光波长1064nm,脉宽20ps、1ns、10ns条件下的激光损伤阈值分别为2GW/cm²和1064,19,3.5GW/cm²,并探讨了影响晶体激光损伤阈值的本征因素和外部因素。在低重复频率的强激光作用下,晶体的损伤过程主要是激光的电场效应起作用,其损伤阈值与晶体的热学性能（如比热、热膨胀系数等）和机械性能有密切的联系。LATF晶体的熔点为217℃,热稳定性高于LAP晶体（140℃）;室温下LATF晶体的比热(340.70J·K^-1·mol^-1)比LAP晶体的比热(158.99J·K^-1·mol^-1)大得多,且其热膨胀各向异性比LAP晶体小,因此在脉冲激光作用下,LATF晶体具有比LAP晶体更高的抗光伤潜力;通过热膨胀张量主轴化方法,绘制出了LATF晶体在（010）平面内热膨胀系数随方向的分布,其各向异性热膨胀特性要求晶体生长过程中应尽量减少温度的波动。测定了LATF晶体的介电常数和介电损耗随频率和温度的变化曲线,结果发现:介电常数随频率的增大而减小且在100kHz以上出现饱和,此现象主要是界面极化作用所致,即移动的电荷载体被其它的物理势垒所阻断,限制了物质局域性极化的发生;在特定频率下,晶体的介电常数随温度几乎不发生变化,突出了晶体内部较高的化学均匀性和稳定性;低频时的高介电损耗源自于分子偶极矩的空间电荷极化机制而高频时的低介电损耗则表明了晶体样品拥有较高的光学质量和稳定性。3.LATF晶体表面形貌与生长机理的研究利用原子力显微镜,对LATF晶体在不同生长条件下的表面形貌及微观生长机制进行了深入研究。LATF晶体{101}面的生长机制主要包括螺旋位错生长和二维成核生长两种机理,其中以二维成核生长为主,主要结论如下:（1）生长台阶可以分为三类:一类是基台阶,高度与（101）面面间距相当;另一类是聚并台阶和大台阶,高度与基台阶的高度成倍数关系,台阶的聚并不仅会造成一些不稳定粗糙台阶片段生成,同时还会影响到最终的界面形态演变,台阶聚并往往是由于溶液中杂质通过改变固液界面处吸附层特点进而影响到生长单元的叠加而引起的,但杂质对台阶运动的作用具有选择性,若生长组分供给充足,受到杂质影响不明显则能够表现出非常直的台阶流;最后一类是没有完全聚并而仍保持基台阶性质的“准聚并台阶”,其产生可以采用Chernov的台阶运动波理论来定性解释,台阶间距越大,台阶生长速度越快,因此台阶密度最小的区域台阶推进速度最快,逐渐追及前面的基台阶群导致台阶密度增大。平行于b轴直台阶列的普遍存在是晶体生长的一个重要特征,是由晶体结构决定的本征生长现象,其形成与晶体的微观基元及其在晶体中的排列方式密切相关。b轴方向上存在着较大的极化场和结合力,使得b轴成为晶体快速生长的方向,一旦有临界晶核形成,生长基元便会迅速地沿b轴堆积形成微观直线台阶,而后靠台阶列的切向移动逐层生长铺满整个晶面。（2）螺位错生长丘的形状多数呈椭圆状,但当不同区域生长条件的变化促使螺位错周围的相变驱动力差异较小时,会形成长方形的生长丘,即沿不同方向运动的台阶生长速率的各向异性得到了充分体现。b轴方向是晶体的快速生长方向,存在较强的偶极引力作用。沿b轴伸长的长方形生长丘说明了晶体生长既受到生长条件的影响,也要受到晶体结构的制约。螺位错推出的生长台阶一般具有不同的台阶高度和台阶间距,但少数情况下,台阶运动存在伯格效应,吸附的结晶分子在台阶表面扩散协调后同步向前运动,表现出生长台阶的稳定性。（3）观察到了晶体的二维核产生、发展最后融合成“堆垛层”的连续变化过程,提出了一种新的成核-侧向推展模型（二维核叠层生长模型）来描述此特殊生长现象。此前,在对LAP晶体生长机理的实验观察中,并未发现二维核叠层生长现象,晶体的生长丘多由螺位错发展而成。二维核融合的过程中没有在连接处留下痕迹,不会导致缺陷,融合后的新二维岛中,越靠近岛中心,高度越低,中心最低处与岛周边高度最大相差50nm,这说明二维岛向外扩展的同时也从四面向岛中央逐层延伸。这种四周高而中心凹下去的二维核生长丘分布易于吸附或沉积杂质粒子,最终可能会产生晶体缺陷。台阶推移速度与平台宽度之间的关系可以用表面扩散控制生长模型来解释,拥有宽平台的台阶会比拥有窄平台的台阶运动速度快,成为成核最有利的位置。同时,二维核提供了进一步成核的台阶源,新核在其上面继续形成加速了宽台阶的生长,逐渐形成阶梯状向外扩展的二维岛甚至导致台阶悬垂形貌形成。（4）二维核的形貌与生长温度和过饱和度之间存在密切关联:较低温度下二维核大都呈圆形和椭圆形,随着温度的升高和过饱和度的降低,二维核形貌由圆形转变成椭圆形再到扇形,4.LATF晶体缺陷的研究借助光学显微镜和原子力显微镜,结合化学腐蚀法,对LATF晶体的缺陷及其形成机制进行了系统的观测与研究。LATF晶体中生长缺陷主要包括位错、空洞、溶液包裹体、开裂和楔化等。位错缺陷主要是受到生长条件的波动和杂质浓度的影响而形成的;空洞主要是由杂质对晶体生长的阻碍作用所致;溶液包裹体的形成与失去稳定性的聚并台阶及大台阶有关;导致开裂产生的原因包括温度波动、过饱和度变化太大、晶体的各向异性热膨胀特性及包裹体的存在等;引起晶体楔化的因素可以归结为杂质离子、溶液的pH值、过饱和度及生长温度。针对晶体缺陷的不同形成机制,采取了高温处理溶液新方法等有效减少或消除LATF晶体缺陷的相应措施。更多还原

【Abstract】 Nonlinear optics （NLO） crystal materials has become forefront of current research in view of its vital applications in areas such as high-speed optical communication, optical signal processing, optical modulation, optical switching, frequency shifting and high-density data storage. Organic NLO crystals have already become a hotspot in the field of nonlinear optical crystal material research, as they have large nonlinear coeffieicents, inherent ultrafast response times, high optical thresholds for laser power and easy processability as compared with inorganic crystals.L-arginine phosphate monohydrate （LAP） is discovered by Shandong University as a very promising UV organic NLO crystal material and has been given more emphasis by scientists. Due to the incomparable high laser damage threshold and more than 90% conversion efficiency, LAP crystal behaves important value in developing intense laser frequency shifting and phase conjugating mirrors and attracts more and more worldwide researchers’s attention to develop rapidly and widely.Stimulated by the discovery of LAP crystal, our group has synthesized L-arginine trifluoroacetate （LATF） crystal as another NLO crystal material exhibiting high damage threshold for the fisrt time. LATF is synthesized by dissolving equimolar quantities of L-arginine and trifluoroacetate acid in deionized water. Primary experimental results indicate that LATF belongs to the monoclinic system, P2₁ space group, powder SHG efficiencies is 2.5 times higher than that of KDP, ultraviolet transparency cutoff is 232 nm, its specific heat is 2 times larger than that of LAP, and the optical damage threshold of LATF at low repeat frequency strong laser is higher than that of LAP and DLAP. Therefore, LATF crystal is a new valuable and potential NLO crystal material. In this dissertation, the bulk crystal growth, structure, defects, surface morphologies, mechanisms, characterization and appraisal properties of LATF crystal are systemically investigated. The outline of this dissertation is as follows:1. Investigation of bulk crystal growth of LATF single crystals The growth velocity of LATF crystal oriented various directions has large anisotropy, c-axis is much slower than that of b-axis and a-axis, which bring difficulty to provide crystal samples for experimental measurement. Through adjustment of the solution composing, the rule of prevention sheet growth of crystals has been discovered, and combined with a continuous optimized seed-crystal method, the key technical question of obtaining excellent bulk LATF crystal has been resolved. In addition, through the investigations of nucleation kinetics, the metastable zonewidth and induction period are determined, the nucleation parameters such as interfacial energy of crystal-solution, critical radius and critical free energy barrier are all calculated. Buck LATF crystals with maximum size up to 50×27×7 mm³ and high optical quality have been grown by using the temperature lowering method. The crystal morphology was also analyzed, the optimal growth direction is oriented along b axis and the major flat facets are indexed as （101）, （100） and （001）.2. Investigations of crystal structure and properties of LATF crystalsThe structure of LATF crystal is determined and characterized by X-ray diffraction, Fourier transform infrared （FT-IR）, Fourier transform Raman and Fourier transform nuclear magnetic resonance （FT-NMR） techniques. The introduction of CF₃COOH optimizes the orientation of L-Arginine, namely L-Arginine and CF₃COOH arrange in order along c-axis. The optically active L-Arginine with the asymmetrical guanidyl and carboxyl groups to combine with the alkyl radical possessing F₃C tetrahedra of CF₃COOH and forms the high nonlinearity of the crystal. The NMR studies found that there is hydrogen-bonding and static interactions exist between L-Arginine and CF₃COOH, and may result in the conformational change of L-Arginine.Through the investigations of optical, thermal and electric properties, LATF crystal has been proved to be an excellent new NLO crystal material.The SHG efficiency of LATF crystal powder has been estimated as 2.5 times higher than that of KDP; the transparent region is in optical range of 232-2000 nm, with 232 nm being the UV cutoff wavelength, besides negative solvatochromism, i.e., hypsochromic （blue） shift occurs with increasing solvent polarity which might indicate a reduction in dipole moment on excitation; the principal refractive indices of crystal were measured by classical V-prism method and the dispersion curves obtained from the fit in terms of the Sellmeier analytical equation were plotted; the phase-matching curves of crystal were simulated and calculated theoretically and the equation for the effective nonlinear coefficients were presented; the laser damage threshold is 2 GW/cm² for laser wavelength 1053 nm, pulse width 20 ns and 1064, 19, 3.5 GW/cm² for laser wavelength 1064 nm, pulse width 20 ps, 1 ns, 10 ns, the essential and external factors that affect the laser damage threshold were both studied. For low repeat frequency strong laser, the electric field effect plays a great role in damage process and the laser damage threshold of crystal is correlated with its thermal （specific heat and thermal expansion coefficient） and mechanical properties.LATF crystal is thermally stable up to 217℃, which is substantially higher than that of LAP （140℃）; the specific heat of LATF is larger than that of LAP and the thermal expansion coefficients of LATF are less anisotropic than that of LAP, Therefore, LATF has a higher laser damage potential than LAP; According to the method obtaining the values of principal expansion coefficients, projection of thermal expansion quadric along the b-axis is presented.The dielectric constant and dielectric loss were recorded and analyzed both as function of frequency and temperature. The dielectric constant is relatively high in the lower frequency region and decreases with increasing frequency and becomes almost saturated beyond 100 kHz. This may be due to the interfacial polarization, in which the mobile charge carriers are interdicted by a physical barrier which restrains generating a localized polarization of the material. The variation of dielectric constant with temperature is small, which infers that the crystals are of good chemical homogeneity. The higher values of dielectric loss at low frequencies originates from space-charge polarization mechanism of molecular dipoles, and the characteristic of low dielectric loss at high frequencies clarifies that the grown samples possess enhanced optical quality with lesser defects. 3. Investigations of surface morphologies and growth mechanisms of LATF crystalsThe surface morphologies and growth mechanisms of LATF crystals grown from various conditions were investigated using by atomic force microscopy （AFM）. Crystals grow mainly by layer mode including dislocation-controlled mechanism and two-dimensional （2D） nucleation growth which dominates during the crystal growth.（1） The growth step patterns consist of three types, namely the elementary steps, macrosteps and quasi-macrosteps. The step height of elementary steps is about 0.64 nm, matching the interplanar distance of d₁₀₁ and the macrosteps’s height is multiple as that of elementary steps. The step bunching is always introduced by foreign impurities or particles. However, the impurities have a selective affect on the step movement. The straight step trains will show ultimately if the growth component is ample and the influence by the impurities is not evident. Quasi-macrosteps are practically formed by highly dense steps but remain the properties of elementary steps and its formation can be explained by Chernov’s kinematic waves of steps theory. Steps with low density usually advance faster than steps with narrow separations, the elementary steps behind gradually overtake and pile up at the step bunches and finally the highly dense steps come into being.The ubiquity of straight steps oriented along b axis is one of the primary growth characteristics. Its formation is correlated with the microcomponent of crystal and their arrangement pattern. Since large polarized field and binding energy exist along the b axis, crystal growth oriented this direction advances rapidly. Once a critical nucleus forms, it will engender the formation of b-oriented straight steps inevitably, and then the step trains spread along the tangent direction and overspread the whole crystal facet.（2） Most of the spiral dislocation growth hillocks are elliptical, rectangular shaped hillocks which indicate the anisotropy of step advancement are seldom seen only when the driving force surroundingthe the spiral dislocation differs slightly. The b-axis is the preferential growth direction, which contributes to the formation of rectangular shaped hillocks, crystal growth is confined not only by growth conditions but also by crystal structure. The steps generated by spiral dislocation have dissimilar heights and spacing, however, under few growth conditions, the Berg’s effect which indicates the stability of step movement can also be discovered.（3） The sequence of nucleation, growth, coalescence of islands and expansion of a "stack" has been observed. A new birth and spread （terraced nuclear） model is put forward to explain this new phenomenon. Previously, this new birth and spread phenomenon was not found during LAP studies, a majority of the hillocks were primarily generated by spiral dislocation.In the multiple nuclei growth, the 2D nuclei can gradually merge into a uniform and perfect growth layer when they meet with one another. No defects exist at the junction of the 2D nuclei. The height in the center of the 2D islands is lower than that of far from the center and maximal height difference is 50 nm, which shows that these 2D islands extend inward to the center and expand outward simultaneously. Thus, impurities were easily absorbed and deposited at the bottom of the hillocks and defects will appear ultimately.The correlation between step speed and terrace width can be explained by surface-diffusion-controlled growth coupled with the up-step diffusion bias model. The growth rate of a step with a wider terrace is faster than that of a step having a narrow terrace and the steps with larger terraces are the preferential growth sites. At the same time, the emergence of the 2D islands provides step sources for further growth and growth of the wide steps is expedited when several 2D nuclei grow on top of them. Thus, the segments proceed to grow faster than other segments, and the overhang morphology of steps appears in the end.（4） The morphologies of 2D nuclei depend both upon supersaturation and upon temperature. More specifically, at the low temperature, 2D nuclei often have circular and elliptical shape. With the increase of temperature and decrease of supersaturation, the islands change from circular to elliptical shape, and further become sector-shaped.4. Investigations of LATF crystal defectsMake use of opton optical microscope and atomic force microscopy, combined with chemical etching method, the results of the observations and discussions of growth defects and its formation mechanisms were presented in the dissertation. The main defects of LATF crystals are growth dislocations, hollow cores, inclusions, cracks, tapering. Growth dislocation defects are mainly influenced by disturbance of growth conditions and distribution of impurities; Hollow cavities result from the adsorbed impurities that block up crystal gowth; the formation of liquid inclusions is correlated with the macrosteps, once the growth surface loses its stability, mother solution will be trapped resulting in liquid inclusions; cracks in crystals are related with the disturbance of temperature, large diversity of supersaturation, anisotropic thermal expansion properties and the presence of inclusions; impurity particles, the pH value, supersaturation and growth temperature of the solution may cause tapering defects. In order to avoid or decrease these defects, corresponding measurements such as a newly disposing high temperature solutions method were taken according to the distinct formation mechanisms results.更多还原