【作者】 孙志华；

【导师】 许东； 张光辉；

【作者基本信息】 山东大学 ， 材料学， 2010， 博士

【摘要】 强激光技术在惯性约束核聚变等领域的应用已引起了国际上广泛的关注,许多发达国家将其列入优先发展的科技计划中；重复频率为0.1 Hz、峰值功率高达300 TW的脉冲激光也已经实现了运转。超高强度激光的不断升级发展,对所使用材料的光学性能、尺寸等提出了更加严格的要求,优化并提高材料的抗激光损伤能力也是一个不容忽视的关键科学技术问题。L-精氨酸磷酸盐(LAP)晶体是山东大学在国际上首创,并受到国内外高度认同的一种性能优异的紫外非线性光学材料。该晶体尤其是氘化后的DLAP晶体不仅实现了90%以上的高转换效率,而且具有其他晶体无法比拟的高激光损伤阈值。日本科学家的研究结果表明在波长为1053 nm,脉冲时间为1 ns时DLAP晶体的激光损伤阈值高达87GW/cm2,远高于其他非线性光学晶体,几乎是目前作为惯性约束聚变激光频率转换材料KDP晶体的5倍。因此,LAP和DLAP晶体被人们认为是极其重要的强激光频率转换和波前整形的光学材料。LAP和DLAP晶体具有如此优异的抗激光损伤能力,引起了国际上许多非线性光学材料专家的浓厚兴趣。为了揭示LAP晶体具有高激光损伤阈值的原因,国内外都曾开展了一些相关的研究工作,但迄今为止仍没有得到很好的解释。本课题组一直致力于探索LAP晶体具有高激光损伤阈值的原因。通过大量的生物化学文献调研发现：精氨酸磷酸(Phosphate Arginine,简称PA)本身就是无脊椎动物体内能量存储和运输的载体,在生物能传递的整个过程中表现出分子构象易变的特点。LAP晶体的组成基元与无脊椎动物体内的PA完全相同,激光辐射是否也会引起LAP晶体的分子结构和构象变化,是否与生物能的存储和传递过程存在着某些关联均引起了我们的关注。因此,探讨生物能和强激光辐射能量引起两者分子结构和构象变化之间的关联,是本论文探索LAP晶体具有高激光损伤阈值原因的主要出发点,在此基础上开展了新型L-精氨酸盐晶体的制备和表征工作。论文的主要研究内容包括：(一)、总结、分析了PA在无脊椎动物体内能量存储和传输过程中分子构象变化的规律。PA是无脊椎动物体内能量存储和传递的载体。在生物能的存储和运输过程中,PA体现出分子构象易变的特点。能量储存在PA内时,精氨酸分子与磷酸基.团的键合作用导致其分子构象由伸展型转变为弯曲型；而在能量传递过程中,分子构象则恢复为伸展型。(二)、结合L-精氨酸系列晶体的结构,研究了激光辐射作用下LAP晶体组成基元和分子构象的变化特点。采用X-射线单晶衍射分析了L-精氨酸系列晶体中L-精氨酸分子的构象,发现也具有分子构象易变的特点。借助Mercury解析了LAP晶体的平面方程,结果表明胍基平面和羧基平面之间的夹角为29°。通过与其他晶体的比较发现,不同类型阴离子键合作用导致两组平面相对于碳骨架发生了空间旋转。阴离子的作用力越强,胍基平面和羧基平面之间的夹角越小,L-精氨酸分子构象弯曲的程度越大。LAP晶体的磷酸根是具有畸变四面体的离子构型,存在两个P-O单键和两个P=O双键。在拉曼谱图中,磷酸基团O-P-O对称伸缩振动V1表现为929、953和973 cm-1等特征振动峰；P04对称变角振动V2为413和394 cm-1两条谱线；P04反变角振动V3表现为518和533 cm-1等主要振动峰。利用微区拉曼光谱技术研究了LAP晶体磷酸根的不对称结构在激光辐射下发生的变化。结果发现：V1在973 cm-1处的振动峰消失,另两个峰的相对强度减弱；V2在413 cm-1处的振动谱线基本消失,而V3在518 cm-1附近的振动峰相对强度减弱,这些变化是激光辐射能在晶体内作用的结果。(三)、初步探讨了生物能传输过程中和激光辐射作用下PA与LAP晶体分子构象变化之间的关联,分析了构象变化对晶体激光损伤的影响。激光辐射导致LAP晶体中磷酸根引起的拉曼振动分裂峰数目减少,表明晶体内磷酸基团的一致性增加,呈现出由畸变四面体向正四面体构型转变的趋势。L-精氨酸受到的键合力减弱,其分子构象则向伸展型转变,这与生物能传递过程中PA分子构象的变化类似。但与生物体内明显的转变相比,晶体中的分子构象变化目前在实验上还很难直接观测到。主要采用基于密度泛函理论的第一性原理模拟了不同的分子构象对LAP晶体的电子态密度分布和能带结构的影响。初步的结果表明：单纯的分子构象变化没有改变晶体的能级结构,对晶体的激光损伤不会产生明显的影响,能量通过分子构象变化的传递有利于其抵抗激光的损伤。但当分子构象的变化在晶体内部引起缺陷时,则将影响晶体的电子态密度,甚至引入缺陷能级,降低晶体的激光损伤阈值。(四)、研究了其他因素在L-精氨酸系列晶体损伤过程中的作用,重点讨论了在不同激光条件下,比热、热膨胀、热扩散和热传导等热学性质对晶体抗激光损伤能力的影响。实验测得L-精氨酸系列晶体具有较大的比热。LAP、L-精氨酸三氟乙酸(LATF)和L-精氨酸双三氟乙酸(LABTF)晶体在293 K时的比热分别为158.99、340.1和560.8 J.mo1-1·K-1,高于一般的无机晶体,较大的比热是它们在脉冲激光下具有优良抗光损伤性能的主要原因之一热膨胀性能也是不容忽视的重要因素。实验测得LATF沿着三个主轴方向的热膨胀系数分别为51.4×10-6、7.5×10-6和16.4×10-6K-1,LABTF晶体的三个相应值为98.7×10-6、-8.6×10-6和70.4×10-6K-1。与LATF相比,LABTF晶体的热膨胀系数具有更大的各向异性,且沿b轴方向热收缩,引起它在激光辐射时更容易出现炸裂式损伤,对应较低的激光损伤阈值。LAP晶体沿二次轴方向的热传导系数为0.59 W·m-1·K-1,测得LATF晶体沿6轴方向的热传导系数为1.264 W·m-1·K-1,均低于多数无机晶体材料。较差的热传导性能导致LAP和LATF晶体在连续激光或高重复频率激光作用下比无机晶体更容易发生损伤。(五)、制备并表征了几种L-精氨酸盐及其他氨基酸衍生物新晶体。为系统地研究L-精氨酸盐晶体的结构与性能之间的规律,开展了相关新晶体的制备与性能表征工作。在L-精氨酸和三氟乙酸体系中,制备了两种激光损伤阈值较高的非线性光学晶体：LATF和LABTF.生长了尺寸为35x17×9.5mm3、光学质量优良的LATF单晶,在较短的周期内采取降温法和溶剂蒸发法获得了尺寸分别为32x21×4 mm3和20x29x5 mm3的LABTF单晶。两种晶体的生长习性差异较大,前者具有层状生长和生长速率各向异性的特点,后者则克服了层状生长的缺点。从结构的角度入手,分析了影响两种晶体生长习性的因素。研究了LATF晶体中多种形式的缺陷,如生长条纹、包裹体、开裂和柱面弯曲等,这些缺陷严重影响了晶体的光学质量,降低了晶体的激光损伤阂值。我们结合晶体生长过程分析了各种缺陷的形成原因,并采取有效的措施消除或抑制了晶体内产生缺陷。对两种晶体的性能进行了测试表征,发现LATF、LABTF晶体的紫外截止波长分别为232和237 nm,在紫外、可见和近红外波段内透光性能良好；两种晶体的粉末倍频效率相当,约为KDP晶体的2.5倍。LATF晶体具有较高的热稳定性和更优异的抗光损伤能力。两种晶体都能够对多种波长的激光实现倍频转换,作为非线性光学晶体材料具有潜在应用价值。制备并生长了L-赖氨酸三氟乙酸(LYTF)和L-赖氨酸乙酸(LYAC)两种氨基酸衍生物晶体,获得尺寸分别为28×12x6 mm3和17×10x4 mm3的体块单晶。它们的粉末倍频效应分别为KDP的1倍和0.5倍,并对两种晶体的各种性能进行了初步的研究。本论文对LAP晶体具有高激光损伤阈值的原因进行了探索研究,发现激光辐射引起的晶体分子结构和构象的变化,与生物能传递过程中PA的构象变化之间存在相似性,这为进一步探索晶体的激光损伤机理提供了重要依据。开展了新型L-精氨酸和L-赖氨酸晶体的制备与性能表征工作,重点进行了LATF和LABTF单晶生长和性能研究,为其器件设计和开发应用奠定基础。更多还原

【Abstract】 Techniques used in superpower laser have attracted considerable attention because of their wide application in the inertial confinement fusion field. Recently, pulse laser with the maximum power intensity to 300 TW at 0.1 Hz repetition rate has been accomplished successfully. The upgrading and development of such high-power laser will strictly demand that optical materials should not only possess higher quality and larger size, but also have more outstanding resistance to laser-induced damage, which is one of the most important issues.L-Arginine phosphate monohydrate (abbreviated as LAP) crystal was firstly discovered by Shandong University as a promising ultraviolet nonlinear optical (NLO) material and has obtained a great deal of authorizations. The most attractive properties of LAP crystal, especially its deuterated analogue namely DLAP crystal, such as high conversion coefficient (maximum to 90%) and extremely high laser-induced damage threshold make it more outstanding than other NLO crystals. Research results achieved by Japanese scientists indicate that laser-induced damage threshold of DLAP crystal is 87 GW/cm2 with wavelength of 1053 nm and pulse-width at 1 ns, which is about 5 times higher than that of KDP crystal. Therefore, LAP and DLAP crystals are considered as extremely promising NLO materials in frequency conversion and wave-front shaping fields.Many experts all of the world express their interests in the reason that LAP crystal and its deuterated analogue have such high resistance to laser-induced damage. Many investigations have been carried out on this issue. However, it has not obtained very comprehensive achievements up to now. The innovation and exploration for the laser induced damage mechanism of LAP crystal has become the research topic.Our research group has concentratively devoted to exploration for the reason that LAP possesses high laser-induced damage threshold. Through sufficient literature investigations, we find that phosphate arginine (abbreviated as PA) works as the medium carrier for bioenergy transport and storage in invertebrates, whose particular structural features are responsible for the biological functions. In the whole process, arginine presents its conformational flexibilities and varieties. LAP crystal has the same structural units as PA in invertebrates. It is the question attracts our attention whether laser irradiation will arouse conformational variations of L-arginine molecule in LAP crystal. Thus, our exploration for the reason that LAP possesses high laser-induced damage threshold is established on the following issue:relationship between biological flexibilities and stimulated changes of arginine in crystals. Meanwhile, a series of novel crystals in L-arginine and amino acid derivate family have been prepared and characterized in order to develop NLO materials with excellent merits. The dissertation mainly comprises the following aspects:(Ⅰ) The dissertation has investigated and summarized the structural variations of PA molecule during bioenergy storage and transport process in invertebrates.PA is considered as the bioenergy carrier in invertebrates, which provides enough bioenergy for biological activities. It exhibits obvious flexibilities of conformations during bioenergy storage and conversion process. It is found that the bonding interactions of arginine with phosphate group make its conformation change from an extended state to a folded state, which will be reversible while the bioenergy transfers to others.(Ⅱ) The dissertation investigates structural diversifications induced by laser irradiation in LAP in combination with crystal structures of L-arginine derivates.The conformational features of L-arginine molecules in crystals have been investigated by the X-ray single-crystal diffraction. The plane equations resolved by Mercury indicate that dihedral angle between guanidyl group and carboxyl group is 29°in LAP crystal. In comparison with other crystals, the two above-mentioned planes are rotated against carbon chain due to the different electrovalent bonding interactions. L-Arginine molecular conformations are also influenced by crystal components, charge distribution and hydrogen bonds, etc.In LAP crystal, phosphate group has an ionic configuration with distorted tetrahedron. The asymmetric structure with two P=O bonds and two P-O bonds will be affected by laser irradiation. The Raman spectral lines at 929,950 and 973 cm-1 are assigned to symmetric stretching mode of O-P-O bonds. The characteristic lines at 413 and 394 cm-1 are caused by symmetric deformation of PO4. The asymmetric deformation bands of PO4 group are located at 518 and 530 cm-1. The varieties of phosphate groups in LAP crystal have been investigated by micro-zone Raman spectral technique. It is found that the characteristic lines at 973 and 413 cm-1 disappear. The relative intensities of 518 cm-1 and other vibrational bands have also been weaken, which are resulted from laser irradiation.(Ⅲ) The relationship has been discussed between biological flexibilities of PA and stimulated changes of arginine in LAP crystal. The influences on laser-induced damage have also been discussed.Absence of these splitted peaks implies that the distorted phosphate tetrahedron trends to become more regular and homogenous. Thus, molecular conformation of L-arginine changes due to weak effect of phosphate group. Energy transport in LAP crystal leads to varieties of its component and molecular conformation. In bioenergy transport process, the zwitterionic conformation of L-arginine changes from an extended state to the folded state, which has been proved by many methods. However, the similar variety induced by laser irradiation is quite difficult to observe directly by experiments. Using ab initio total-energy calculations and density functional theory (DFT), we mainly discuss the influences caused by the different molecular conformations on total density of states and band structures. It is found that conformational varieties do not change band structures of LAP crystal, which will not obviously affect the laser-induced damage threshold. However, the band structures will be influenced because of intrinsical defects induced by conformational varieties.(Ⅳ) Thermal properties of NLO crystals in L-arginine family, such as specific heat, thermal expansion and thermal conductivity, are systematically discussed as important influencing factors on their laser-induced damage thresholds under different laser conditions.Larger specific heat of L-arginine crystals is responsible for higher resistance to laser-induced damage. The respective specific heat values of LAP, LATF. and LABTF are 158.99,340.1 and 560.8 J·mol-1·K-1, which are much larger than inorganic materials, plus higher laser-induced damage thresholds under laser irradiation with low repetition rates. Properties of thermal expansion are also contributive to their resistance to laser-induced damage. The measured thermal expansion coefficients of LATF along three principal axes are 51.4×10-6,7.5×10-6 and 16.4×10-6 K-1, while the corresponding values of LABTF are 98.7×10-6,-8.6×10-6 and 70.4×10-6K-1. In contrast with LATF, LABTF crystal has more anisotropic coefficients and the corresponding properties are quite abnormal along b-axis. Thus, LABTF is easily cracked under laser irradiation, accordingly exhibiting low laser-induced damage threshold.Thermal conductivity coefficient of LAP crystal along two-fold axis is 0.59 W·m-1·K-1. The measured thermal conductivity coefficient of LATF along b-axis is 1.264 W·m-1·K-1, which is inferior to those of inorganic materials. Thus, LAP and LATF become easily destroyed under laser with high repetition rates exhibiting weak resistance to laser-induced damage.(V) Several novel NLO crystals in L-arginine and other amino-acid derivates have been prepared and characterized in the dissertation.LATF and LABTF, two organic crystals exhibiting excellent laser-induced damage thresholds, have been discovered and prepared from L-arginine plus trifluoroacetic acid system. Bulk LATF crystals with maximum size up to 35×17×9.5 mm3 and high optical qualities have been grown by the temperature lowering method. LABTF single-crystals with sizes of 32×21×4 mm3 and 20x29x5 mm3 have also been obtained by temperature lowering and solvent evaporation methods, respectively. Crystal structures of LATF and LABTF are considered to reveal differences of their growth habits. Defects of stripes, inclusions, cracks and tapering, etc in LATF crystals are investigated and analyzed, which inhibit crystal qualities and laser-induced damage threshold. Corresponding measurements have also been adopted to improve crystal qualities.The properties of LATF and LABTF crystals have been investigated, and it is found that the respective ultraviolet cutoff wavelengths for LATF and LABTF are 232 and 237 nm with excellent transparent efficiencies in Uv-vis-NIR region. Their powder SHG efficiencies are equally estimated to be 2.5 times higher than that of KDP. The experimental figures reveal that LATF possesses much higher thermal stability and more outstanding resistance to laser-induced damage than that of LABTF crystal. All the outcomes indicate that LATF is a promising organic NLO crystal with high laser-induced damage threshold.Crystals of other amino-acid derivates, named L-lysine trifluoroacetate (LYTF) and L-lysine acetate (LYAC) with respective sizes of 28×12×6 mm3 and 17×10×4 mm3, have been prepared by the solvent evaporation and temperature lowering method. The powder SHG efficiency of LYTF is about 1 time higher than that of KDP, while LYAC is estimated to be 0.5 times higher than that of KDP. Their structural, thermal and optical properties have also been investigated.All the outcomes reveal that variations of molecular conformations induced by laser irradiation are similar to those of PA in bioenergy process, which provides an important foundation for laser-induced damage mechanism of LAP crystal. The experimental results suggest that LATF and LABTF crystals are promising NLO materials with high laser-induced damage threshold.更多还原