节点文献
新型有机功能材料的双光子吸收特性及超快动力学研究
【作者】 闫永丽；
【导师】 钱士雄；
【作者基本信息】 复旦大学 ， 光学， 2008， 博士
【摘要】 在光科学和光电子技术得到高速发展的当今时代,具有优异非线性光学性质的材料是一类令人注目且有重要应用潜力的新材料。双光子过程以其特有的光响应特性、非线性性质、三维处理能力和极高的空间分辨本领而在生物、物理、化学、医学、微电子技术等众多领域显示出变革性的应用潜力。目前双光子技术已应用于高密度数据存储、单分子检测、医疗诊断以及三维精细加工等领域的研究和开发,取得了重要的成果。为使这种新技术得到更广泛的应用,具有更优异性能的材料是实际应用的前提和基础,所以合成出具有较大双光子吸收截面和强的双光子荧光材料已经成为光学及其交叉学科中最诱人和最活跃的研究领域之一。而飞秒激光器的广泛使用和飞秒光谱学的日臻完善使人们对新型材料中的超快动力学过程的探测成为可能。由于这些快过程中包含着分子结构以及光与物质相互作用的丰富而重要的信息,对它们的深入研究就能为合成和优化新型材料,探索新颖器件和应用提供坚实和可靠的基础。因而,对具有双光子吸收能力材料的非线性光学和超快光动力学研究也就成为近年来光科学领域的前沿课题之一。本论文通过吸收光谱,荧光光谱,z扫描,飞秒泵浦-探测和时间分辨荧光等技术对一系列具有双光子吸收特性的有机化合物的非线性光学性质和超快光动力学特性进行了系统的研究。通过对不同化合物的实验测量结果的比较,分析了分子结构对材料的非线性光学性质以及光动力学特性的影响。取得的主要创新成果包括:1)通过光克尔门技术对时间分辨荧光的测量,我们发现,具有多枝结构的分子不同波长处的荧光弛豫特性很不相同,而枝数较少的分子的荧光弛豫则不随探测波长的变化而变化,且多枝结构分子的各向异性弛豫时间仅为720fs。研究结果表明枝数较多的分子中存在强烈的分子内耦合作用,会增强双光子吸收能力,这一观点在z扫描实验中也得到了证实。2)通过z扫描技术测量了多枝结构分子的双光子吸收截面,并研究了它们的溶剂效应。我们发现分子在极性大的溶剂中的双光子吸收截面也较大,在DMF中的双光子吸收截面是在THF中的7倍;其增强机制主要来源于分子内枝与枝之间的相互作用,对枝数很少的分子,分子内相互作用很弱,因而溶剂效应也不显著。3)泵浦-探测实验表明聚合物的激子迁移有较强的方向选择性,只能沿着聚合物主链的方向进行;在分子中引入强的电子给体、电子受体或者扩展分子的枝数都是很好的增强分子双光子吸收截面的方法。4)通过对两种金属酞菁衍生物的比较研究,我们发现酞菁中心金属原子对能量弛豫有着非常重要的影响。具有开壳层结构的金属原子(如钴)置于酞菁大π环中心时,最外层非成对电子与π轨道耦合会导致能级分裂,不仅加速了激发态的弛豫,还能有效淬灭荧光发射。5)我们还研究了三种结构的二芳基乙烯类分子的光致变色性能及变色对分子光学性质的影响,我们发现,含有电子给体基团的分子双光子吸收性能较好;分子变色后双光子吸收能力有明显的增强。
【Abstract】 Along with the progresses of the optical science and optical electronics in the past decades, the materials with excellent nonlinear properties are currently of considerable significance and they show a variety of potential applications. Two-photon absorption has shown great potential applications in biology, physics, chemistry, medicine, micro-electronic technology and many other fields due to its unique optical response, nonlinear property, 3D processing capabilities, and high spatial resolution. At present, two-photon technique has been applied in high-density data storage, single-molecule detection, medical diagnosis, three-dimensional microfabrication and so on. Of course, the materials with excellent properties are the basis for wider application. Therefore, to get the materials with large two-photon absorption cross section as well as strong two-photon fluorescence has become one of the most attractive topics. With the maturity of the femtosecond laser system and the great achievement in the field of the ultrafast spectroscopy, ultrafast processes of the novel materials can be investigated in the picosecond and femtosecond domains. Due to the fact that the ultrafast processes can reflect a lot of significant information about the interaction between light and matter, the investigation of these processes are meaningful for the synthesizing and optimizing the new materials, as well as exploring the novel devices. Therefore, the study on the nonlinear optical properties and ultrafast dynamics of the materials with intense two-photon absorption ability, are become one of the frontier and hot topics in the optical science.We employed the UV-Vis and fluorescence spectroscopy, z-scan technique,femtosecond pump-probe and time-resolved fluorescence experiments tosystematically study the nonlinear optical properties and the ultrafast response of aseries of organic compounds, and we analyzed the relationship between the structureand the optical properties. The main innovative results and conclusions are as follows:1) We successfully measured the time-resolved fluorescence and anisotropy throughthe optical Kerr gate method. The fluorescence relaxations of multi-branchedmolecule show great differences at different detection wavelength, and theanisotropy relaxation time is only 720fs for the molecule possessing mostbranches, while the relaxations do not change with the detection wavelength inmolecules with few branches. The results show that it is the strongintra-molecular coupling within the multi-branched molecules which will enhance the two-photon absorption ability. The conclusions were confirmed from the z-scan experimental results.2) By using z-scan technique, we measured the two-photon absorption cross section of these molecules and studied the solvent effect. The largest value of two-photon absorption cross section for a given molecule was observed in the most polar solvent. The two-photon absorption cross section for the largest molecule in DMF is seven times larger than that in THF. The mechanism of the enhancement mainly comes from the strong interactions among branches. For the smallest molecule, the intra-molecular interaction is weak, thus the solvent effect is not remarkable.3) The pump-probe experimental results indicate that the exciton migration in oligomers has a strong choice of direction, they can only happen along the direction of backbone. Introducing strong electron donor, electron acceptor or an expansion are all good methods for improving two-photon absorption ability.4) Through the comparative study of two metal phthalocyanine derivatives, we found that the central metal atom in phthalocyanine has a very important influence on the energy relaxation. When a metal atom with open-shell structure (eg. cobalt) was put in theπcenter, the non-paired electron will couple with theπorbital and cause the splitting of energy level, thus speeding up the excited state relaxation as well as reducing the fluorescence emission.5) We have also studied three kinds of dithienylethene derivatives which are known as good candidate of photochromic materials. We found that the molecules containing electron donor show better two-photon absorption ability in comparison with others. After the irradiation of UV light, the two-photon absorption ability of the close-circle materials is enhanced.