【作者】 杜鑫；

【导师】 杨延强；

【作者基本信息】 哈尔滨工业大学 ， 光学， 2012， 博士

【摘要】 超短脉冲激光技术和时间分辨光谱技术的发展使得人们可以在超快时间尺度上获取无序分子材料中分子相干振动动力学及有序晶体材料中晶格相干振动（即相干声子）动力学的信息。对分子及晶格相干振动动力学的研究在许多方面都有着重要的价值，比如研究物质中各种元激发之间的相互作用；监控化学反应过程；研究分子及声子所处环境的变化等。飞秒时间分辨相干反斯托克斯拉曼散射（飞秒TR-CARS）光谱技术是以一种既能研究电子基态分子动力学又能研究电子激发态分子动力学的有力工具，而且已经被用于研究几种纳米晶体材料中相干声子动力学。飞秒TR-CARS技术的理论和实验研究已经取得了众多成果，但也存在着很多问题，比如其机理研究还不够系统和深入，实验研究还存在一定的狭隘性。本论文对飞秒TR-CARS技术进行了深入的理论分析，搭建了飞秒TR-CARS光谱实验系统，将宽谱带的超连续白光用作斯托克斯光，利用超连续白光的啁啾特性来实现对不同频率振动模式的选择激发，避免了采用复杂的激光系统。研究了无序分子材料甲苯和PVK中电子基态分子相干振动动力学以及有序晶体材料蓝宝石中相干声子动力学。目前，人们对飞秒TR-CARS技术的理论分析还不够深入，现存的理论模型还不够完善，对一些实验结果的模拟和解释尚存在一定困难。本文利用半经典理论分析方法深入分析了飞秒TR-CARS过程的机理，结合飞秒激光脉冲的特点，经过系统的公式推导，建立了利用飞秒TR-CARS技术激发并探测相干振动的理论模型，就飞秒TR-CARS实验中经常出现的量子拍频信号进行了理论解释，得到了飞秒CARS信号及量子拍频信号随探测脉冲相对同步的泵浦/斯托克斯脉冲对的延迟时间演变的理论公式，为合理分析和解释实验结果、获得关于拉曼模式振动动力学的信息提供了重要的理论依据。在分子振动动力学研究领域，对振动模式之间相干耦合问题的研究还存在很多不足，这影响了分子动力学的全面发展。本文利用飞秒TR-CARS光谱技术，以小分子材料甲苯液体和高分子聚合物材料PVK固态薄膜为研究对象，研究了电子基态分子振动动力学，选择激发并探测了高频振动区3000cm-1左右的C-H伸缩振动模式，在两种样品的飞秒TR-CARS信号中都观察到了振动模式之间相干耦合引起的量子拍频现象。利用建立的理论模型对实验结果进行了分析和解释，并对动力学曲线进行了理论拟合，得到了受激振动模式的振动动力学信息，并表明了分子中不同位置处同种化学键的振动模式之间可以发生相干耦合，这对深入理解量子相干并发展量子相干技术具有重要意义。对晶体中声子的研究是固体物理学中一项重要的课题，国内外利用飞秒TR-CARS技术对块状晶体中相干声子动力学的研究还未见报道，对相干声子基础研究的缺乏制约了其应用研究的发展。本文利用飞秒TR-CARS光谱技术研究了块状蓝宝石晶体中相干声子动力学，首先同时激发并探测了蓝宝石晶体中300~600cm-1范围内的多个声子模式，观察到了多个模式相干耦合的量子拍频，又选择激发并探测了较低频的379cm-1和418cm-1声子模式，通过理论拟合获得了这两个模式的动力学信息。此项工作对于丰富人们对相干声子的认识，发展相关领域的应用研究具有重要意义。缺陷对晶体的许多性质都有着重要的影响，因此发展对晶体缺陷的无损检测技术是非常必要的。晶体中缺陷的存在会影响声子的动力学行为，本文对经不同辐照剂量γ射线辐照过的蓝宝石晶体中较高频的645cm-1和750cm-1声子模式进行了选择激发和探测，研究了蓝宝石晶体中缺陷对相干声子动力学参数的影响，发现两种模式相干声子的失相时间都随着射线辐照剂量的增加而减小，这是由于随着γ射线辐照剂量的增加使蓝宝石晶体中缺陷密度增加，相干声子被缺陷散射加速了其失相过程。提出了飞秒TR-CARS光谱技术可以作为一种对晶体缺陷进行无损检测的新技术。本论文的工作丰富了飞秒TR-CARS过程理论研究内容，拓宽了飞秒TR-CARS光谱技术的研究范围，对于发展新技术也具有重要价值。更多还原

【Abstract】 With the development of ultrashort pulse laser and time-resolved spectroscopytechnique, people can gain information about the molecular dynamics in the disorderedmolecular materials and coherent lattice vibrational, i.e. coherent phonon dynamics inordered crystalline materials in the ultrafast time-regime. The investigations onmolecular and lattice coherent vibration dynamic is of great value in many fields, suchas studying the interaction between element excitations, monitoring chemical reactionprocess, studying the changes of molecular and phonon surrounding environment, etc.Femtosecond time-resolved coherent anti-Stokes Raman scattering (TR-CARS)technique has proved to be a valuable tool to study the dynamics of molecular systemsboth in electronic ground state and excited state. It has been widely used to studycoherent phonon dynamics in nano-materials. So far, there are many achievements onthe theoretical and experimental investigations of femtosecond TR-CARS. However, thetheoretical investigations are not enough in-depth, and there are a lot of deficiency inexperimental investigations. Therefore, in the present thesis, the theory of femtosecondTR-CARS are analysed in depth. Based on the temporal chirp characteristics of thewhite-light continuum, we have designed the experimental system of femtosecondTR-CARS. The ultrabroadband white-light continuum (WLC) is used for the Stokespulse, due to the temporal chirp characteristics of the WLC, We have achieved theselective excitation of vibrational modes, no complicated laser system is required. Wehave studied the molecular dynamics in electronic ground state in small moleculetoluene and polymer PVK, and coherent phonon dynamics in bulk sapphire crystals.At present, the theoretical investigations of femtosecond TR-CARS technique arenot enough in-depth, there are still certain lacks of theoretical models, and there are a lotof difficulties in analyzing and explain experimental results. We analyse femtosecondTR-CARS process in depth by semiclassical theory. Considering the characteristics ofthe femtosecond laser pulses, we have built the theoretical model of femtosecondTR-CARS, explained the quantum beat signals in emtosecond TR-CARS procss. Thiswork provides important theoretical basis for analyzing and explaining the experimentalresults, and obtaining dynamic information of vibrational modes.In the field of molecular vibrational dynamics, it is insufficient on the study ofcoherent coupling between vibrational modes. This affects the integrated developmentof molecular dynamics. By femtosecond TR-CARS technique, we have achieved theselective excitation of CH stretching vibrational modes near3000cm-1in liquid tolueneand PVK film samples, and and probe the dynamics process of the vibrational modes.The theoretical formulas are in good agreement with the experimental results. Thedephasing times of the excited modes are obtained. Our investigations demonstrate that there exists coherent coupling between vibrational modes of the same kinds of chemicalbonds at different positions in molecules. This conclusion is especially helpful for us tounderstand quantum coherence and develop quantum coherence techniques.It is a subject of special significance for the study of phonon in crystals. At present,however, there is no report on coherent phonon dynamics in bulk crystals studied byfemtosecond TR-CARS technique. In order to fill the gap of research in this field, wehave studied the coherent phonon dynamics in sapphire crystals by femtosecondTR-CARS. The multiple phonon modes in300~600cm-1region are simultaneouslyexcitated and probed. The quantum beat signal is observed. We have also achieved theselective excitation of the379cm-1and418cm-1phonon mode in the low-frequencyregion, and detected the coherent phonon dynamics process. The experimental resultsare fitted using the theoretical model, the dephasing times of the two phonon modes areobtained. This work is improtant for understanding coherent phonon and developingapplied research.Defects will influence many properties of crystals.Therefore, it is important todevelop non-destructive detection technique of lattice defects. Phonon dynamicsDefects in crystals will influence phonon dynamics. We have achieved the selectiveexcitation of the645cm-1and750cm-1phonon mode in the high-frequency region, andstudied the ultrafast dynamics of coherent phonons in sapphire crystals irradiated with60Co γ-rays for three different doses. The dephasing times of the two modes bothdecrease with increasing γ-ray irradiation dose, which is due to that the dephasingprocesses of coherent phonons are accelerated because of the scattering of phonons bythe defects introduced by γ-ray irradiation. This investigation indicates that fs-CARStechnique is a powerful tool to study the effect of defects on the dynamics of coherentphonons, and it provides the possibility of developing a new non-destructive detectiontechnique to predict the damage degree of crystals.This work is of great valable for enriching the theory contents of femtosecondTR-CARS process, broadening the research scope of femtosecond TR-CARStechnology, and developing new technologies.更多还原