【作者】 孙玉萍；

【导师】 王传奎；

【作者基本信息】 山东师范大学 ， 原子与分子物理， 2010， 博士

【摘要】 超强超短激光脉冲技术的发展以及高功率高强度的同步辐射光源和新型X射线自由电子激光的相继出现,引起了人们极大的兴趣,再次激发了我们对强场激光与物质相互作用的研究热情,非线性光学以及X射线光谱学取得了突飞猛进的发展。本论文的主要目的是探索高强度的激光脉冲与物质相互作用的微观机制,解释和预测非线性光学以及X射线光谱学领域中各种新颖的物理现象。本论文的研究内容包括有机分子材料的光限幅特性、X射线自由电子激光在原子介质中的传播特性和共振X射线散射光谱等,取得了一系列创新点,不仅对该领域理论的发展,而且对指导实验工作有重要的意义。主要内容和结果如下:1、双光子吸收光限幅效应我们采用时域有限差分法(FDTD)和预估校正法求解全波矢的Maxwell-Bloch方程组,模拟了周期量级飞秒脉冲激光在强双光子吸收介质4,4’?二甲氨基二苯乙烯中的动力学传播过程,并研究了该分子介质的光限幅特性。数值计算结果表明,对于超短脉冲,介质主要发生的是一步双光子吸收(TPA),在脉冲传播过程中,该介质表现出较好的光限幅特性。我们利用脉冲输出—输入光强关系,初步计算了分子的动态TPA截面。当光场强度不足以使介质发生TPA饱和时,其TPA截面随入射光强增加线性减小。当光强较强时,光电离对TPA过程有明显的影响。随介质电离率的增加,光限幅的动力学窗口变宽。2、反饱和吸收光限幅效应我们研究了皮秒脉冲序列在富勒烯C6 0分子体系中的传播特性,以及基于反饱和吸收的光限幅机理。当每个子脉冲与介质相互作用时,单重态和三重态系统之间的粒子数转移几率很小,整个体系可以分为两个独立的子系统:单重态子系统和三重态子系统。在无脉冲作用时域,被激发到单重激发态的粒子数可以通过系统内交叉过程跃迁至三重态,两个子系统耦合在一起。脉冲序列中前面的子脉冲主要发生的是单重态之间的线性吸收;由于非线性积累效应,后面的子脉冲与介质作用时主要发生的是最低三重态的激发态吸收。脉冲序列内子脉冲不同的吸收机制导致了介质的光限幅效应。基于反饱和吸收的光限幅材料三重态必须具有较长的能级寿命和较强的光吸收截面。我们提出了一种新型的测量三重态能级寿命的实验方案,即改变脉冲序列的入射频率,通过研究脉冲能量透射率的变化间接测量三重态的能级寿命。3、X射线自由电子激光(XFEL)在原子介质中的动力学过程我们通过数值求解Maxwell-Bloch方程首次模拟了XFEL在原子介质中的传播过程,从激光场对介质做功的角度揭示了XFEL与介质相互作用的微观机制。XFEL的传播伴有受激共振拉曼散射过程发生,斯托克斯(Stokes)电场强度在XFEL传播过程中逐渐增强,其增益机制由放大的自发辐射转化为无粒子数反转激光。在脉冲传播过程中,由于较强的受激辐射过程,俄歇电子的相对产率降低。XFEL与介质较强的非线性相互作用导致XFEL在传播过程中出现明显的压缩和减速现象。4、共振X射线散射光谱与瑞典MAX-Lab实验组和意大利的Elettra实验组合作,我们从实验和理论两方面对多原子分子的共振散射光谱进行了充分研究。共振激发可以得到较强的光谱信号,共振散射光谱技术可以通过对物质进行选择性激发来研究物质内部局域部分的电子结构和化学成键信息。首先,我们建立了液相下的共振X射线拉曼散射(RXS)理论,研究了液相丙酮的RXS光谱。液相环境分子间的静电相互作用是共振非弹性X射线拉曼散射(RIXS)光谱展宽的主要机制,而对共振弹性X射线拉曼散射(REXS)光谱影响较小。汤姆逊散射大大增强了REXS中0-0跃迁谱线的强度,而实验样品的自吸收效应对REXS光谱影响较小。乙烯分子的共振俄歇散射(RAS)光谱显示,由于多个简正模的共同激发,与光电子谱(PES)相比,RAS光谱可以映像出更多的振动信息。5、X射线诱导荧光谱(XIFS)的反冲分裂我们提出了一种新型的探测反冲效应的实验方案—X射线诱导荧光(或二次吸收)光谱的反冲分裂。在强X射线场作用下,光电离过程产生的分子离子会获得较大的反冲动量,离子动量的各向异性导致了荧光发射(或二次吸收)的各项异性。我们以氮气分子为例说明该一般性理论,研究表明这种基于反冲效应的荧光光谱的多普勒分裂约为10μeV。实验上可以借助先进的同步辐射光源或新型的XFEL,通过傅里叶转换光谱技术或激光吸收光谱技术进行测量。本论文分为九章,第一章为综述,回顾了人工光源的发展历史,介绍了超短超强激光脉冲技术的发展和应用,并对非线性光学现象和X射线光谱学做了简单介绍。在第二章和第三章中,介绍了本论文研究过程中用到的基本理论和数值计算方法。第四章到第八章是基于上述基本理论和方法所做的研究工作。最后,在第九章中对主要研究内容和工作创新点进行了总结,并对所研究领域的发展做了展望。更多还原

【Abstract】 New generations of ultrashort and intense laser pulses as well as high power synchrotron radiation sources and x-ray free-electron lasers evoke people’s curiosity and motivate us to study the interaction between strong laser field and the matter, which has promoted a rapid development in nonlinear optics and X-ray spectroscopy. The main aim of this thesis is to explore the micro-mechanisms of the interaction between the intense laser pulses and the media, and to explain and predict new phenomena in nonlinear optics and X-ray region.The objectives of this thesis mainly focus on following aspects: the optical limiting behavior of the organic molecular media, the propagation of X-ray free-electron laser pulses in atomic media, resonant X-ray scattering spectroscopy, and so on. Our studies achieve many novel results in nonlinear optics and X-ray spectroscopy, which is significant not only for the further development of theories but also for the guidance of the experimental work. The main contents and results are summarized as follows:1. TPA induced optical limiting effectBy solving the full-wave Maxwell-Bloch equations with FDTD and predictor-corrector numerical methods, we simulate the propagation of the few-cycle femosecond laser pulses in a strong TPA medium ( 4,4’? bis(dimethylamino) molecules), and investigate the optical limiting behavior of this TPA medium. Our numerical results show that for ultrashort pulses one-step coherent TPA is dominating, and the medium exhibits a good optical limiting performance during pulse propagation. The dynamical TPA cross section of the 4,4’? bis(dimethylamino) molecule is calculated using the output-input peak-intensity relation. It is found that the dynamical TPA cross section is a linearly decreasing function of the incident intensity when the incident intensity is not high enough to induce the TPA saturation. The photoionization has an obvious influence on the TPA process increasing the input pulse intensity. The dynamic optical limiting window becomes wider when the photoiozation rate is larger.2. RSA induced optical limiting effectWe have studied the dynamics of picosecond pulse trains propagating in fullerene C6 0, and given an insight into the mechanism of the optical limiting behavior based on reserve saturable absorption. Due to the short duration of the subspulse, the intersystem crossing (ISC) between the singlet states and triplet states has no time to occur during single short pulse, and the whole system studied can be devided into two independent singlet and triplet subsystems. However, in the time domain between the subpulses, the population of the singlet excited state will relax to the lowest triplet state through the ISC process. Thus two subsystems are coupled with each other. The front subpulses are mainly affected by the linear absorption between singlet states, while the latter subpulses experience the excite-state absorption due to the accumulative nonlinearity. Different absorption mechanisms result in the optical limiting effect. The optical limiting materials should have long excited-state lifetime and a large photon absorption cross section. We suggest a new experimental method to measure the lifetime of the triplet state, in which the lifetime of the triplet state can be determined by scanning the transmittance versus the repetition of the pulse train.3. The dynamics of the XFEL in the atomic mediaWe simulate the propagation of x-ray free-electron lasers (XFELs) through the atomic media (Ar or Mg) for the first time by numerically resolving the Maxwell-Bloch equations. The mechanisms of the interaction between XFEL and the media have been explained in terms of the work function of the XFEL field. The propagation of XFEL field is accompanied by the stimulated resonant Raman scattering (SRRS). The intensity of Stokes field increases during XFEL propagation, and the main reason for the gain of the Stokes field is qualitativleky changed from the amplified spontaneous emission (ASE) to lasing without inversion (LWI). During pulse propagation, the relative Auger yield is suppressed due to the strong stimutated emission. The compression and slowdown of XFELs are observed because of a strong nonlinear interaction between XFELs and the media.4. Resonant X-ray scattering spectroscopyWe explore the resonant X-ray scattering spectroscopy from the polyatomic molecules both theoretically and experimentally in collaboration with MAX-Lab and Elettra (Italy) experimental group in Sweden. Due to the resonant condition, we can obtain stronger singnal of the spectra. Resonant scattering spectroscopies can be used to study the electronic structures and chemical bonding information of the local section by exciting the molecules selectively. First, we develop the theory of resonant X-ray Raman scattering (RXS) in liquid phase and study the RXS spectra from liquid acetone of superhigh resolution at the first time. The static interaction between the molecules in liquid is the major mechanism of the spectral broadening of RIXS. This static broadening has little influence on the REXS band. The 0-0 spectral line of REXS is enhanced strongly due to Thomson scattering. The effect of self-absorption on REXS spectrum is suppressed because of off-resonant absorption by carbon. Second, we study the resonant Auger scattering (RAS) spectra from ethylene molecule. Multimode simulation results are in a good agreement with experimental spectra measured in MAX-Lab laboratory. RAS show much stronger vibrational heating of the ethylene molecule in comparison with the photoelectron spectrum (PES) due to strong multi-mode core-excitation.5. Recoil splitting of X-ray induced optical fluorescenceWe propose a new experimental scheme of detecting the recoil effect, which is called recoil splitting of X-ray induced optical fluorescence or absorption lines. In x-ray regime, the ionized atoms or molecules obtain a large recoil momentum due to the large momentum of the photoelectron. The anisotropy of the recoil velocity distribution of the ions leads to the anisotropy of the subsequent optical fluorescence or absorption. The relevant theory is developed and illustrated with the nitrogen molecule. It is found that the order of the recoil induced Doppler splitting is about 10μeV. In experiment, this phenomenon can be observed using Fourier or laser absorption spectroscopic techniques with the help of power synchrotron radiation sources and x-ray free-electron lasers.This thesis is divided into nine chapters. The first chapter is an overview, which gives a brief review of the history of artificial light sources, the development and application of the ultrashort and intense laser pulses. The developing process of the nonlinear optics and X-ray spectroscopy is also described in this section. In the second and third chapters, the fundamental theories and numerial calculation methods used in our study are outlined. From the fourth chapter to eighth chapter, my own study works are shown here. At last, the conclusions of the contents and novel results, and the prospect of future work are given.更多还原