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题名	气体高次谐波产生过程中若干问题的研究
作者	袁晓龙
文献子类	博士
导师	李儒新
关键词	超强超短激光 Super-intense ultra-fast laser 高次谐波 High-order harmonic generation (HHG) 激发态 Excited state 电子多次散射 Multiple electron rescatterings 原子核运动 Nuclear motion
其他题名	Research on some issues of the high-order harmonics generation in gas medium
英文摘要	近年来随着激光技术的超快发展和日趋成熟，超强超短激光脉冲系统在世界范围内的实验室中广泛建立，并在一定程度上实现了商业化。目前，激光脉冲的峰值功率已经达到拍瓦量级，激光脉冲的时域脉宽已经达到周期量级，这为强场激光物理研究提供了前所未有的实验条件，造就了进入新世纪以来强激光与物质相互作用研究的大发展，一些传统微扰理论所无法解释的极端非线性物理现象相继出现，例如：阈上电离、高次谐波及阿秒脉冲等。利用光学参量放大等激光技术可以将钛宝石激光器输出的激光脉冲转化到中红外波段，中红外驱动激光不仅可以提高高次谐波辐射的截止光子能量，还可应用于固体中电子超快动力学的研究。拍瓦量级激光脉冲更是将强场激光物理推进到相对论机制下，使人类可以在实验室中研究激光聚变快点火、天体物理、新型加速器等。 本论文瞄准飞秒激光脉冲与气体相互作用产生高次谐波过程的研究热点，重点研究了驱动激发态介质产生增强高次谐波、中红外激光驱动高次谐波过程中电子多次散射效应和H2分子原子核运动导致的谐波光谱展宽。主要研究内容及成果包括以下几个方面： 1 实验上利用泵浦探测机制实现驱动激发态气体介质产生增强高次谐波，椭圆偏振泵浦光激发产生基态与激发态相干叠加态，探测光驱动该预制备激发态介质产生高次谐波。通过控制泵浦光与探测光的相对时间延迟，观察到一个寿命在几十皮秒左右的谐波信号增强平台区，与驱动没有预激发的基态介质产生的高次谐波相比谐波强度被增强大约一个数量级。同时发现可以通过一定程度上增强泵浦光强度来增加激发态原子的布居数，进而逐步增强谐波强度。通过数值计算激光驱动相干叠加态体系的含时薛定谔方程的结果可以很好的解释该增强现象。最后，讨论了利用高次谐波信号进行气体等离子体中电子与原子碰撞探测的可能。 2 理论上研究中红外激光驱动高次谐波产生过程中电子多次散射效应。半经典模型分析电子轨道给出一个清晰的物理图像。同时配合对含时薛定谔方程计算所得的偶极矩加速度进行时频分析，发现电子多次散射随着驱动激光波长的增加贡献越来越大。强场近似下量化计算电子第一次散射辐射的高次谐波与电子多次散射辐射的高次谐波强度比值证明了上述结论。 3 实验上研究分子高次谐波产生过程中的核运动效应。H2分子高次谐波与其它具有质量较重原子核的分子产生的高次谐波光谱相比出现明显展宽。对比三种电离势相似的气体介质(H2, N2, Ar)产生的谐波强度发现H2分子高次谐波强度明显比较低。驱动激光诱导的H原子核运动会导致H2分子高次谐波强度的抑制与光谱的展宽。非波恩-奥本海默近似下含时薛定谔方程数值模拟给实验结果一个很好的理论支持。; Recently, several super-intense ultra-fast laser systems have been widely set up in some laboratories of the world, with the fast development and mature of advanced laser technology. Many techniques used in those above laser systems have been commercialized. Nowadays, the peak power of laser pulse has been achieved the magnitude of petawatt and the temporal width of laser pulse has been achieved the few cycle level, which can provide unprecedented experimental conditions for the high field laser physics and promote the development of the research of the interaction between laser and matter. Extreme nonlinear physical phenomena have been appeared, such as the above threshold ionization (ATI), the high-order harmonic generation (HHG), the attosecond pulse generation, etc., which cannot be explained by some traditional perturbation theory. The wavelength of Ti: sapphire laser system output can shift into infrared wavelengths using optical parametric amplification. The infrared driving laser not only improve the cutoff photoenergy of HHG but also can be widely used in the electron ultrafast dynamics of solid. The high field physics can be pushed into the mechanism of the theory of relativity using the petawatt laser system, which can make it possible to research laser fusion fast ignition, astrophysics, and new accelerator in laboratories. The research on the HHG from the interaction between femtosecond laser and gas medium is the main subject of this Ph. D thesis. Enhanced high-order harmonics generation from excited gas mediun, contribution of multiple electron rescatterings on high-order harmonics generation in the mid-infrared wavelength regime, and effect of nuclear motion on spectral broadening of high-order harmonics generation have been emphatically studied. Main research progress and results of this thesis are as follows： 1 We experimentally demonstrate an effective scheme to generate the enhanced high-order harmonics from a coherent superposition state of ground state and excited states in rare gas atoms. The pump pulse is used to preparing excited states and the probe pulse is used to drive excited atoms to generate high-order harmonics. By controlling the time delay between the pump and probe pulses, a surprising enhancement plateau of the high-order harmonics generation is observed, with a half lifetime of dozens of picoseconds. In the plateau, the harmonics intensity is enhanced by nearly one order of magnitude compared to the case without pre-excitation. The gradual enhancement process is also presented when the pump intensity is increasing for improving the population of excited states. The results of TDSE simulation can well explain the experimental phenomenon. Moreover, we discuss the probability of the electron-atom collision detection by using the harmonics signal. 2 We theoretically investigate multiple electron rescatterings in high-order harmonics generation with a wide range of driving laser wavelengths. In order to obtain a clear and intuitive insight, the time-frequency analysis of the dipole acceleration calculated by the numerical solution of the time-dependent Schr?dinger equation is performed and compared with the classical electron trajectory calculation. The result shows that in the mid-infrared regime, the high-order electron trajectory associated with multiple rescatterings plays a more important role than the usually referred-to “long and “short” electron trajectories. To provide quantitative evidence, the strong-field approximation is used to calculate the yield ratio of the high-order harmonics generation from the first rescattering and the multiple rescatterings. 3 High-order harmonics generation in molecular targets is experimentally investigated in order to reveal the role of the nuclear motion played in the process. An obvious broadening in the harmonics spectrum from the H2 molecule is observed in comparison with the harmonics spectrum generated from other molecules with relatively heavy nuclei. We also find that the harmonics yield from the H2 molecule is much weaker than the yield from those gas targets with the similar ionization potentials, such as Ar atom and N2 molecule. The vibrational motion of nuclear induced by the driving laser pulse can lead to the HHG yield suppression and the spectrum broadening. Moreover, the one-dimensional (1D) time-dependent Schr?dinger equation (TDSE) with the non-Born-Oppenheimer (NBO) treatment is numerically solved to provide a theoretical support to our explanation.
学科主题	光学
内容类型	学位论文
源URL	[http://ir.siom.ac.cn/handle/181231/30983]
专题	中国科学院上海光学精密机械研究所
作者单位	中国科学院上海光学精密机械研究所
推荐引用方式 GB/T 7714	袁晓龙. 气体高次谐波产生过程中若干问题的研究[D].

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