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题名	激光等离子体产生太赫兹及高次谐波的研究
作者	李娜
文献子类	博士
导师	刘鹏
关键词	强激光场 Intense laser fields 激光等离子体 laser induced plasma 太赫兹辐射 terahertz radiation 团簇 cluster 高次谐波 high harmonics generation
其他题名	Terahertz radiation and high-order harmonic generation from plasma induced by intense laser fields
英文摘要	超快强激光场与气体原子分子相互作用表现出丰富的非微扰论强场特性，如阈上电离、高次谐波和等离子体光丝产生太赫兹辐射等。对这些现象的研究进展带来了阿秒脉冲产生、分子轨道成像等。超强超短激光技术的迅猛发展，极大地提高了人们认识客观世界的时间分辨和空间分辨能力。本论文针对本研究方向上的两个课题：等离子体太赫兹辐射的机理和团簇高次谐波辐射中的电子动力学，开展研究。在激光等离子体产生太赫兹的研究中，首次从实验上证实等离子体振荡电流对太赫兹光谱的贡献，并修正了等离子体-光电流理论模型。在团簇产生高次谐波的研究中，分析了团簇内电场对高次谐波的影响，实验首次观测到团簇产生谐波的相位匹配峰，确定团簇产生高次谐波产率相比于原子的增强倍数。这些结果为宽带太赫兹光谱的调控、利用体相材料产生增强高次谐波并揭示其机理提供了思路和可能。 本论文的主要内容和创新点包括： 1、独立搭建真空分子束太赫兹辐射和检测装置，实现激光等离子体密度和空间尺寸的调节，研究了双色强激光场驱动氮气分子束所产生的宽带太赫兹光谱的变化规律。在较低等离子体密度，发现太赫兹光谱依赖于等离子体共振频率的频移现象，验证了等离子体振荡对太赫兹光谱的贡献。在较高等离子体密度，发现太赫兹频带趋于常数，通过引入等离子体衰减常数修正现有等离子体-光电流模型，可以对实验结果较好地解释。 2、基于以上研究，提出并实验验证了调控太赫兹的频率和偏振度的方案。根据太赫兹光谱随短程等离子体密度和长度的依赖关系，可以通过改变等离子体密度来控制其中心频率，通过改变碰撞频率来控制太赫兹谱宽；该实验条件下的短程等离子体密度还可以用来精细调节驱动双色激光场的相位，从而改变太赫兹波的偏振态，通过增加背压，太赫兹从线偏振变为椭圆偏振态。 3、对团簇产生高次谐波的研究中，利用线性无碰撞吸收理论推导出团簇在强场电离后内部的电子运动产生瞬态诱导场。进一步模拟发现内电场与激光场的相干叠加形成增强的内电场，提高高次谐波的产率，并改变电子波包的量子轨迹，造成团簇和原子相位匹配位置的分离。实验上检测到团簇和原子各自独立的相位匹配条件，验证了团簇内的电子振荡，分析得到团簇高次谐波振幅的提高（团簇比气体原子高5个量级）和单原子产率的提高（团簇内原子比气体中单个原子高4倍）。; Interaction of intense laser fields with gaseous atoms/molecules reveals characteristic phenomena such as above-threshold ionization (ATI), high harmonic generation (HHG) and terahertz (THz) radiation from the laser induced plasma. Progress of the study brings the development of attosecond methodology and molecular orbital imaging which push the extreme of understanding the universe in time and space. This thesis aims at two topics: mechanism of THz radiation from laser induced plasma and electron dynamics in the HHG of atomic clusters. For the former, the contribution of plasma current during THz emission is observed in experiment for the first time and the amendment to the available plasma - photocurrent model is made. For the latter, the formation of internal field of Xe clusters is deduced using the linear theory of collisionless absorption and the phase matching condition of clusters is experimentally isolated from atoms for the first time. The enhancement of HHG from clusters over atoms is identified. The results provide the basis for tuning the THz spectrum and effectively boosting the efficiency of HHG. 1. The candidate set up a vacuum chamber for the generation and detection of THz radiation from a molecular beam inside. Through adjusting the density of laser induced plasma and its penetrating length, the THz spectra as a function of the changing plasma conditions are recorded and analyzed. At the low density of plasma, the central frequency of THz is found to follow the variation of plasma oscillation frequency determined by the plasma density. This identifies the plasma frequency plays a dominant role on the THz spectrum as the skin depth of plasma is larger than its length. At the high density, observed THz frequency turns to be a constant, which cannot be explained by the plasma - photocurrent model. A plasma decaying factor is introduced into the model to give a consistent simulation result. 2. Based on the mechanism and the experimental set-up, methods of controlling THz frequency and polarization state are proposed. The plasma density and length can be adjusted to control the central frequency of THz emission. Also, the plasma density can modify the phase difference of the pumping two-color fields and tunes the ellipticity of THz wave from linearly polarized to be elliptically polarized. 3. For the HHG from clusters, the inside movement of tunneling ionized electrons is deduced by applying the linear non-collisional absorption theory. The electrons are found to oscillate owing to the induced polarization by external field. At certain conditions, the oscillation induced field enhance the external laser field, resulting the enhancement of HHG and modification of phase accumulation of electron wavepacket comparing with atoms. Experimentally an isolated phase matching peak for clusters is identified, confirming the theoretical prediction. The enhancement of HHG from clusters is determined to be 5 orders higher than atoms and 4 times for averaged atom response.
学科主题	光学工程
内容类型	学位论文
源URL	[http://ir.siom.ac.cn/handle/181231/30962]
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	李娜. 激光等离子体产生太赫兹及高次谐波的研究[D].

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