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题名	相对论激光与薄靶作用过程中的高次谐波和类光栅结构
作者	郁亚红
学位类别	博士
答辩日期	2014
授予单位	中国科学院上海光学精密机械研究所
导师	沈百飞
关键词	超短超强激光脉冲，等离子体，高次谐波
其他题名	High harmonics generation and grating-like structure in ralativistic laser-foil interaction
中文摘要	随着激光啁啾脉冲放大（CPA）技术的应用，激光强度大幅度提高。近十年来，激光脉冲强度达到相对论强度，脉冲宽度短至数十甚至数飞秒(fs， s)，强激光与等离子体的相互作用的研究迅速发展。目前，激光在稀薄等离子体中驱动的尾场加速已经可以得到能量达到数GeV( eV)的准单能电子束。通过激光与固体等离子体靶相互作用，可以得到高强度的高阶高次谐波及单个阿秒脉冲。这种超短的激光脉冲可应用于操控原子、分子世界，物质的探测以及生物制药等等。相对论激光与靶相互作用过程中产生的高次谐波可由两种理论模型给出解释：相干尾波辐射机制（CWE）和相对论振荡镜模型(ROM)。本论文基于激光与等离子体相互作用，做了以下几方面的工作： 1. 通过解析和PIC模拟研究了相对论激光与等离子体薄靶相互作用过程中产生的高能辐射。已有的研究指出当相向传播的两束圆偏光垂直入射到薄靶的两个表面时，靶内电子被压缩在一定的空间范围内，形成一个稳定的高密度电子薄层。层内电子在横向上剧烈振荡并且辐射高次谐波。激光与靶之间存在一个匹配条件，在此条件下才能高效的产生高阶辐射。为了进一步增强高阶部分的辐射强度，引入第三束高频低强度的线偏光。解析和模拟结果均表明高阶辐射的强度与圆偏光和第三束线偏光之间的相对相位差有关。当改变两束光之间的相位差时，高频辐射的强度随着相对相位差的值周期变化。这不仅可以用于产生强的高阶辐射，也提供了一种测量激光相对位相的方法。 2. 两束相向传播的圆偏振激光垂直入射到一等离子体薄靶时，我们发现靶内电子的密度分布出现类光栅的结构。在不考虑离子运动的情况下（部分电离的重离子），该高密度分层结构（其密度峰值相对于初始电子密度高一到两个数量级）的演化周期为半个激光周期。在激光作用结束后，电子密度分布会逐渐回到均匀的状态。而对于轻离子靶（电子被完全电离），在考虑离子的运动的情况下，在激光作用过程中离子和电子均会出现分层结构。并且在激光与靶作用结束之后，电子会在较短的时间内与离子运动实现同步，两者之间这种同步的分层结构可保持几十个激光周期。等离子体的初始温度会对这种分层现象有一定的影响。我们的研究表明当超薄靶被两束激光约束在一定的空间范围内，靶内可以形成纳米级别的类似光栅的结构。其光栅常数仅为激光波长的几百分之一。 3. 此外，在激光与稀薄等离子体相互作用方面我们也作了一些初步的工作，主要集中在相位突变的激光对尾波场加速的注入机制的一些研究。无相位突变的平面激光与自由电子作用后，电子恢复到初始状态。控制相位突变的参数，当电子与相位突变激光作用后，使得电子获得的动量主要集中在激光传播的前向方向。在激光尾波加速中，这样的电子更容易被等离子体空泡捕获而得到加速，使得电子注入更容易。在相同的激光条件下，可以在更低密度下实现电子的自注入，从而得到更长距离的加速，获得更好的加速。 本文的主要内容集中在第一部分和第二部分的工作，第三部分简单给出了初步尝试的结果。
英文摘要	The chirped pulse amplification(CPA) and subsequent developments make the laser pulse short to fs scale with relativistic intensity, which promotes the rapid development of laser-plasma interaction research. Electron beams with energy over GeV, quasi-monoenergetic electron spectrum, and high stability are beautifully experimentally obtained by several groups. High order harmonics and picosecond pulses have been produced when applying an electromagnetic field across the plasma, which play important roles in industry, material science, medicine, electron dynamics on the atomic timescale and so on. The generation of high order harmonics in laser-foil interaction is based on two models: coherent wake emission(CWE) and relativistic oscillating mirror(ROM). This thesis has the following parts, all of which are based on the theory of relativistic laser plasma interaction. 1. High harmonic generation (HHG) from relativistic laser-foil interaction is investigated analytically and through particle-in-cell (PIC) simulations. Previous work has shown that when two counter-propagating circularly polarized (CP) laser pulses interact with a thin foil, electrons can be well confined spatially to form a high density layer. The layer electrons oscillate in certain transversal direction and radiate intense high order harmonics. It is demonstrated here that there is a critical foil thickness, only below which can high harmonics be generated efficiently. Furthermore, to enhance the intensity in higher order region, the third linearly-polarized (LP) short-wavelength laser pulse with much lower intensity is introduced. Analysis and simulations both show that the enhancement is determined by the relative phase between the driving CP laser pulses and LP pulse. The enhancement at high order is quite considerable and very sensitive to the relative phase , thus offering not only a way to efficiently produce HHG but also a new method to measure the phase of intense high-frequency laser pulses. 2. An interesting layered structure of multiple high density layers are formed when two counter-propagating circularly polarized laser pulses with the same polarization direction irradiate on an ultra-thin foil. This structure changes periodically. When the ions are static, the peak density of this layered structure is two orders of magnitude of the original density. After the interaction, the electron distribution goes back to the original situation. For light atoms most of which electrons may be fully ionized, the evolution of electrons’ distribution is similar with the former one and the evolution of the ions is quite interesting. At the end of the laser-foil interaction, the ions in the foil are compressed to several high density layers. Then the electrons catch up with the ions, and form similar layered structure soon. This structure can be hold in dozens of laser cycles. This interesting structure may have potential applications in high density grating making. 3. After the interaction with a phase jump laser pulse, the electron gains great momentum in the direction of propagation. These electrons are more easily trapped in laser wake field. That is to say, electrons can be injected and trapped into the wake field at a lower density and be accelerated to a higher energy.
语种	中文
内容类型	学位论文
源URL	[http://ir.siom.ac.cn/handle/181231/15879]
专题	上海光学精密机械研究所_学位论文
推荐引用方式 GB/T 7714	郁亚红. 相对论激光与薄靶作用过程中的高次谐波和类光栅结构[D]. 中国科学院上海光学精密机械研究所. 2014.

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