【作者】 于承新；

【导师】 刘世炳；

【作者基本信息】 北京工业大学 ， 物理学， 2013， 博士

【摘要】 当飞秒强激光脉冲和原子相互作用时,将发生一系列的极端非线性现象,包括阈上电离,高次谐波的产生等。本文研究了飞秒强激光脉冲在场电离介质中的传播、激光能量的转换、高次谐波产生及其特征,以及强场绝热参数的推广等问题。本文取得的主要进展如下：1.在强场原子过程中,由于采用了强场近似,库伦势能的作用通常被忽略掉。通过将库伦场引入到激光脉冲在场电离介质中的传播方程中,研究了库伦效应在能量转换过程中扮演的角色。对于激光在介质中的传播过程,库伦场修正的引入使得自由电子的运动发生改变,从而影响了电子局域密度分布。这又影响了激光脉冲能量通过电子加速过程的发生转换和损耗。当激光脉冲开始传播,比如大约一个光波长时,激光能量主要通过原子场电离和电子加速形式发生损耗。随着脉冲的进一步传播,辐射损失逐渐变得重要。这是因为,当脉冲变得较弱时,大量电子进入到库伦场主导的区域,并与原子核发生碰撞。通过激光辅助逆轫致吸收过程,导致该形式的能量损耗。2.在强场原子过程中,通过唯象的方式将电子离子复合引入到电离速率方程中得到了电离-复合速率方程。利用这个推广的速率方程,重新推导了飞秒强激光脉冲在气态场电离介质中的传播方程。发现了强场物理过程中的一些新的物理行为。通过加入电子-离子复合机制,研究了飞秒激光脉冲在场电离介质的传播。电子-离子复合使得介质的饱和电离被延迟,有一些原子将会经历多次电离,从而导致更多的激光能量在原子电离过程被消耗掉。能够发生复合的场电离电子在激光能量转换过程中起到的作用是显著的,它们在激光前部吸收能量,然后到激光脉冲的后部释放出来,从而改变了激光的时域形状。3.通过引入半周期操控激光脉冲,并适当地调节相对延迟时间,使得操控脉冲对自由电子的运动路径进行调控,从而使其能够以最大的动能返回到原子核附近,然后被原子核俘获,释放出高能光子。数值计算表明,通过优化半光周期操控脉冲与驱动激光脉冲的相对时间延迟,在合适的相位延迟条件下,半光周期操控脉冲能够显著地增加原子电离速率及增强电子的返回动能,从而产生最大截止频率的高次谐波辐射。在操控脉冲存在的情况下,高次谐波的截止阶次达到了177次,而对于单束驱动激光脉冲,谐波截止阶次仅有53次。为了深入理解电子波包的动力学,引入了以Gabor变换为基础的相空间分析。分析表明,对于最优的延迟时间,大量电子由驱动激光电离出来,并被加速一段时间,操控激光使得这些电子在到达计算边界前速度减为零,从而使得这些电子有更长的加速距离。然后这些电子在组合激光脉冲作用下朝着原子核运动,当它们回到原子核附近时,以较大的动能与原子核发生复合,辐射出高次谐波。4.在强场物理中,Keldysh绝热参数在不同电离机制区分中扮演着不可替代的重要角色。然而随着激光强度的增加,由于隧穿势垒的消失,该参数失去了它的地位。通过引进库伦势垒,将该参数进行推广,使得它能够将激光物质相互作用区域分为三个部分。结果表明,推广的参数在弱场极限下与传统的Keldysh参数等价,另外,还给出了需要考虑相对论效应的一个判据。并且对每个问题分别编制了相应的计算程序。程序语言为FORTRAN,数据的处理靠MATLAB完成。更多还原

【Abstract】 The interaction of an intense femtosecond laser pulse with atoms and moleculesgives rise to a variety of nonlinear phenomena, including above-threshold ioniza-tion, the generation of high-order harmonics (HHG), etc. In this thesis, the prop-agation of an ultraintense femtosecond laser pulse through a feld-ionized medium,the laser energy transfer, the plateau extension of the frequency spectrum of HHGand the generalization of Keldysh adiabatic parameter are investigated. The mainachievements are as follows:1. By bringing the Coulomb potential into the laser propagation through a feld-ionized media, we investigate the role of the Coulomb efect plays in the en-ergy conversion processes. The results shows that the abstraction of Coulombpotential alters the local density distribution of ionized electrons, and a newmechanism of laser loss called laser-assisted inverse bremsstrahlung absorp-tion is introducing into the laser propagation equation.2. A recombination rate of electron-ion in the strong-feld atomic process is phe-nomenologically introduced into the ionization rate equation, and therefroman ionization and recombination rates equation (IRRE) is obtained. By usingthe extended IRRE, the propagation equation of an intense femtosecond laserpulse in the gaseous medium is re-derived.3. A manipulatable half-cycle light pulse, homochromy with the driving fem-tosecond laser, is applied to enhance the harmonic emission by optimizingthe delay time between the driving and manipulating light pulses in thestrong-feld atomic process. We fnd that, by numerical computation, thehalf-cycle manipulating light can signifcantly increase the atomic ionizationrate supplementarily and enhance the return kinetic energy of the electrons by a suitable phase delay. The results show that the harmonic order of themaximum or cut-of photon energy emitted in the presence of the half-cyclemanipulating light pulse is risen to the177th which is a signifcant increasecompared with the53rd order harmonics in the case of single driving lightpulse.4. By accounting for the Coulomb potential, we present a generalized version,which can divide the interaction into three domains that have evident bound-aries. The results show that the generalized parameter is equivalent to thetraditional Keldysh parameter in the weak feld limit.The corresponding computer programs are compiled using the FORTRAN languageand the data are processed by MATLAB.更多还原