短脉冲激光辐照下SiO2损伤微观机理简化模型

 从电子密度速率方程出发，建立短脉冲激光辐照下SiO₂材料中导带电子增长简化模型，计算了SiO₂中光致电离速率和电子雪崩速率，得到SiO₂激光损伤阈值与脉冲宽度的关系，计算分析了光致电离和碰撞电离两种电离机制在导带电子累积过程中的不同作用。结果表明：脉冲较长，碰撞电离几乎能提供全部的导带电子，激光损伤阈值与脉宽的0.5次方成正比；脉冲较短时，导带电子主要由碰撞电离产生，光致电离提供碰撞电离的初始电子，激光损伤阈值随着脉宽的减小，先增加后减小。     

低于阈值激光功率密度下Mg原子的非序列双电离

基于经典系综模型,研究了Mg原子在低于重碰撞阈值激光功率密度下的非序列双电离(NSDI)。当少周期量级线性偏振激光的功率密度为3.0×10~(13) W·cm~(-2)时,末态关联电子动量分布中的电子对主要分布在第一、第二和第四象限,第一象限中的电子对呈现了明显的关联行为。分析了重碰撞和双电离之间的延迟时间,发现不同的延迟时间对应着不同的电离过程,延迟时间对电子出射过程具有显著影响;延迟时间小于半个周期的NSDI事件,双电子倾向于反方向出射,而延迟时间大于半个周期的,存在双电子同方向出射的可能。     

飞秒光物理与介观光学研究获新进展

 北京大学“飞秒光物理与介观光学”创新研究群体采用周期量级飞秒激光系统和冷靶反冲离子动量谱仪装置,首次在实验中通过改变周期量级飞秒激光的载波相位,实现对一氧化碳分子二价电离态的非对称解离通道的控制,同时也实现分子三价离子的非对称解离通道的控制。通过实验证实在一氧化碳分子电离和解离过程中,电子再碰撞激发引起的解离以及电子再碰撞引起电离是主要过程,这两个过程是相互竞争的,并可通过改变激光脉冲载波相位进行调控。该研究工作表明,通过超快光场控制电子再散射波包与分子离子的相互作用,可以在分子电子态水平上实现对分子高阶电离和解离通道过程的操控。研究工作发表在Physical Review Letters 106,073004（2011）。
研究工作得到国家自然科学基金委“创新研究群体”项目,国家重大基础研究计划（973）项目和北京大学人工微结构和介观物理国家重点实验室的资助。     

低于再碰撞阈值下强激光场原子非序列双电离

在最近的实验和理论研究中,我们探讨了氩原子和氖原子在红外强激光场中低于再碰撞阈值的非序列双电离问题。在研究中,我们发现在非序列双电离过程中,氖原子的电子关联表现为在激光偏振面内肩并肩出射,而对于氩原子的电子关联行为表现为在激光偏振面内的背对背出射,我们采用三维半经典模型（考虑电子隧道电离）很好地解释了实验结果。在阈值附近,我们发现电子在激光场中的多次散射以及电子再碰撞激发后电子隧道电离是氩原子反关联行为的主要原因,而电子在激光场作用下的单次碰撞是电子关联行为的主要原因。通过测量双电离过程中产生电子的横向电子动量分布,观察到了库伦聚焦效应,我们认为这是非经典的关联行为。最后,我们给出了氩原子和氖原子在激光场中阈值的解析模型,并给出了原子的关联和反关联激光强度区域.     

高功率微波沿面闪络击穿机制粒子模拟

针对高功率微波介质沿面闪络击穿物理过程,首先建立了理论模型,包括:动力学方程、粒子模拟算法、二次电子发射, 以及电子与气体分子蒙特卡罗碰撞模型、电子碰撞介质表面退吸附气体分子机制;其次,基于理论模型,编制了1D3V PIC-MCC程序,分别针对真空二次电子倍增、高气压体电离击穿和低气压面电离击穿过程,运用该程序仔细研究了电子和离子随时间演化关系、电子运动轨迹、电子及离子密度分布、空间电荷场时空分布、电子平均能量、碰撞电子平均能量、碰撞电子数目随时间演化关系、电子能量分布函数、平均二次电子发射率以及能量转换关系。研究结果表明:真空二次电子倍增引发的介质表面沉积功率只能达到入射微波功率1%左右的水平,不足以击穿;气体碰撞电离主导的高气压体电离击穿,是由低能电子（eV量级）数目指数增长到一定程度导致的,形成位置远离介质表面,形成时间为s量级;低气压下的介质沿面闪络击穿,是在二次电子倍增和气体碰撞电离共同作用下,由于数目持续增长的高能电子（keV量级）碰撞介质沿面导致沉积功率激增而引发的,形成位置贴近介质沿面,形成时间在ns量级。     

频域图像下的强场非序列电离过程

自20世纪60年代激光发明以来,激光与物质的相互作用就一直成为物理学领域的一个重要研究方向.通过最近几十年激光技术的发展,大大拓展了激光的频率、强度及脉宽范围,使得复杂体系在激光场中的激发、辐射及电离过程得到更精细而深入的研究.本文总结了处理单色和双色激光场中双电子原子非序列电离的频域理论;归纳了碰撞-电离和碰撞-激发-电离两种机理下原子非序列电离在单色和双色激光场中的动量谱分布,并对动量谱上的干涉条纹利用量子通道相干的理论进行了分析;归纳了前向碰撞和背向碰撞在不同激光场条件下对非序列电离的不同贡献,以及高频激光场在非序列电离中所起的作用.     

强激光与高密度气体相互作用中电子和离子加速的数值模拟

在现有的一维粒子模拟程序的基础上发展了带光电离和碰撞电离及蒙特卡罗两体碰撞的模拟程序(1D PIC-MCC). 用此程序模拟研究了短脉冲激光与He气靶相互作用时电子和离子的加速过程. 研究表明当强激光与过临界密度的微米厚度的平面靶相互作用时，靶前表面物质将被激光脉冲前沿迅速离化；新生的电子被激光场有质动力加速成为高能电子，这些电子穿入到靶内，通过电子碰撞电离离化靶内物质；一部分高能电子穿透靶后，会在靶的后表面形成强的电荷分离场，该场迅速离化靶后表面物质，同时使得后表面离子得到加速. 部分穿透靶的超热电子将被电荷分离场重新拉回靶内，在靶的前后表面振荡. 一些振荡电子在此过程中得到电荷分离场加速，离开前表面，在前表面也形成电荷分离场，使前表面离子得到加速. 关键词： 激光等离子体 光电离和碰撞电离 电子加速 离子加速     

等离子体屏蔽对He2+与H原子碰撞电离微分截面的影响

应用经典径迹Monte Carlo（CTMC）方法研究了He²⁺与H原子在等离子体环境下的碰撞电离过程,计算了在5—400 keV/u的能区随等离子体屏蔽作用变化的碰撞电离总截面和一阶微分截面.等离子体中带电粒子之间的相互作用采用Debye-Hückel模型来描述.由于等离子体屏蔽效应的存在,靶中束缚态电子能级及其经典微正则分布以及入射离子与靶电子的相互作用都发生了变化,而这些变化会直接影响碰撞电离过程.研究发现,碰撞电离总截面随等离子屏蔽的增加而增大,特别是在10 keV/u以下的低能区电离截面有量级的增加.对随能量变化的一阶微分截面,在低能碰撞过程中,屏蔽作用增加,微分截面呈量级增加,高能碰撞微分截面呈倍数增加.同时,屏蔽作用导致电离电子向高能方向移动,随着碰撞能量的增加两体碰撞机制的贡献越来越大,并在较高的出射电子能量出现了一个新的峰.对无屏蔽的自由原子碰撞过程,CTMC方法计算出的电离总截面在碰撞能量大于70 keV/u的较高能区在实验误差内与实验测量结果符合很好,而在较低的能区比实验值小30%—50%. 关键词： 重粒子碰撞电离 等离子体屏蔽效应 经典径迹Monte Carlo方法 Debye-Hückel模型     

Nb,Ag原子的K壳层和L壳层电离截面的理论计算

电子离子碰撞电离截面是模拟激光等离子体的超热电子的能谱和产额的主要过程之一.基于相对论性的电子离子碰撞的K壳层电离截面理论,计算了Nb、Ag的K壳层和L壳层电子碰撞电离截面,结果和最近的文献实验数值和其它理论数值进行了比较,计算结果比其它模型更加准确,与最近的实验结果也吻合较好,该结果可用来模拟激光等离子体的超热电子能谱和产额.     

飞秒激光在石英玻璃中诱导微爆炸的理论研究

利用有限元方法建立了二维模型，研究了飞秒激光作用下石英玻璃中导带电子的产生、激光 能量的沉积、导带电子和能量扩散等微观过程. 计算了导带电子扩散引起的局部净电荷及其 形成的静电场分布，初步揭示了微爆炸的演化过程. 关键词： 雪崩击穿模型 微爆炸 飞秒激光 有限元方法     

Correlated multielectron dynamics in ultrafast laser pulse interactions with atoms

We present the results of the detailed experimental study of multiple ionization of Ne and Ar by 25 and 7 fs laser pulses. Whereas in multiple ionization of Ar different mechanisms, involving field ionization steps and recollision-induced excitations, play a role, for Ne only one channel, where the highly correlated instantaneous emission of up to four electrons is triggered by a recollisional electron impact, is found to be important. Using few-cycle pulses we are able to suppress those processes that occur on time scales longer than one laser cycle.     

Single and double ionization of the hydrogen molecule in an intense few-cycle laser pulse

In this paper, we present ab initio two-electron model calculations of laser-induced single and double ionization of the hydrogen molecule in a linearly polarized laser field with static nuclei located along the polarization axis. Within the model, the center-of-mass motion of the two electrons is restricted along the polarization axis of the field, while the relative electron motion is unrestricted. The results of numerical simulations allow us to identify and characterize the mechanisms leading to single and double ionization in an intense few-cycle laser pulse. The role of the rescattering mechanism on the ionization processes is analyzed in particular.     

少周期激光脉冲与气体作用产生的离化电流和THz波辐射

本文通过理论和数值模拟,研究少周期激光脉冲电离气体原子产生的离化电流 以及相应的THz波辐射.研究表明,少周期激光脉冲离化气体后能产生较大的离化电流, 因而可以产生较强的THz辐射.不同的少周期激光脉冲相位导致电离出的 电子初始速度和电离起始时刻不同,从而产生的离化电流有所不同, 辐射的THz波随激光脉冲的相位成周期性变化.该理论得到一维PIC数值模拟的验证. 对于给定的激光脉冲相位,离化电流和THz辐射振幅并没有随入射激光振幅的增加而单调增加, 而是存在一些极值点.与均匀分布气体相比,当气体分布具有一定梯度时, 辐射表现相似的规律,但频谱会发生一定的变化.     

Carrier-envelope-phase effect in nonsequential double ionization of Ar atoms by few-cycle laser pulses

A classical ensemble method is used to investigate nonsequential double ionization(NSDI) of Ar atoms irradiated by linearly polarized few-cycle laser pulses. The correlated-electron momentum distribution(CMD) exhibits a strong dependence on the carrier-envelope phase(CEP). When the pulse duration is four cycles, the CMD shows a cross-like structure, which is consistent with experimental results. The CEP dependence is more notable when the laser pulse duration is decreased to two cycles and a special L-shaped structure appears in CMD. Recollision time of returning electrons greatly depends on CEP, which plays a significant role in accounting for the appearance of this structure.     

Phase-dependent excitation and ionization in the multiphoton ionization regime

We theoretically study the dependence of atomic excitation and ionization on the carrier envelope phase of few-cycle laser pulses in the multiphoton ionization regime. Our theoretical results for the hydrogen atom based on the solution of the 3D time-dependent Schr?dinger equation show that the strong phase dependence can be seen in not only total ionization, but also bound-state population under the weak laser intensity regime.     

Experimental ionization of atomic hydrogen with few-cycle pulses

We present experimental data on strong-field ionization of atomic hydrogen by few-cycle laser pulses. We obtain quantitative agreement at the 10% level between the data and an ab initio simulation over a wide range of laser intensities and electron energies.     

Control of atomic ionization by two-color few-cycle pulses

We have studied the ionization of Rydberg atoms by few-cycle radio-frequency pulses and used two-color fields to control the ionization dynamics. We show that the number of times that electrons are emitted during a pulse can be limited and that the duration of the electron emission can be shortened. These results, once they are transposed to the optical domain, may inspire new strategies for the production of single attosecond pulses.     

Ionization of atoms by few-cycle EUV laser pulses: carrier-envelope phase dependence of the intra-pulse interference effects

We have investigated the ionization of the H atom by intense few-cycle laser pulses, in particular the intra-pulse interference effects, and their dependence on the carrier-envelope phase (CEP) of the laser pulse. In the final momentum distribution of the continuum electrons the imprint of two types of intra-pulse interference effects can be observed, namely the temporal and spatial interference. During the spatial interference electronic wave packets emitted at the same time, but following different paths interfere leading to an interference pattern measurable in the electron spectra. This can be also interpreted as the interference between a direct and a scattered wave, and the spatial interference pattern as the holographic mapping (HM) of the target. This HM pattern is strongly influenced by the carrier-envelope phase through the shape of the laser pulse. Here, we have studied how the shape of the HM pattern is modified by the CEP, and we have found an optimal CEP for the observation of HM.     

Interference oscillations in the angular distribution of laser-ionized electrons near ionization threshold

We analyze the two-dimensional momentum distribution of electrons ionized by few-cycle laser pulses in the transition regime from multiphoton absorption to tunneling by solving the time-dependent Schr?dinger equation and by a classical-trajectory Monte-Carlo simulation with tunneling (CTMC-T). We find a complex two-dimensional interference pattern that resembles above threshold ionization (ATI) rings at higher energies and displays Ramsauer-Townsend-type diffraction oscillations in the angular distribution near threshold. CTMC-T calculations provide a semiclassical explanation for the dominance of selected partial waves. While the present calculation pertains to hydrogen, we find surprising qualitative agreement with recent experimental data for rare gases [A. Rudenko, J. Phys. B 37, L407 (2004)].     

Effect of Time-Dependent Ionization on Propagation of a Few-Cycle Circularly Polarized Laser Pulse in Two-Level Medium

Influence of multiphoton ionization on the propagation and spectrum of few-cycle circularly （elliptically） polarized laser pulses in an open two-level medium （two-level plus continuum model） is investigated based on the conventional two-level model proposed by Slavcheva and Hess （Phys. Rev. A 72 （2005） 053804）, and the propagation dynamics of an arbitrary elliptically polarized laser pulse is reduced into that of right and left circularly polarized laser pulses. When the laser intensity is high enough to cause ionization, there are significant impacts of ionization on the pulse reshaping and on the higher order spectral components, and the impacts for the open two-level model are different from those for the dosed two-level model.