脉冲激光偏振方向对氮分子高次谐波的影响－－基于含时密度泛函理论的模拟

采用含时密度泛函方法，结合赝势模型和电子交换相关作用的广义梯度近似，模拟了氮分子在超强飞秒激光脉冲作用下的高次谐波产生现象，并研究了激光脉冲偏振方向对氮分子高次谐波的影响.结果表明氮分子的高次谐波谱具有典型原子谐波谱的特征；谐波谱强度随着θ(激光偏振方向与分子轴向夹角)的增大而减小.这与J.Itatanl在Nature上报道的实验结果基本一致. 关键词： 含时密度泛函理论 激光偏振方向 高次谐波 氮分子     

脉冲激光偏振方向对氮分子高次谐波的影响——基于含时密度泛函理论的模拟

采用含时密度泛函方法,结合赝势模型和电子交换相关作用的广义梯度近似,模拟了氮分子在超强飞秒激光脉冲作用下的高次谐波产生现象,并研究了激光脉冲偏振方向对氮分子高次谐波的影响.结果表明氮分子的高次谐波谱具有典型原子谐波谱的特征;谐波谱强度随着θ(激光偏振方向与分子轴向夹角)的增大而减小.这与J.Itatanl在Nature上报道的实验结果基本一致.     

脉冲激光偏振方向对氧分子高次谐波的影响——基于含时密度泛函理论的模拟

采用含时密度泛函方法,结合赝势模型和电子交换相关作用的广义梯度近似,模拟了氧分子在超强飞秒激光脉冲作用下的高次谐波产生现象,并研究了激光脉冲偏振方向对氧分子高次谐波的影响.结果表明氧分子的高次谐波谱具有典型原子谐波谱的特征;当θ=45°(激光偏振方向与分子轴向夹角)时,谐波谱强度最大.这与文献报道的实验结果基本一致.     

相对论圆偏振激光与固体靶作用产生高次谐波

超短超强激光与固体靶表面等离子体相互作用可以通过高次谐波的方式产生从极紫外到软X射线波段的相干辐射,获得飞秒甚至阿秒量级的超短脉冲,可用于观测原子或分子中的电子运动等超快动力学过程.本文实验研究了相对论圆偏振飞秒激光与固体靶相互作用的高次谐波产生过程,实验结果表明,在较大入射角下,圆偏振激光也可以有效地产生高次谐波辐射.通过预脉冲控制靶表面的预等离子体密度标长,发现高次谐波的产生效率随密度标长的增加而单调下降.进一步通过二维粒子模拟程序,分析了激光的偏振以及预等离子体密度标长对高次谐波产生的影响,很好地解释了实验观测结果.     

强激光和超薄等离子体薄膜相互作用产生高次谐波

通过一维粒子（particle in cell,简称PIC）模拟，研制了超薄等离子体薄膜在两列旋转方向相反的圆偏振激光脉冲作用下的高次谐波辐射。在10^21W/cm^2强度的激光作用下，可以获得200次以上的高次谐波。当激光脉冲宽度较短时，由于等离子体薄膜被压缩所导致的多普勒频移对高次谐波产生了显著的频率调制。而这种多普勒频移可以通过对抽运光脉冲引入频率啁啾来补偿。     

双色反向旋转圆偏振激光场下氩原子高次谐波的产生

通过二阶分裂算符法数值求解了二维含时薛定谔方程,研究了双色反向旋转圆偏振激光场作用下氩原子的高次谐波的产生.研究表明,双色反向旋转圆偏振激光场的幅值比对高次谐波发射的效率有较大影响,当幅值比为1.4时高次谐波的发射效率最高,并应用时频分析方法、经典三步模型以及利萨茹图形等解释了高次谐波发射的物理机制.研究了不同幅值比情况下谐波场的旋转偏振特性,发现对不同频段的谐波谱的电场进行叠加,谐波场的偏振特性随幅值比的不同而发生变化,但谐波场的旋向性与入射激光脉冲的旋向性相同,不受幅值比变化的影响.     

超强飞秒激光脉冲作用下氢分子的高次谐波行为——基于含时密度泛函理论的模拟

在含时密度泛函理论框架下,采用经典和量子相结合的模型,模拟了氢分子在超强飞秒激光脉冲作用下的高次谐波行为. 结果表明氢分子的高次谐波谱具有“下降—平台—截止”的结构和偶次谐波被禁闭仅出现奇次谐波的选择性特征;不同的激光偏振方向会导致高次谐波的截止频率和谱线强度的变化. 关键词： TDDFT 超强飞秒激光 高次谐波 2分子')" href="#">H₂分子     

圆偏振激光附加太赫兹场作用下的高次谐波发射和孤立阿秒脉冲的产生

本文通过数值求解二维含时薛定谔方程,研究了圆偏振激光和太赫兹组合场作用下,H2+的高次谐波发射和孤立阿秒脉冲的产生。研究发现,无论在圆偏振激光场的 或 方向附加一个太赫兹场,都可以产生一个比较平滑的连续谐波谱。通过时频分析我们发现,高次谐波的贡献主要来自于短轨道。适当选取一些级次的连续谐波进行叠加,可以得到129 as或83 as的孤立阿秒脉冲。     

对称分子H2+在强短波激光场中高次谐波椭偏率性质的研究

通过数值计算,研究了强短波(400—600 nm)激光场中H₂⁺分子高次谐波辐射的椭偏率性质.研究表明,H₂⁺分子在不同激光强度、不同激光波长、不同核间距及不同取向角下高次谐波的椭偏率性质是不同的;特别是在两中心干涉区,激发态在高次谐波产生中起着重要作用,但在不同取向角下,激发态对谐波椭偏率的影响不同;分析表明,这些不同的影响源于沿平行和垂直于激光偏振方向辐射的高次谐波的相对产量,以及激发态对平行和垂直谐波产量的影响;此外,椭偏率的测量检验了强场近似和平面波近似在强短波激光场中是成立的,并对强短波激光场中分子动力学做了更充分的研究.     

正交偏振双色激光场作用下生成的孤立阿秒脉冲

我们理论研究了正交偏振双色激光场作用下的高次谐波发射和孤立阿秒脉冲的产生.当y方向加一束中红外激光脉冲(12.5 fs/2000 nm),x方向加一束强度较弱的激光脉冲(12 fs/800 nm)时,我们得到从250 eV到350 eV的超连续谐波平台,在平台范围内叠加50 eV的谐波,可以得到一个脉宽约为97as的孤立阿秒脉冲. 通过时频分析,我们解释了高次谐波发射的物理机制.     

Scaling law and polarization of second harmonic emission from laser generated plasma

Measurements are described of the intensity and polarization of the second harmonic emission from a plasma generated by 1.054 μm, 35 ps laser pulses at oblique incidence. It is shown that the measured scaling cannot be explained by a simple power law and that at high intensities the degree of polarization of the second harmonic radiation does not depend on the polarization of the incident laser light.     

飞秒激光脉冲在氮气中产生高次谐波的实验研究

报道了以氮气为介质产生高次谐波的实验结果。实验中主要研究了不同的气体密度、不同入射激光能量和不同偏振对高次谐波辐射强度及谐波级次的影响。并从理论上进行了定性分析。分析结果表明：要想得到较高次、较强的谐波辐射，必须反复进行实验，确定最佳的激光能量和气体密度，以减少由于自由电子密度的提高而导致的相位失配的影响。     

Generation of attosecond pulses with ellipticity-modulated fundamental

We have studied experimentally and theoretically high-order harmonic generation using a laser field with a time-dependent ellipticity. We show that the harmonic emission can be confined into a narrow temporal window, in which the fundamental polarization is quasi-linear. This allows a single attosecond pulse (200 as) with a fundamental field obtained from 10 fs pulse to be generated. PACS 42.65.Ky; 42.65.Re; 32.80.Wr     

Theory of multiphoton and tunnel ionization in a bichromatic field

The imaginary-time method [6, 7] is used to calculate the multiphoton and tunnel ionization probabilities for atoms in a laser radiation field part of which is converted into the second harmonic. We assume that the first harmonic has a linear or elliptical polarization and the second harmonic is polarized linearly, with its polarization vector making an arbitrary angle with that of the first harmonic. The mean momentum of the photoelectrons knocked out from atoms is shown to depend on the phase shift between the first and second harmonics and their mutual polarization and to be identically equal to zero for a monochromatic field. An important difference between the case of elliptical polarization and the case of linear polarization of both harmonics is the absence of conditions under which the conditions for dominance of one of the two generation mechanisms considered here can be identified during the generation of terahertz radiation from the region of optical breakdown in a gas.     

Single-atom-response to a Partially Coherent Intense Laser Field

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Measurement of the polarization of high-order harmonics from aligned N(2) molecules by spatial interferometry

The polarization of high-harmonics from aligned N(2) molecules was measured by observing the visibility of spatial interference between two high-harmonics generated separately. The minimum visibility was observed at an angle of 60 degrees between the polarization of the harmonic generation laser field and the molecular orientation. In this case, the angular shift of harmonic polarization is 15 degrees from the molecular orientation. Our measurement of the visibility variation matches the theoretical prediction based on the harmonic field calculation for aligned N(2) molecules.     

Polarization state of high-order harmonic emission from aligned molecules

High harmonic emission in isotropic gases is polarized in the same direction as the incident laser polarization. Laser-induced molecular alignment allows us to break the symmetry of the gas medium. By using aligned molecules in high harmonic generation experiments, we show that the polarization of the extreme ultraviolet emission depends strongly on the molecular alignment and the orbital structure. Polarization measurements give insight into the molecular orbital symmetry. Furthermore, molecular alignment will allow us to produce attosecond pulses with time-dependent polarization.     

Toward single attosecond pulses using harmonic emission from solid-density plasmas

We report on investigations of high-order harmonic generation from solid surfaces in the coherent wake emission regime with relativistically intense few-cycle (8 fs) laser pulses. Significant spectral broadening compared to previous experiments with many-cycle pulses and the appearance of substructures on the harmonics are observed that strongly fluctuate from shot-to-shot. Measurements in which the linear polarization was rotated or ellipticity of the laser pulse was varied exhibit a strong dependence of the harmonic emission on the polarization state of the incident pulse. We show that the observed spectral features are ultimately connected to the sub-cycle electron dynamics in the laser-solid interaction and thus proof of the few-cycle nature of the observed harmonic emission. Using a simple model we have investigated the factors that play an important role in the shape of the emitted spectrum.     

基于高次谐波产生的极紫外偏振涡旋光

突破传统涡旋光场束缚,发展短波极紫外涡旋光场是实现阿秒脉冲偏振控制的有效途径.本研究利用自制的平场光栅光谱仪和超快时间保持的单色仪,以800 nm,35 fs高斯或具有偏振奇点的涡旋光脉冲驱动诱导氩原子产生高次谐波,分别获得相应的高次谐波光谱以及谐波谱单阶光源的分布.实验结果表明,基于高次谐波产生实现近红外波段的涡旋光束特性转移到极紫外波段,优化后的极紫外涡旋可以实现每秒108光子数输出.同时发现极紫外波段的涡旋场和高斯场高次谐波产生具有相似相位匹配机制.基于高次谐波产生的极紫外波段的偏振涡旋光为探究和操控原子分子量子态的含时演化动力学以及形成阿秒矢量光束提供了重要的方法和技术手段.