高次谐波的Guo-Åberg-Crasemann理论及其截断定律

将Guo-Åberg-Crasemann形式散射理论推广到高次谐波产生过程, 获得了高次谐波产生概率公式. 利用这一公式, 计算了不同惰性气体原子的高次谐波谱. 理论分析和数值计算显示高次谐波有新的截断定律q_cħω = (9 -4√2) U_p + (2√2-1) I_p ≈ 3.34 U_p + 1.83 I_p, 其中, U_p 为电子的有质动能, I_p 为原子电离能, ħω 为激光光子能量, q_c 为高次谐波的截断阶数. 这一截断定律与近期Popmintchev等 (Popmintchev et al. 2012 Science 336 1287) 的实验观测符合得很好.     

Single Ultrashort Attosecond Pulse Generation via Combination of Chirped Fundamental Laser and an Ultraviolet Controlling Pulse

We theoretically study the high-order harmonic generation (HHG) from a hydrogen atom in an intense few-cycle chirped fundamental laser in combination with an ultraviolet (uv) controlling pulse.The high-order harmonic spectrum is calculated by solving the time-dependent Schr¨odinger equation using the split-operator method.In our calculation,we present the difference of the high-order harmonic spectrum from one-dimensional (1D) model hydrogen atom and three-dimensional (3D) real hydrogen atom.We found that the plateau of the high-order harmonic generation from the 1D case and 3D case are all extended effectively to I p + 35U p due to the presence of the chirped laser pulse and the HHG supercontinuum spectrum is generated by adding an ultraviolet controlling pulse at a proper time,but the efficiency of the HHG for 3D case is more higher at the near cut-off region than the 1D case.Therefore,the generation of the attosecond pulse by synthesizing the harmonics near cut-off region have some slight differences between 1D and 3D simulations.As a real 3D case study,we show that an isolated 18 as pulse with a bandwidth of 232.5 eV is generated directly by optmizing the combination laser fields.     

On the cutoff law of laser induced high harmonic spectra

The currently well accepted cutoff law for laser induced high harmonic spectra predicts the cutoff energy as a linear combination of two interaction energies, the ponderomotive energy U_p and the atomic biding energy I_p, with coefficients 3.17 and 1.32, respectively. Even though, this law has been there for twenty years or so, the background information for these two constants, such as how they relate to fundamental physics and mathematics constants, is still unknown. This simple fact, keeps this cutoff law remaining as an empirical one. Based on the cutoff property of Bessel functions and the Einstein photoelectric law in the multiphoton case, we show these two coefficients are algebraic constants, 9 - 42≈ 3.34 and 22- 1 ≈ 1.83, respectively. A recent spectra calculation and an experimental measurement support the new cutoff law.     

空间非均匀啁啾双色场驱动下氦离子的高次谐波以及孤立阿秒脉冲的产生

本文理论上研究了初态为基态与第一激发态等权叠加的一维氦离子在空间非均匀啁啾双色场驱动下氦离子的高次谐波发射及孤立阿秒脉冲的产生. 研究表明, 一维氦离子在空间非均匀啁啾双色场驱动下发射的高次谐波相对于均匀场情况截止位置得到明显扩展, 得到了光滑的超连续谱,并应用半经典三步模型解释了高次谐波发射的物理机理. 通过小波变换的方法对连续谱进行了时频分析, 并且与电子的经典运动轨迹进行了对比分析, 结果显示在空间非均匀场中长量子轨道消失, 短量子轨道加强. 讨论了空间非均匀啁啾双色场中时间延迟对谐波和孤立阿秒脉冲产生的影响, 发现适当调整时间延迟值可以得到较大延展的光滑的超连续谐波谱, 本方案中时间延迟为t₀=1.6πup/ω₁时得到了最大延展, 通过对谐波中600次到680次(80次)谐波合成得到32 as的孤立脉冲.     

不同取向角下CO2分子波长依赖的垂直谐波效率

通过数值计算, 研究了强激光场中CO₂ 分子在不同波长和取向角下产生的高次谐波辐射的效率. 发现CO₂ 分子的垂直谐波效率在较小的和中间的取向角时倾向于与平行谐波效率可比或更高, 而在较大的取向角时, 垂直谐波效率远低于平行谐波效率. 进一步的分析表明, CO₂ 分子的结构对其垂直谐波效率有重要的影响, 且该影响与波长有关. 建议对于较复杂的分子, 应该在分子的轨道成像实验中考虑垂直谐波的贡献.     

采用适宜靶原子提高高次谐波的转化效率

通过对影响谐波转化效率因素的分析,提出选择适宜的靶原子对象,即选择在高能区具有较强连续态-基态耦合强度的靶原子,可以优化高次谐波的发射条件.本文的数值实验结果表明,利用该方案,可以把平台区尾部谐波的转化效率提高两个数量级.     

晶体环境下高次谐波谱的截止频率分析

通过数值求解中红外飞秒光脉冲与一维单电子多阱势相互作用的含时薛定谔方程, 研究了晶体环境下的高次谐波发射光谱, 发现了晶体谐波谱的截止位置公式.研究发现, 不同于气体环境下的谐波谱截止频率规律, 晶体谐波谱的截止频率与激光电场的峰值振幅及晶格参数成线性关系. 鉴于准经典力学的三步模型对于气体谐波截止频率的解释, 进一步采用准经典力学方法验证了晶体谐波截止位置规律的正确性. 关键词： 中红外飞秒光脉冲 晶体 高次谐波 截止频率     

不同核轴取向的O2的高次谐波

分子的高次谐波是强场超快物理的重要研究课题. 采用建立在形式散射理论基础上的频域方法计算了O₂在线偏振激光场下的高次谐波, 探讨了核轴被准直在与激光传输方向垂直的平面内时, 高次谐波随核轴与光电场偏振方向所成夹角θ₀的依赖关系. 结果表明: 各次谐波都是在θ₀约为45°时强度最大, 并有较宽的峰值宽度; 当偏离此角度, 高次谐波的强度变小; 到达平行或垂直取向时, 降到最低. 分析表明, 这是由于高次谐波的强度取决于分子基态的电子在动量空间中的电场方向的布居. 针对核轴被准直在激光传输方向与电场偏振方向所确定的平面内的情况, 计算了高次谐波随θ₀的依赖关系, 结果与前一种情况基本相同. 分析发现, 当核轴被准直固定后, 分子绕核轴旋转的角度ψ没有固定, 所以最后的高次谐波强度需要对不同的ψ 时的高次谐波的贡献求和平均. 平均后相当于波函数相对于核轴旋转对称, 从而导致O₂的高次谐波仅与θ₀有关, 而与核轴被准直在哪个面上无关.     

Controlling continuum wavepacket interference by two-color laser field in over-the-barrier ionization regime

Continuum wavepacket interference is investigated by numerically solving the time-dependent Schröodinger equation for the interaction of hydrogen atoms with laser fields. The obtained wavepacket evolution indicates that, in the over-the-barrier ionization regime (10¹⁶ W/cm²), the continuum–continuum (CC) interference of ionizing electrons becomes the main process in highorder harmonics generation (HHG), compared with continuum-bound (CB) transition, as reported by Kohler et al. [Phys. Rev. Lett. 105(20), 203902 (2010)].We propose a two-color laser field scheme for controlling the quantum trajectories of ionizing electrons and for extending the CC harmonic energy. As a result, a high energy platform occurs in the HHG spectrum, which entirely originates from the CC harmonics, with a cutoff adjustable by the relative phase of the two-color fields. This provides further understanding of the dynamic feature of atoms and molecules in super intense laser fields and provides an opportunity to image the atomic or molecular potential.     

High-order harmonic generation from wurtzitic and hexagonal BN

In this letter, the high-order harmonic generation (HHG) of wurtzitic and hexagonal boron nitride (h-BN) under ultrafast intense laser field was studied by solving an extended multiband semiconductor Bloch equations (SBEs). The results showed that for both the wurtzitic and hexagonal structures, the cutoff energy of the HHG was extended linearly by increasing the field strength, and the efficiency was suppressed as the wavelength of laser increased. It was worth noting that for hexagonal structure, the efficiency of HHG was 1–2 orders of magnitude higher than that of the wurtzitic structure. At the same time, the hexagonal structure significantly improved the cutoff energy of HHG from BN.

     

非均匀激光场中氢分子离子高次谐波的增强

通过数值求解非波恩-奥本海默近似下的一维含时薛定谔方程,研究了蝴蝶结型纳米结构基元中氢分子离子高次谐波的产生.研究表明,在蝴蝶结型纳米结构基元内部产生的非均匀场的空间位置对高次谐波的发射有较大影响.当非均匀场的空间位置从30 a.u.平移到-30 a.u.时,高次谐波的截止位置被延展且形成光滑的超连续的谐波谱,并应用时频分析方法、经典三步模型以及电离概率等解释了高次谐波发射的物理机理.研究了高次谐波谱对非均匀场空间位置的依赖性与载波包络值的关系,发现随着载波包络值的变化,非均匀场在不同空间位置处的高次谐波谱变化趋势相同.     

Controlling the contributions to high-order harmonic generation from different nuclei of N_2 with an orthogonally polarized two-color laser field

The generation of high-order harmonic and the attosecond pulse of the N2 molecule with an orthogonally polarized two-color laser field are investigated by the strong-field Lewenstein model.We show that the control of contributions to high-order harmonic generation(HHG) from different nuclei is realized by properly selecting the relative phase.When the relative phase is chosen to be φ= 0.4π,the contribution to HHG from one nucleus is much more than that from another.Interference between two nuclei can be suppressed greatly; a supercontinuum spectrum of HHG appears from 40 e V to125 e V.The underlying physical mechanism is well explained by the time–frequency analysis and the semi-classical threestep model with a finite initial transverse velocity.By superposing several orders of harmonics,an isolated attosecond pulse with a duration of 80 as can be generated.     

利用43飞秒的强激光脉冲实现单个阿秒脉冲输出的新机理

研究了多个光周期的长脉冲(43 fs)激光与氖原子相互作用产生的高次谐波的辐射特性. 通过计算机数值模拟，发现当激光场的强度超过饱和光强时，原子将会在几个光周期内被迅速电离，使得原子的高次谐波谱在截止区呈现出平滑的多平台结构. 同时，在高频波段离子产生的高次谐波的影响将变得十分显著. 通过选取合适的子平台进行频率叠加，在消除了离子的高次谐波的影响后，可以获得单个的阿秒软X射线脉冲. 关键词： 高次谐波 阿秒脉冲     

联合双原子在组合脉冲辐照下的高次谐波发射

首先采用经典的“三步模型“理论,分析了一维普薛耳-特勒势(P-T势)模型原子在组合脉冲作用下的动力学行为,进而利用该组合脉冲辐照联合双原子模型,通过调节组合脉冲的场幅、相位以及设定原子的核间距,使原子有适当的电离和较大的复合效率,因而在截止位置远达Ip 8Up处获得了效率高达10-9的高次谐波发射.     

正交偏振双色激光场作用下高次谐波的椭偏特性研究

通过数值求解二维含时薛定谔方程,理论研究了正交偏振双色激光场作用下H2+分子高次谐波发射和孤立阿秒脉冲的产生. 数值结果显示,改变激光场的强度比,可以调控高次谐波强度以及谐波椭偏率. 结果表明随着场强比的增加,y方向的谐波强度不断提高. 当场强比为1:2.5时,x方向和y方向分别仅有奇次谐波和偶次谐波产生,并且偶次谐波的强度比奇次谐波强度高2-3个数量级. 此时,奇次谐波的椭偏率最大可以达到0.3,偶次谐波的椭偏率接近于0. 此外,通过改变分子的准直角可以控制谐波的椭偏率,基于这一现象,利用椭圆偏振高次谐波的平台区合成了椭偏率为0.05的椭圆偏振阿秒脉冲链.     

电荷共振增强电离和离解电离在H~+_2谐波辐射中的贡献

理论研究了电荷共振增强电离和离解电离在H~+_2谐波辐射中的贡献.结果表明:在少周期激光场下,谐波辐射只由电荷共振增强电离贡献产生,谐波光谱呈规则的奇次谐波.在多周期激光场下,谐波辐射由电荷共振增强电离和离解电离共同贡献产生,但是电荷共振增强电离在谐波辐射中起主要作用.并且低阶谐波呈现偶次谐波.最后,通过分析含时核运动,电离几率以及谐波辐射时频分析图解释了少周期和多周期激光场驱动H~+_2辐射谐波的过程.     

High field physics and extreme nonlinear optics

Despite the fact that high order harmonic generation (HHG) aims at serving as a table-top light source for imaging applications with extremely high spatial resolution, a general lack of accurate conversion efficiency measurements exists in the field. Here, we present such a measurement for a HHG setup with a semi-infinite gas cell. By combining measurements with a calibrated photodiode sensitive in the extreme ultra-violet (XUV) and spatially resolved spectral measurements of HHG spectra, we are able to determine conversion efficiencies of the HHG process as well as brilliance values for individual harmonics. The method is explained in detail and applicable to any target geometry.     

红外激光场中共振结构原子对极紫外光脉冲的压缩效应

通过数值求解一维原子的含时薛定谔方程, 研究了具有共振结构的原子在双色场(红外激光(IR)+极紫外光(XUV)) 驱动下发射高次谐波的特征. 研究结果表明, 具有共振结构的原子所发射的高次谐波与无共振结构原子(简称为一般原子)发射的高次谐波有明显不同, 共振结构的原子除了在某一能量附近(原子的共振能量+电离能)高次谐波的强度有很大提高外, 它还对XUV光的响应较一般原子表现得更为敏感, 即使XUV光的强度较弱, 也能够明显提高XUV光脉冲中心频率附近的谐波强度, 更重要的是通过调节双色场的时间延迟, 能使输入的XUV光的脉宽得到明显的压缩, 通过时间-频率分析给出了发生这种现象的原因. 由此提出了通过滤波-连续反馈的方式可使XUV光的脉冲从200 as压缩至120 as左右.     

量子路径控制生成宽频软X射线连续辐射谱

数值研究了双色激光脉冲与一维氩原子相互作用的谐波发射功率谱.双色激光脉冲由800 nm和1600 nm的两束激光组成,脉冲持续时间为9.7 fs.计算结果表明,相对相位为π时,谐波截止频率从单色场情形的58 eV大幅度地展宽到316 eV,获得频宽为93 eV的软X射线连续辐射谱.此外,还采用短程的普薛耳－特勒势和长程的软化库仑势进行比对,数值结果表明该连续辐射谱的存在具有普适性.通过半经典的“三步”模型和时频分析小波变换方法对该连续辐射谱进行分析,发现其主要由单一的“短”量子路径生成,直接叠加频率从22 关键词： 红外双色激光脉冲 连续辐射谱 阿秒脉冲     

Infrared spectroscopy of adsorbates on low-area surfaces; The advantages of a photons-in photons-out technique

The technique of double modulation infrared spectroscopy permits the simultaneous measurement of (I_p - I_s) and (I_p + I_s) where I_p and I_s are the intensities of radiation polarized parallel and perpendicular to the plane of incidence at the sample. This particular technique allows the infrared spectrum of a surface adsorbate to be measured in the presence of substantial quantities of bulk-phase adsorbant. We have used this technique to study the infrared spectra of NO and CO adsorbates on Pt surfaces and of CO adsorbates on a Pt electrode surface in the presence of 1 M H₂SO₄. The results of these studies shed new light on the origin of the well established wavenumber shifts which are associated with structural changes in the adlayers formed from simple diatomic molecules.