Extension of Harmonic Cutoff and Generation of Isolated Sub-30 as Pulse in a Two-Color Chirped Laser Field

We theoretically investigate high-order harmonic and isolated attosecond pulse generation in a two-color chirped laser field,which is synthesized by a 9 fs/800 nm fundamental chirped pulse and a 9 fs/1600 nm controlling chirped pulse.Our numerical results show that,by using this method,not only is the harmonic cutoff significantly extended to the 948th order harmonic,but also the bandwidth of the supercontinuum spectrum is effectively broadened to about 1342 eV.In addition,due to the introduction of the chirp,the long quantum path is suppressed and only the short one is selected,and then an isolated 28 as pulse with a bandwidth of 155 eV is obtained directly.     

Isolated sub-30-attosecond pulse generation using a multicycle two-color chirped laser and a static electric field

We present a theoretical investigation of high-order harmonic generation in a chirped two-color laser field, which is synthesized by a 10-fs/800-nm fundamental chirped pulse and a 10-fs/1760-nm subharmonic pulse. It is shown that a supercontinuum can be produced using the multicycle two-color chirped field. However, the supercontinuum reveals a strong modulation structure, which is not good for the generation of an isolated attosecond pulse. By adding a static electric field to the multicycle two-color chirped field, not only the harmonic cutoff is extended remarkably, but also the quantum paths of the high-order harmonic generation （HHG） are modified significantly. As a result, both the extension of the supercontinuum and the selection of a single quantum path are achieved, producing an isolated 23-as pulse with a bandwidth of about 170.6 eV. Furthermore, the influences of the laser intensities on the supercontinuum and isolated attosecond pulse generation are investigated.     

中红外组合激光场产生低于50阿秒的单个脉冲

研究了氦离子在中红外组合激光场作用下的高次谐波辐射和孤立阿秒脉冲的产生. 研究结果表明,当主脉冲强度相对低时,谐波截止在657阶次处,形成一个宽度为287 eV的连续谱. 当主脉冲强度相对高时,可使谐波截止拓展到1795阶次,连续谱加宽到834 eV. 在两种情况下,我们实现了长量子路径选取,并且产生转换效率较高的连续谱. 特别是对于相对高的激光强度,叠加450—1590阶次内任意87 eV的谐波都可以直接得到50 as以内的单个脉冲.     

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.     

Generation of an isolated sub-30 attosecond pulse in a two-color laser field and a static electric field

We theoretically investigate high-order harmonic generation(HHG) from a helium ion model in a two-color laser field,which is synthesized by a fundamental pulse and its second harmonic pulse.It is shown that a supercontinuum spectrum can be generated in the two-color field.However,the spectral intensity is very low,limiting the application of the generated attosecond(as) pulse.By adding a static electric field to the synthesized two-color field,not only is the ionization yield of electrons contributing to the harmonic emission remarkably increased,but also the quantum paths of the HHG can be significantly modulated.As a result,the extension and enhancement of the supercontinuum spectrum are achieved,producing an intense isolated 26-as pulse with a bandwidth of about 170.5 eV.In particular,we also analyse the influence of the laser parameters on the ultrabroad supercontinuum spectrum and isolated sub-30-as pulse generation.     

利用多周期空间非均匀啁啾场产生单个短阿秒脉冲

提出了一种直接生成单个短阿秒脉冲的有效方案.采用数值求解一维含时薛定谔方程,理论研究了多周期空间非均匀啁啾场驱动预激发氦离子的高次谐波辐射和阿秒脉冲的产生.结果表明：在空间非均匀啁啾场作用下,谐波截止可扩展至1050阶次,并且高于300阶次以上的谐波是连续的.由于连续谱来自单一量子路径的贡献,滤出连续谱上880-1120阶次的谐波,直接产生了一个脉宽为11.4 as的单个脉冲.通过增加驱动脉冲的持续时间,可获得一个脉宽短至10 as的单个脉冲.     

Single quantum path control by a fundamental chirped pulse combined with a subharmonic control pulse

In this paper, we study the issue of single quantum path control and its role in attosecond pulse generation. By carrying out the time-dependent Schrödinger equation analysis for the harmonic emission from a single He atom irradiated by the two-color laser field, consisting of a short 800 fundamental chirped pulse and a subharmonic 800–2400 nm control pulse, we find that the most favorable condition for attosecond generation is at the fundamental chirp parameter β = 4.55 together with the zero-phase 2000 nm control pulse, in which the single quantum path (short quantum path) is selected to contribute to the harmonic spectrum exhibiting an ultrabroad supercontinuum of a 337 eV bandwidth. Finally, an isolated attosecond pulse as short as 39 as is thus generated directly.     

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     

Harmonic Extension and Enhancement Using a Two-Color Chirped Pulse and an Ultrashort Ultraviolet Pulse

A promising method to both extend the harmonic energy and enhance the harmonic intensity is presented when a He atom is exposed to the combination of a two-color chirped pulse and an ultrashort ultraviolet pulse. It is found that: (1) with the introduction of the two chirps, the harmonic energy has been remarkably extended; (2) by properly adding an ultrashort ultraviolet pulse to this synthesized two-color chirped pulse, the harmonic yield is enhanced by 3 orders of magnitude compared with the two-color chirped case; (3) the harmonic spectrum higher than the 96th order shows an ultrabroad supercontinuum with a 395 eV bandwidth, and the proper superposition over the harmonics directly produced a series of intense 40 as pulses which is 3 orders of magnitude higher than that in the two-color chirped case.     

利用蝴蝶型纳米结构下的极化门方案输出单个阿秒脉冲

理论研究了在蝴蝶型纳米结构下运用多周期极化门方案驱动He原子输出高次谐波以及阿秒脉冲的特点.结果表明,由于纳米结构表面的等离子共振增强现象,谐波截止能量得到延伸.同时,由于激光场呈现空间非均匀性,长量子路径对谐波的贡献被减弱.并且,在极化门控制下,谐波平台区的贡献只来源于单一的谐波辐射能量峰,进而形成一个147 eV的超长平台区.最后,通过叠加平台区的谐波,可获得一个持续时间在30 as的超短脉冲.     

附加谐波脉冲生成强的39阿秒孤立脉冲

采用多周期800 nm激光组合它的27次谐波脉冲生成超短、宽频的孤立阿秒脉冲.研究表明,脉宽为1 fs的27次谐波脉冲可以有效地控制电子动力学过程.将其加入到单色激光场的特定时域,可以控制电离概率在半个光学周期内迅速提升,使得谐波的转化效率相对于单色场情形增强4个数量级,并实现单一的短量子路径选择,出现频宽为108 eV的超连续辐射谱,叠加该连续谱140次到210次谐波获得了脉宽为39as的强、短孤立脉冲.与文献[7]中采用5 fs/800 nm激光附加紫外阿秒脉冲的方法相比,该方案生成的连续谱频带增宽一 关键词： 组合激光脉冲 连续辐射谱 阿秒脉冲     

Ultra-broadband supercontinuum generation with a nonlinear chirped pulse for controlling quantum path

We theoretically investigate the high-order harmonic generation in helium atom driven by a nonlinear chirped laser pulse with few-cycle duration. By employing appropriate chirp to the driving pulse, an efficient electric field waveform of controlling quantum path for ultra-broadband supercontinuous harmonics is realized, and then an isolated sub-50 as pulse with bandwidth of 739 eV can be significantly obtained.     

非平行偏振双色场驱动产生脉宽稳定的单个宽谱阿秒脉冲

提出一种直接得到脉宽稳定的单个宽谱阿秒脉冲的新方法.利用波长为800 nm脉宽为5 fs的超短脉冲叠加上偏振方向与主脉冲成π/3,脉宽同样为5 fs的二次谐波脉冲驱动氦原子,可以得到宽度达到50 eV的超连续谱.当二次谐波的强度大于10¹⁴ W/cm²时,超连续谱的位置以及谱宽几乎不会随着二次谐波脉冲的强度的改变而改变.对85—125 eV的超连续谱进行滤波可以直接得到100 as左右的单个阿秒脉冲,这个性质对于实验上获得单个宽谱阿秒脉冲而言是非常有利的. 关键词： 阿秒脉冲 超连续谱     

Isolated attosecond pulse generation from atom radiated by a three-color laser pulse

We theoretically investigate high-order harmonic and attosecond pulse generation from helium atom in a three-color laser field, which is synthesized by 10 fs/800 nm Ti-sapphire laser and a two-color field consisting of 30 fs/532 nm and30 fs/1330 nm pulses. Compared with harmonic spectrum generated by a monochromatic field, the harmonics generated from the synthesized three-color field show a supercontinuum spectrum with a bandwidth of 235 eV, ranging from the 154 th to the 306 th order harmonic. This phenomenon can be attributed to the fact that the ionization of atoms as well as motion of ionized electron can be effectively controlled in the three-color field. Therefore, an isolated 46-as pulse can be generated by superposing supercontinuum from the 160 th to the 210 th order harmonics.     

High-order harmonic generation of the N2 molecule in two-color circularly polarized laser fields

The generation of high-order harmonics and the attosecond pulse of the N₂ molecule in two-color circularly polarized laser fields are investigated by the strong-field Lewenstein model. We show that the plateau of spectra is dramatically extended and a continuous harmonic spectrum with the bandwidth of 113 eV is obtained. When a static field is added to the x direction, the quantum path control is realized and a supercontinuum spectrum can be obtained, which is beneficial to obtain a shorter attosecond pulse. The underlying physical mechanism is well explained by the time-frequency analysis and the semi-classical three-step model with a finite initial transverse velocity. By superposing several orders of harmonics in the combination of two-color circularly polarized laser fields and a static field, an isolated attosecond pulse with a duration of 30 as can be generated.     

利用三束中红外空间非均匀激光场产生超长平台区

数值研究了氦离子在三束中红外激光场下发射高次谐波以及阿秒脉冲的特点. 计算结果表明, 通过适当调节三束激光场的延迟时间和相位角, 不仅谐波发射的截止能量得到了延伸, 而且单一的量子路径也被选择出来对谐波发射起作用, 随后通过适当的引入空间非均匀参数,谐波截止能量得到了进一步扩展,形成了一个超长的1773eV的平台区. 并且我们发现当选择氦离子的基态和第一激发态为叠加初始态时,谐波的强度被增强了4-6个数量级. 最后, 通过叠加谐波谱上的谐波, 可获得一系列脉宽为32as左右的阿秒X射线光源. 并且这些光源的强度比氦离子基态作为初始态时增强了4-6个数量级.     

利用三束中红外空间非均匀激光场产生超长平台区

数值研究了氦离子在三束中红外激光场下发射高次谐波以及阿秒脉冲的特点. 计算结果表明, 通过适当调节三束激光场的延迟时间和相位角, 不仅谐波发射的截止能量得到了延伸, 而且单一的量子路径也被选择出来对谐波发射起作用, 随后通过适当的引入空间非均匀参数,谐波截止能量得到了进一步扩展,形成了一个超长的1773eV的平台区. 并且我们发现当选择氦离子的基态和第一激发态为叠加初始态时,谐波的强度被增强了4-6个数量级. 最后, 通过叠加谐波谱上的谐波, 可获得一系列脉宽为32as左右的阿秒X射线光源. 并且这些光源的强度比氦离子基态作为初始态时增强了4-6个数量级.     

Chirped polarization-gating technique for isolated attosecond pulse generation

We propose a scheme to generate broadband high harmonic supercontinuum and single attosecond pulse emission in time domain by employing a chirped polarization-gating laser field. It is shown that this scheme can significantly extend the high harmonic cutoff to higher energy region, and the produced isolated attosecond pulse is shorter than 100 attoseconds after proper dispersion compensation, when a 7 fs driving laser pulse is used.     

利用偏振门方案和静电场组合控制阿秒脉冲的产生

本文理论研究了5 fs,800 nm的左旋圆偏振和右旋圆偏振激光组合的偏振门方案叠加静电场作用下氦原子的高次谐波发射及阿秒脉冲的产生.研究发现,加入的静电场可以控制量子轨道,进而抑制量子干涉效应.当静电场系数α为0.1时,高次谐波谱出现双平台结构且比较平滑,通过小波分析,发现对第二平台有贡献的主要是一个短轨道,通过叠加第二平台区域50 eV(160 eV～210 eV)的连续谐波,我们得到了80 as的阿秒脉冲.     

Theoretical investigation of high-order harmonic generation from asymmetric molecular ions in the chirped laser field

High-order harmonic generation from the asymmetric helium hydrogen molecular ion exposed to a few-cycle chirped laser field is theoretically investigated by numerically solving the reduced one-dimension non-Born–Oppenheimer time dependent Schrödinger equation. The numerical results show that the plateau of the high-order harmonic spectrum is dramatically broadened and smoother with the introduction of the chirp. Further study presents that only the short quantum path contributes to harmonic emission and the long quantum path disappears in the chirped laser. In addition, an 80-as isolated attosecond pulse is obtained by superposing the harmonics from the 100th to the 130th order in the chirped field. The time-frequency distributions and the electronic probability distributions are presented to explain the underlying physical mechanism.