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.     

啁啾激光与半周期脉冲形成的组合场驱动原子产生单个阿秒脉冲

提出了由波长为800 nm、脉冲宽度为5 fs的啁啾激光与半周期脉冲形成组合场,并利用这种组合场驱动一维模型氦原子获得单个阿秒脉冲. 通过数值求解一维氦原子的含时薛定谔方程,发现氦原子在组合场驱动下高次谐波谱的截止位置可以扩展到I_p+21.6U_p. 对第二平台区域不同范围内高次谐波的叠加都能得到单个阿秒脉冲,最短可达37 as,特别是对平台区域的前端进行叠加不仅能够得到较短的单个阿秒脉冲,而且与截止位置附近高次谐波构造的阿秒脉冲相比,强度提高了3个数量级. 关键词： 啁啾激光场 半周期脉冲 高次谐波 阿秒脉冲     

两束同色激光场和中红外场驱动氦原子在等离激元中产生的单个阿秒脉冲

通过数值求解一维氦原子由两束同色激光场和中红外形成组合场中的含时薛定谔方程,研究了氦原子在纳米等离激元中发射高次谐波的性质以及合成阿秒脉冲的特点.研究表明,在等离激元中氦原子在组合场驱动下发射的高次谐波相对于均匀场情况下截止位置会得到明显扩展,但等离激元对处在连续态电子的吸收效应会对高次谐波截止位置影响较大,通过改变激元的相对位置能明显提高其中一个轨道对谐波的贡献,抑制另一些电子轨道的贡献.经典分析表明,两个电子轨道发生并合,从而实现单个阿秒脉冲的输出.与原子在均匀场驱动的情况相比,阿秒脉冲的宽度明显缩短,最短可实现28 as的单个脉冲输出.     

原子在两色组合激光场中产生的单个阿秒脉冲

利用分裂算符法求解含时薛定谔方程,对一维氦原子处于红外场（IR）与紫外场（UV）组合的两色激光场中产生的高次谐波进行研究,分析了由高次谐波产生的阿秒脉冲的特征. 发现在组合场中可以通过增加红外场的强度,来缩短阿秒脉冲的宽度,通过调节组合场中两束激光的时间延迟,提高一个电子轨道对谐波的贡献,而抑制另一个电子轨道的贡献,从而得到低于100 as的单个脉冲. 关键词： 强激光场 组合场 高次谐波 阿秒脉冲     

利用两束同色激光场和半周期脉冲驱动原子产生单个阿秒脉冲

利用分裂算符方法数值求解一维氦原子的含时薛定谔方程,研究了氦原子在两束同色激光场与半周期脉冲(Hcps)形成的组合场驱动下所发射高次谐波的特点.研究结果表明,氦原子在这种组合场驱动下,高次谐波谱的平台区域能得到很大的扩展,其截止位置可延伸到I_P+9.6U_P,通过构造截止位置附近的高次谐波谱能够得到脉宽为63 as的单个阿秒脉冲.经过分析后发现,半周期脉冲的加入不仅使高次谐波谱平台能够得到扩展,同时还抑制了电子长路径对高次谐波的贡献. 关键词： 两束同色激光场 半周期脉冲 高次谐波 阿秒脉冲     

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

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1773 e V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32 as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.     

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287eV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834eV.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87eV的谐波都可以直接得到50as以内的单个脉冲.     

双色激光脉冲辐照下38 as孤立短脉冲的产生

通过数值求解一维含时薛定谔方程, 本文研究了具有特定波长的双色激光脉冲与氦原子相互作用产生的高次谐波和阿秒脉冲, 这里双色激光脉冲由5 fs较低强度基频钛宝石主脉冲与另一束较高强度的1330 nm 红外附加脉冲构成. 研究发现, 若两束脉冲之间的相对相位选择合适, 可以获得宽带连续辐射的高次谐波谱, 叠加该连续辐射谱可获得脉宽为38 as的孤立短脉冲. 进一步研究发现, 不同于以往孤立阿秒脉冲研究中选出长、短量子路径之一作为辐射源, 这里单阿秒脉冲来源于长、短两个量子路径的贡献, 只是这两个量子路径在很宽的谐波次数变化范围内辐射时刻比较集中. 关键词： 双色激光脉冲 阿秒脉冲 量子路径     

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.     

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.     

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     

HeH2+分子多通道谐波发射的理论研究

利用非波恩奥本海默近似模型,理论研究了强激光场下HeH2+分子发射高次谐波的特点。计算结果表明,HeH2+分子的谐波截止能量要大于经典预言值。理论分析表明,延伸的谐波截止能量是由于激发态激光诱导电子跃迁所产生的多通道电子电离-回碰过程所导致的。随后利用双色空间非均匀激光场机制,谐波截止能量进一步延伸,形成了1个由单一量子路径贡献而成的482eV的超长平台区。该辐射能量相当于单色激光场场强被增强了3倍的结果。并且高振动激发态会产生强度更高的谐波光谱。最后,通过适当的叠加谐波平台上的谐波, 可获得一系列脉宽在30as的超短X射线光源。     

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.     

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.     

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.     

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.