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.     

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,H+2的高次谐波发射和孤立阿秒(1 as=10-18s)脉冲的产生.研究发现,无论在圆偏振激光场的x或y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.     

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。     

圆偏振激光脉冲附加静电场或太赫兹场时H2+高次谐波产生

本文通过数值求解二维含时薛定谔方程理论的研究了圆偏振激光脉冲(circularly polarized laser pulse, CPLP)作用下H2+的高次谐波产生(high-order harmonic generation, HHG)。通过对比CPLP附加静电场和CPLP附加太赫兹场(terahertz, THz)时的高次谐波发射谱发现,当在CPLP的方向附加静电场时,谐波谱出现双平台结构,且布满调制,通过电子波包概率分布图像发现,电子三次与母核复合,量子轨道之间产生了较严重的干涉。当附加相同强度的THz场时,电子仅两次与母核复合,量子轨道之间的干涉减少,最终得到了86eV到208eV带宽为122eV的连续谱,相比于附加静电场,谐波谱变得连续且平滑,实现了量子轨道调控。最后,利用半经典三步模型、时频分析和电离几率等对谐波发射物理机制进行了深入研究。     

利用超短紫外光源增强阿秒脉冲的强度

提出了一种利用超短紫外光源来增强阿秒脉冲强度的方法. 计算结果表明,当适当的加入一束125 nm光源到双色正交激光场时,不仅高次谐波的强度比原双色场时增强了2个数量级,而且选择单一的量子路径对谐波发射起作用,进而形成了一个152 eV的超长平台区. 随后通过激光参数的优化,发现激光脉宽和偏振角对于谐波强度的增强不太敏感. 最后,通过叠加谐波可获得脉宽仅为38 as 的孤立阿秒脉冲,其强度比原双色场情况下产生的阿秒脉冲增强了2个数量级.     

利用两束同色圆偏振激光场产生水窗区间阿秒脉冲

数值研究了氦原子在两束同色圆偏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,当适当调节两束激光场的波长,相位以及时间延迟时,不仅高次谐波谱的平台区域能得到很大的扩展,而且谐波的强度也有较大的增强,形成了一个500eV的超长平台区.最后,通过适当的叠加谐波平台上的谐波,可获得一系列脉宽为35as左右的水窗区间阿秒脉冲.     

空间非均匀场下核间距对不对称分子谐波发射的影响

数值研究了空间非均匀场下核间距离对于不对称分子HeH~(2+)发射高次谐波以及阿秒脉冲的影响.计算结果表明,通过适当调节HeH~(2+)分子离子的核间距离以及空间非均匀参数,不仅谐波发射的截止能量得到了延伸,而且单一的短量子路径也被选择出来对谐波发射起作用.随后适当引入第二束控制激光场,谐波截止能量得到了进一步扩展,形成了两个带宽分别在431 eV和372 eV的连续平台区.最后,通过适当的叠加谐波,可获得脉宽为31 as-65 as的一系列阿秒X射线光源.     

利用相位啁啾单色中红外激光场产生高次谐波

本文通过求解薛定谔方程,理论探索了高次谐波及孤立阿秒脉冲在相位调制的单色中红外激光场中的产生.研究结果表明,通过加入含时相位啁啾小量,能够有效的提高高次谐波的产生效率,获得超宽平台谐波谱及超短孤立阿秒脉冲.特别是在相位为0.3π的基础上,同时加入啁啾小量βt,在β=0.3时,可得到带宽为822 e V的超连续高次谐波平台.最后通过叠加第二平台高次谐波,可得到带宽仅为2.7 as的孤立阿秒脉冲.且脉冲强度比没有加入啁啾小量的情况下显著增强.     

空间非均匀场下核间距对不对称分子谐波发射的影响

数值研究了空间非均匀场下核间距离对于不对称分子HeH2+发射高次谐波以及阿秒脉冲的影响。计算结果表明,通过适当调节HeH2+分子离子的核间距离以及空间非均匀参数,不仅谐波发射的截止能量得到了延伸,而且单一的短量子路径也被选择出来对谐波发射起作用。随后适当引入第二束控制激光场,谐波截止能量得到了进一步扩展,形成了两个带宽分别在431eV和372eV的连续平台区。 最后,通过适当的叠加谐波,可获得脉宽为31as-65as的一系列阿秒X射线光源。     

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     

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.     

Broadband-Isolated Attosecond Pulse Generation by Two-Color Elliptically Polarized Laser Pulses

We numerically solve the two-dimensional Schrödinger equation and investigate isolated attosecond pulse generation by combining a left and a right elliptically polarized pulse together. It is found that the plateau of the harmonic spectra can be extended and the quantum paths can be controlled by adjusting ellipticity. A broadband width of about 220 eV is generated for the case that ellipticity equals 0.3, and time-frequency time–frequency analysis shows that only one long quantum path contributes to high-order harmonic generation. By superposing a bandwidth of 60 eV near the cutoff of the harmonic plateau, an attosecond pulse with the duration of about 62 as could be generated.     

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.     

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.     

任意夹角的双色偏振激光作用下孤立阿秒脉冲的产生

本文选取脉宽为12 fs波长为2000 nm和800 nm的两束线性偏振激光, 适当调节两束激光的偏振方向的夹角θ, 发现当θ=π/2时, 高次谐波谱的第二级平台出现了“漏斗形”凹槽.当θ为π/6时, 得到了290 eV的超宽连续谱的高次谐波发射, 利用小波变换合理的解释了高次谐波形状及截止位置. 在该段连续谱上任意截取70 eV宽度的频率, 都可以得到脉宽约为60 as的孤立阿秒脉冲. 适当减小截取范围可以得到线性偏振的脉宽94 as的孤立阿秒脉冲. 这为实验中产生能灵活调节的孤立阿秒脉冲提供了一种方案.     

60 fs长脉宽双色场作用下孤立阿秒脉冲的产生

通过数值求解一维含时Schrödinger方程,研究了具有较长脉宽双色激光脉冲与氢原子相互作用产生的高次谐波和阿秒(as)脉冲.这里的双色激光脉冲由一束基频钛宝石主脉冲与另一束红外附加脉冲构成.研究发现,当合成脉冲的脉宽选为12 fs时, 选取合适的附加脉冲波长,合成电场的振幅在始末端时间段能被大幅抑制, 仅中间部分的电场对谐波谱平台区和截止位置起主要贡献. 通过分析合成脉冲电场始末端时间段抑制的机理,进一步扩展了合成脉冲脉宽到60 as, 并得到160 as的孤立短脉冲.这是迄今为止在孤立阿秒脉冲产生研究中所采用的最长脉宽. 该方案中的合成脉冲等效于单一5 fs短脉冲的作用, 却克服了5 fs脉冲低输出能量导致的阿秒脉冲能量低的困难.     

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.     

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.     

利用三色组合脉冲激光获得孤立阿秒脉冲发射

孤立阿秒脉冲因可以跟踪和控制原子及分子内电子的运动过程而备受关注.本文从理论上开展了氦原子在3束飞秒脉冲激光组合场辐照下产生的高次谐波和阿秒脉冲辐射的研究.组合激光场由16 fs/1600 nm,15 fs/1100 nm和5.3 fs/800 nm的钛宝石脉冲构成.与前两束脉冲合成的双色场产生谐波谱相比,附加钛宝石脉冲的三色场产生的高次谐波发射谱呈现出高转换效率及宽带超连续特性,超连续谱范围覆盖从230—690次谐波,傅里叶变换后实现了128 as高强度孤立短脉冲的产生.该结果归因于合成的三色场呈现出高功率及少周期的中红外飞秒脉冲激光特性,可以有效控制原子电离以及复合发生在中红外飞秒脉冲的一个有效光学周期内.