吸收多个远紫外光子生成的高次谐波的多重截止结构

本文通过数值求解含时薛定谔方程研究He+在远紫外和红外激光场作用下产生高次谐波的过程.在电子隧穿并从红外激光场获得能量后,其在与母离子碰撞过程中可能吸收额外的远紫外光子,并导致高次谐波中出现以远紫外光子能量为间隔的多重截止结构.我们进一步通过傅里叶频谱分析的方法证实了这一结构产生的物理机制;并分析了高次谐波的多重截止频率强度和远紫外强度的关系.我们的研究为产生高能量谐波提供了一种方案.     

利用单光子和双光子共振电离增强He原子谐波强度

理论研究了He原子在吸收紫外光子后对高次谐波的强度的影响。结果表明：当紫外光光子能量满足He原子基态与激发态的单光子和双光子共振跃迁时,谐波强度有超过2个数量级的增强。当紫外光光子能量满足He原子基态与激发态的多光子共振跃迁时,谐波强度增强明显减弱。分析电离几率表明,单、双光子共振电离在紫外共振电离中起主要作用,因此导致谐波强度的明显增强。同时,在啁啾调频场的作用下谐波截止能量也明显延伸,可以获得一个强度较高的谐波光谱连续区。选取该谐波连续区可获得脉宽为40 as的孤立脉冲。     

双色中红外激光场延迟及相位调控产生水窗区间光子

理论研究了双色中红外激光场时间延迟及相位对高次谐波光谱的影响.结果表明：当激光延迟为-1.0 fs,激光相位为0.2π和0时,谐波截止能量可以得到有效延伸,并且获得一个超宽水窗区间谐波连续平台区.此外,谐波光谱的延伸与控制场波长关系不大,即,在上述激光延迟和相位下,控制场波长在2300 nm到2700 nm区间时都可获得水窗区谐波光谱.最后,叠加光谱连续区的谐波可获得35 as的水窗区孤立阿秒脉冲.     

时-空域波形优化延伸高次谐波截止能量的研究

在高次谐波发射三步模型的指导下,理论研究了三色场时-空域波形调控对谐波截止能量的影响．在时域调控下,通过改变三束激光场的强度比、相位和延迟时间获得了最佳的三色场时域波形．在该波形下,谐波截止能量得到最佳延伸．在空间调控下,在上述最佳波形的基础上选择适当的正向激光非均匀效应后,谐波截止能量得到了进一步延伸．并且,随着非均匀参数增大,谐波截止能量也可持续增大．通过分析激光波形、空间非均匀效应和谐波发射过程给出了谐波截止能量延伸的原因．并且,通过叠加部分谐波可获得脉宽为45as的单个脉冲．     

中红外飞秒激光场中氮分子高次谐波的多轨道干涉特性研究

通过系统研究氮分子高次谐波产生过程中的电子超快动力学过程,实验上发现在中红外飞秒强激光场驱动下高次谐波谱的截止区附近存在清晰的谐波谱极小值.进一步研究表明谐波谱极小值对应的光子能量强烈依赖于驱动激光脉冲的光强和波长,而与分子取向角无关,由此推断该极小值来源于氮分子最高占据分子轨道和次最高占据分子轨道产生的高次谐波之间的相消干涉.本研究结果将对极端强场条件下多轨道电子超快动力学研究起到积极推动作用.     

极紫外阿秒脉冲在高次谐波产生过程中引起的非偶极效应

当红外强激光和极紫外(XUV)阿秒脉冲共同作用于原子分子时，电离出去的电子通常会吸收和辐射激光光子而发生能量扩展.讨论了由于XUV阿秒脉冲的短波长与扩展后的电子波包尺度可相比拟时在高次谐波产生过程中引起的非偶极效应.采用H⁺₂作为模型分子，并把分子轴置于激光场的传播方向，通过解二维含时薛定谔方程并比较考虑非偶极效应和采用偶极近似两种方法计算得到的结果，两者相比，前者的谐波强度降低，谐波频率向低级次稍有移动，电子能谱的能带内出现了更多的光电子峰.在相同的光电子能量处，两种方法计算得到的信号强度相差2—5倍.并且这种非偶极效应随着红外基频光光强的增大而增强，随阿秒脉冲波长的增大而减弱. 关键词： 非偶极效应 光场空间不均匀性 阿秒脉冲 高次谐波产生     

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。     

HeH~(2+)分子谐波辐射的空间分布

理论研究了HeH~(2+)分子发射高次谐波的空间分布情况.结果表明:分子谐波发射主要集中在He核和H核附近.当核间距离较小时,He核辐射谐波强度要比H核辐射谐波强度高2~3个数量级.当核间距离较大时,He核辐射谐波强度要比H核辐射谐波强度高4~5个数量级.当空间非均匀激光场引入后,谐波截止能量得到延伸,并且,随着空间非均匀激光场位置由负向到正向移动(-100 a.u.到100a.u.),谐波截止得到进一步延伸.但是He核辐射谐波强度依然大于H核.最后,通过叠加谐波可获得脉宽在60 as的超短脉冲.     

原子在双色组合场中产生高次谐波的转换效率与激光波长的关系

利用分裂算符法求解速度规范下的含时薛定谔方程,研究了一维氦原子处于单色红外场、红外场与紫外场形成的双色组合场中产生的高次谐波谱,分析了在截止位置附近高次谐波的转换效率与激光波长(800—2000 nm)的关系,发现在双色组合场驱动下截止位置附近高次谐波的转换效率随波长的变化为η(λ)∝λ^－x,其中〈x〉的数值取决于激光场的强度,但是只要选取合适场强的组合场就能提高截止位置附近高次谐波的转换效率. 关键词： 双色组合场 分裂算符 高次谐波 转换效率     

HeH2+分子谐波辐射的空间分布

理论研究了HeH2+分子发射高次谐波的空间分布情况。结果表明：分子谐波发射主要集中在He核和H核附近。当核间距离较小时,He核辐射谐波强度要比H核辐射谐波强度高2~3个数量级。当核间距离较大时,He核辐射谐波强度要比H核辐射谐波强度高4~5个数量级。当空间非均匀激光场引入后,谐波截止能量得到延伸,并且,随着空间非均匀激光场位置由负向到正向移动(-100 a.u.到100 a.u.),谐波截止得到进一步延伸。但是He核辐射谐波强度依然大于H核。最后,通过叠加谐波可获得脉宽在60 as的超短脉冲。     

XUV pulse effect on signal modulations of harmonic spectra from H$$_{2}^{+}$$ and T$$_{2}^{+}$$

The effects of signal modulations on the molecular high-order harmonic generations in \(\hbox {H}_{2}^{+ }\) and \(\hbox {T}_{2}^{+}\) have been theoretically investigated. It is found that with the introduction of the XUV pulse, due to the absorption of the extra XUV photons in the recombination process, multiplateaus on the harmonic spectra, separated by the XUV photon energy can be found. Moreover, this multiplateau structure is insensitive to the wavelength of the XUV pulse. In shorter pulse duration, the intensities of the multiplateaus from \(\hbox {H}_{2}^{+}\) are higher than those from \(\hbox {T}_{2}^{+}\); while in longer pulse duration, the opposite results can be found. Finally, by changing the delay time of the XUV pulse, the signal modulations (including the amplitude and the frequency modulations) of the multiplateaus can be controlled.     

Single attosecond pulses from high harmonics driven by self-compressed filaments

We show that isolated subfemtosecond, extreme ultraviolet (XUV) pulses can be generated via harmonic generation in argon by few-cycle infrared pulses formed through filamentation-induced self-compression in neon. Our calculations show that the time structure of the XUV pulses depends sensitively on both the amplitude and the phase modulation that are induced in the driving pulse during the self-compression process.     

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.     

红外激光载波包络相位对氦原子的极紫外光(XUV)吸收谱的量子调控研究

本文通过数值求解双电子含时薛定谔方程, 研究了利用红外(IR)超短超强激光的载波包络相位(CEP)对氦(He)原子的极紫外光(Extreme Ultra-Violet, XUV)吸收谱进行量子调控的可能性. 当XUV作用到He原子上时, 原子存在两个电离通道: 无明显电子关联的直接电离和带强烈电子关联的间接电离(即通过双激发态自发电离). 两个通道相互干涉可在XUV吸收谱中形成人们所熟知的Fano共振线型, 并且谱线的形状由两个通道间的比例决定. 通过引入另外一束IR激光, 我们发现, 原子的XUV吸收谱将发生明显改变, 即伴随着超短脉冲CEP的改变而 呈现出从Fano线型到Lorentz线型的周期性连续变化. 上述结果表明, 通过合理地控制超短脉冲的CEP可以有效地调控两个电离通道之间的量子干涉, 进而为探测和操控原子中的极端超快电子关联提供可能.     

Extreme Ultraviolet Sources Generation by Using the Two-Color Multi-Cycle Weak Inhomogeneous Field

An efficient method for attosecond extreme ultraviolet source generation under the two-color multi-cycle weak pulse has been theoretically presented by using the concept of the plasmonic field enhancement in the vicinity of metallic nanostructures. The results show that by properly choosing the inhomogeneity of the two-color multi-cycle(20 fs) weak pulse(1013W/cm2), not only the harmonic cutoff has been extended, resulting in a broadband XUV continuum, but also the single short quantum path has been selected to contribute to the harmonic. As a result, two isolated XUV pulses with durations of 68 as and 66 as can be obtained.     

Time-resolved measurements of high order harmonics confined by polarization gating

We investigate the temporal confinement of high order harmonic pulses generated by a femtosecond (fs) infrared (IR) pulse with a time varying polarization. We use a set of two birefringent quartz plates to modulate the IR polarization. It produces a short temporal gate of linear polarization where harmonics are efficiently generated during a small fraction of the IR pulse. By rotating one of the plates, the gate width can be continuously varied between 70 fs down to 7 fs. The XUV pulse duration is measured by cross-correlation with a probe IR pulse of 12 fs. When the gate width is decreased, a clear temporal confinement of the XUV emission is observed through the cross correlation signal. This experiment is the first direct experimental evidence in the temporal domain that the polarization gating technique can be used to significantly shorten the harmonic pulse duration. PACS 32.80.Wr; 42.65.Ky; 42.65.Re     

Ponderomotive shearing for spectral interferometry of extreme-ultraviolet pulses

We propose a novel method for completely characterizing ultrashort pulses at extreme-ultraviolet (XUV) wavelengths by adapting the technique of spectral phase interferometry for direct electric-field reconstruction to this spectral region. Two-electron wave packets are coherently produced by photoionizing atoms with two time-delayed replicas of the XUV pulse. For one of the XUV pulses, photoionization occurs in the presence of a strong infrared pulse that ponderomotively shifts the binding energy, thereby providing the spectral shear needed for reconstruction of the spectral phase of the XUV pulse.     

Role of XUV Photons in Atomic High-Order Above-Threshold Ionization Processes in IR+XUV Two-Color Laser Fields

We investigate the above-threshold ionization of an atom in a combined infrared(IR) and extreme ultraviolet(XUV) two-color laser field and focus on the role of XUV field in the high-order above-threshold ionization(HATI)process. It is demonstrated that, in stark contrast to previous studies, the XUV laser may play a significant role in atomic HATI process, and in particular, the XUV laser can accelerate the ionized electron in a quantized way during the collision between the electron and its parent ion. This process cannot be explained by the classical three-step model. Our results indicate that the previously well-established concept that HATI is an elastic recollision process is broken down.     

Attosecond pulse generation from two-electron harmonic emission spectrum

In this paper, we theoretically investigate the high-order harmonic generation and attosecond pulse generation when a two-electron He atom is exposed to the intense laser pulse. It shows that due to the two-electron double recombination mechanism, an extended plateau beyond the classical single-electron harmonic has been obtained on the two-electron harmonic spectrum. Further by using this two-electron harmonic extension scheme combined with the two-color field, two supercontinuum bandwidths with 200 e V have been obtained. As a result, a series of sub-60 as extreme ultraviolet(XUV)pulses have been directly generated.