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.     

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.     

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500 e V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35 as左右的水窗区间阿秒脉冲.     

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.     

Electron‐Nuclear Dynamics in Molecular Harmonic Generation Driven by a Plasmonic Nonhomogeneous Field

Electron (z)‐nuclear (R) dynamics in the molecular high‐order harmonic generation (MHHG) from H₂⁺ driven by the plasmonic nonhomogeneous field, generated by the surface plasmon polaritons in the bowtie‐shaped nanostructure, have been theoretically investigated through solving the two dimensional time‐dependent Schrödinger equation with the Non‐Bohn‐Oppenheimer approximation. It is found that (i) due to the plasmonic enhancement of the laser intensity, the harmonic cutoff can be extended when the spatial position of H₂⁺ is away from the gap center of the nanostructure. However, due to the limit of the gap size, the threshold value of the harmonic cutoff can be obtained at a given position of H₂⁺. (ii) Due to the asymmetric enhancement of the laser intensity in space, the extended higher harmonics are respectively from E(t) > 0 a.u. or E(t) < 0 a.u. for the cases of the positive and the negative spatial position of H₂⁺. As a result, the intensities of the extended higher harmonics are different and can be controlled by changing the carrier‐envelope phase and the pulse duration of the laser field. (iii) In the few‐cycle pulse duration, the MHHG mainly comes from the multi‐photon resonance ionization (MPRI), while as the pulse duration increases, the MPRI, the charge‐resonance enhanced ionization (CREI) and even the dissociative ionization (DI) are contributed to the MHHG. Moreover, as the spatial position of H₂⁺ moves, the contributions of the MHHG from the MPRI, the CERI and the DI can be controlled. (iv) The contributions of the MHHG from the two‐H nuclei have been investigated and found that when E(t) > 0 a.u., the intensities of the harmonics from the negative‐H is higher than those from the positive‐H; while when E(t) < 0 a.u., the intensities of the harmonics from the positive‐H plays the main role in the MHHG. Moreover, the multi‐minima, caused by the two‐center interference can also be found. (v) Finally, by superposing a properly selected harmonics, a single isolated attosecond pulse (SIAP) with the full width at half maximum (FWHM) of 34 as can be obtained.     

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.     

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.     

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。     

交叉型纳米结构下气体位置对阿秒脉冲的影响

理论提出并研究了一种在交叉型纳米结构下运用非均匀极化门方案获得孤立阿秒脉冲的方法.结果表明,当气体沿纳米结构负方向注入时,用于产生阿秒脉冲的高阶谐波只发生在纳米结构的一侧.因此,在此方案下,不仅谐波截止能量得到延伸,而且谐波干涉结构得到减小.随后,通过叠加平台区的谐波,可获得一个持续时间在33 as的孤立阿秒脉冲.     

Single attosecond pulse generation from asymmetric molecules with a multicycle laser pulse

A new scheme for single attosecond pulse generation from asymmetric molecules with a multicycle laser pulse is proposed. It is shown that both even and odd harmonics are generated from the asymmetric molecules, and the attosecond pulses are produced every full cycle of the driving laser field rather than each half-cycle. By filtering the harmonics in the cutoff of the spectrum, a single attosecond pulse can be obtained with a multicycle laser pulse with a duration of 2 times longer than the few-cycle pulse conventionally used.     

Generation of intense water window X-ray pulses by superposition of an initial state of the helium atom driven by a three-color field

We theoretically study the generation of high-order harmonics and attosecond pulses from the superposition of an initial state in helium atom driven by a three-color laser field. It is shown that when the initial state is chosen to be a single electronic state, due to the rapid depletion of the ground state, the efficiency of the spectral continuum will be decreased as the initial state increases. When the initial state is chosen to be the coherent superposition of the ground state and an arbitrary excited state, the efficiency of the spectral continuum can be enhanced compared to that from the single initial state. Further, with the optimization of the three-color laser beam, not only can the harmonic cutoff be extended to the water window X-ray region but also the spectral continuum is contributed by the single harmonic emission peak with the dominant short quantum path. Consequently, two combinations of the laser parameters can be found to generate the water window spectral continuum. By properly superposing some harmonics on the spectral continuum, a number of intense water window pulses with the duration of sub-35 as can be produced.     

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     

类氢离子在强激光场中产生的阿秒脉冲

利用数值方法求解一维模型氢原子和类氢离子在强激光场中的含时薛定谔方程,研究了由它们产生的高次谐波谱构造的阿秒脉冲规律。研究结果表明,对于高电荷离子而言,由于高次谐波平台的显著展宽,使得构造的相应阿秒脉冲的宽度大幅度缩短,从而可以得到脉宽低于 阿秒的超短脉冲。     

利用非均匀正交激光选择谐波发射短量子路径

理论研究了谐波发射的量子路径在非均匀正交激光场下的调控机制.结果表明,在适当的双色激光偏振角和非均匀参数的组合下,不仅谐波截止能量有明显增大;谐波发射的短量子路径可以被单独选择出来对谐波连续区起到贡献作用.并且,该方案在长脉宽激光下依然适用.最后,利用谐波连续区可以获得脉宽在50 as的孤立阿秒脉冲.     

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     

Physics going soft: The emergence of soft condensed matter physics

The recent development of intense, ultrashort, table top lasers in the mid-infrared opens new avenues for research in strong field atomic physics. Electrons submitted to such radiation acquire huge quiver energies, even at moderate intensity and interesting properties arise: first, the wavelength offers a convenient experimental knob to tune the ionisation regime by controlling the Keldysh parameter. Second, many processes like above-threshold ionisation or high harmonic generation, whose characteristics depend directly on this energy, can be pushed to unprecedented limits. Third, the wavelength controls the spectral phase of the harmonics and hence the possibility to improve the generation of pulses in the attosecond regime. Recent studies of rare gas and alkali atoms' photoelectron spectra and harmonic generation at 2 and 3.6 μm have begun to confirm the theoretical predictions. However, unexpected features have also been found showing that strong field interaction still keeps some secrets after more than 40 years of investigation.     

基于半解析自洽理论研究相对论激光脉冲驱动下阿秒X射线源的产生

发展了一种描述相对论激光脉冲与稠密等离子体相互作用产生阿秒X射线源的半解析自洽理论.该理论模型不仅可以获得等离子体界面的振荡轨迹、振荡面电场和磁场等物理参数,而且能够精确计算出激光脉冲驱动下阿秒X射线源的频谱,结果与粒子模拟程序一致.理论计算结果表明阿秒X射线源的辐射特性与等离子体界面随时演化过程相关,在周期量级激光场驱动下等离子体界面振荡振幅呈现中心不对称,通过改变激光场的载波包络相位实现对等离子体界面振荡的控制,获得准单阿秒X射线源.     

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.     

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．