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     

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     

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.     

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     

Asymmetric molecular harmonic assisted generation of the attosecond X-ray pulse

In this paper, we theoretically investigate the asymmetric molecular high-order harmonic emission and the attosecond X-ray pulse generation when the asymmetric molecular HeH²⁺ ion is exposed to an intense chirped pulse. It is found that the molecular harmonics are strongly dependent on the internuclear distance R as well as the chirp parameter, showing a smooth and intense harmonic plateau at R = 8.0 a.u. and β = 0.4. Further, by adding a subharmonic controlling pulse with the optimised conditions, a supercontinuum with a bandwidth of 602 eV can be obtained. Finally, by properly superposing the selected harmonics in the supercontinuum region, a series of isolated X-ray pulses with durations of sub-80 as are directly generated.     

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     

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.     

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.     

Ionization phase-match gating for wavelength-tunable isolated attosecond pulse generation

High-order harmonic emission can be confined to the leading edge of an 800 nm driver laser pulse under moderately intense focusing conditions (7×10¹⁴ W/cm²) (Pfeifer et al. in Opt. Express 15:17120, 2007). Here, the experimentally observed curtailment of harmonic production on the leading edge of the driver pulse is shown to be controlled by an ionization-induced phase-matching condition. The transient plasma density inherent to the process of high-harmonic generation terminates the harmonic emission by an ultrafast loss of phase matching on the leading edge of the laser pulse. The analysis is supported by a reconstruction of the in situ intensity envelope of the driver pulse with attosecond temporal resolution, performed by measurements of the carrier-envelope phase dependence of individual half-cycle harmonic cutoffs. The method opens the way to wavelength-tunable isolated attosecond pulse generation.     

Single attosecond pulse generation with intense mid-infrared elliptically polarized laser pulses

We have studied theoretically high-harmonic-order and single attosecond pulse generation with elliptically polarized laser pulses at wavelengths ranging from the visible to the mid-infrared. Results of ab initio simulations of the time-dependent Schr?dinger equation show that the ellipticity dependence of the high-harmonic signal intensifies with increasing wavelength of the driving pulse and saturates in the mid-infrared. The isolation of single attosecond pulses using the polarization gating method in the mid-infrared is due to an effective suppression of side pulses as compared with an operation at Ti:sapphire wavelengths.     

Effects of pressure and gas-jet thickness on the generation of attosecond pulse

We investigate how the intensity and duration of an attosecond pulse generated from high-order harmonic generation are affected by the pressure and thickness of the gas jet by taking into account the macroscopic propagation of both fundamental and harmonic fields. Our simulations show that, limited by the propagation effects, especially the absorption of harmonics, the intensity of an attosecond pulse cannot be improved by just independently increasing the gas pressure or the medium length. On the other hand, due to good phase-matching conditions, the duration of a generated attosecond pulse can be improved by changing the gas pressure.     

On the feasibility of performing non-linear autocorrelation with attosecond pulse trains

We present experimental results in which a second-order effect, namely two-photon ionization of atomic He induced by a superposition of harmonics, is observed. The harmonics are generated in a Xe gas-jet using a 790-nm 10-Hz femtosecond Ti:sapphire laser and are subsequently focused into a He gas-jet with a Kirkpatrick–Baez arrangement. The superposition is formed by using a thin In filter and it comprises the 7th to 13th harmonics. Solving the time-dependent Schrödinger equation for He in a polychromatic laser field, the He ⁺ ion yield is calculated as a function of the total XUV intensity. Using the calculated yield and taking into account the focusing and transmission properties of the arrangement, the number of He ⁺ ions produced per laser pulse is estimated and is found to be in reasonable agreement with its measured value. The total number of ions produced non-resonantly follows a nearly quadratic dependence on the harmonic intensity, thus establishing the feasibility of a second-order auto-correlation measurement of the superposition of harmonics, i.e., of a direct temporal characterization of attosecond pulse trains.     

超相对论激光和稠密等离子体作用产生阿秒脉冲的优化

利用一维粒子模拟程序研究了超相对论激光脉冲与稠密等离子体相互作用得到的阿秒脉冲.从超相对论近似的角度分析了电子运动行为和高次谐波的产生,发现当等离子体密度一定时,随着无量纲相似参数S的减小,阿秒脉冲的转换效率呈先增大后减小的趋势,因此选择适当的光强就可以得到转换效率较高的阿秒脉冲.当S一定时,随着等离子体密度的增加,阿秒脉冲转换效率有增大的趋势.这说明用适当的光强照射更稠密度的等离子体靶面,可以产生更强的阿秒脉冲.     

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.     

Possibilities for controlling attosecond x-ray pulse generation during molecular ionization by femtosecond laser radiation

We discuss the role of different factors (molecular sizes and configuration, orientation of the molecular axis with respect to the electric field of a laser pulse, the type of the molecular orbital, etc.), which characterize molecules and their state, in the formation of the nonlinear response of a molecule ionized by a high-power femtosecond laser pulse. In numerical experiments within the framework of a two-dimensional model for the H ₂ ⁺ molecular ion, we study possibilities for controlling the process of nonlinear frequency conversion of femtosecond optical radiation into X-ray radiation of attosecond duration by means of preliminary vibrational or electronic excitation of molecules. We demonstrate the possibilities of using the attosecond pulse generation as a diagnostic tool for probing vibration-rotational dynamics of molecules.     

Generation of attosecond pulses with ellipticity-modulated fundamental

We have studied experimentally and theoretically high-order harmonic generation using a laser field with a time-dependent ellipticity. We show that the harmonic emission can be confined into a narrow temporal window, in which the fundamental polarization is quasi-linear. This allows a single attosecond pulse (200 as) with a fundamental field obtained from 10 fs pulse to be generated. PACS 42.65.Ky; 42.65.Re; 32.80.Wr     

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。     

采用双脉冲驱动产生高次谐波阿秒脉冲

提出了采用双脉冲机理来产生阿秒脉冲的方法.研究发现采用双脉冲不仅可以产生单个的阿秒脉冲，从而突破目前产生阿秒脉冲的驱动源（几个飞秒的超短激光脉冲）的能量限制，而且在相同的峰值强度下，双脉冲能够产生强度更高的阿秒脉冲. 关键词： 阿秒脉冲 双脉冲机理 高次谐波     

紧聚焦激光束作用于电子实现单个阿秒脉冲输出

研究了电子在聚焦的圆偏振高斯激光束中的非线性汤姆孙散射过程，在此基础上提出了实现单个阿秒脉冲输出的新机理.通过计算机模拟，发现利用紧聚焦的激光脉冲可以有效地增大辐射脉冲链的最高峰和次高峰的峰值强度比即信噪比，从而将阿秒脉冲链变为单个阿秒脉冲输出.紧聚焦情形下，随着驱动激光强度的增大，辐射信号的脉宽变短，信噪比变大；同时当减小激光束腰半径时，辐射信号信噪比也能得到有效改善.研究还发现，利用几个光周期的极短激光脉冲与电子的相互作用也能实现单个阿秒脉冲输出. 关键词： 阿秒脉冲 紧聚焦 信噪比