Isolated attosecond pulses generated by relativistic effects in a wavelength-cubed focal volume

Lasers that provide an energy encompassed in a focal volume of a few cubic wavelengths (lambda3) can create relativistic intensity with maximal gradients using minimal energy. With particle-in-cell simulations we found that single 200-as pulses could be produced efficiently in a lambda3 laser pulse reflection by means of deflection and phase compression caused by the coherent motion of the plasma electrons that emit these pulses. This novel technique is efficient (approximately 10%) and can produce single attosecond pulses from the millijoule to the joule level.     

Single-pinhole diffraction of few-cycle isolated attosecond pulses with a two-color field

The spatio-temporal characterization of an isolated attosecond pulse is investigated theoretically in a two-color field.Our results show that a few-cycle isolated attosecond pulse with the center wavelength of 16 nm can be generated effectively by adding a weak controlling field. Using the split and delay units, the isolated attosecond pulse can be split to the two same ones, and then single-pinhole diffractive patterns of the two pulses with different delays can be achieved. The diffractive patterns depend severely on the periods of the attosecond pulses, which can be helpful to obtain temporal information of the coherent sources.     

Single attosecond pulse generation in the multicycle-driver regime by adding a weak second-harmonic field

We present a method of producing single attosecond pulses by high-harmonic generation with multicycle driver laser pulses. This can be achieved by tailoring the driving pulse so that attosecond pulses are produced only every full cycle of the oscillating laser field rather than every half-cycle. It is shown by classical and quantum-mechanical model calculations that even a minor addition (1%) of phase-locked second-harmonic light to the 800 nm fundamental driver pulse for high-harmonic generation leads to a major (15%) difference in the maximum kinetic energies of the recombining electrons in adjacent half-cycles.     

Generation of isolated attosecond pulses with a specific waveform two-color laser field

We theoretically propose a new method for generating intense isolated attosecond pulses during high-order harmonic generation (HHG) process by accurately controlling electron motion with a two-color laser field,which consists of an 800-nm,4-fs elliptically polarized laser field and a 1400-nm,～43-fs linearly polarized laser field.With this method,the supercontinua with a spectral width above 200 eV are obtained,which can support a ～15-as isolated pulse after phase compensation.Classical and quantum analyses explain the controlling effects well.In particular,when the pulse duration of the 800-nm laser field increases to 20-fs,sub-100-as isolated pulses can be obtained even without any phase compensation.     

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.     

Isolated attosecond pulse generation with few-cycle two-color counter-rotating circularly polarized laser pulses

Most of the schemes for generating isolated attosecond pulses(IAP) are sensitive to the carrier-envelope phase(CEP)of the driving lasers. We propose a scheme for generating IAP using two-color counter-rotating circularly polarized(TCCRCP) laser pulses. The results demonstrate that the dependence of the IAP generation on CEP stability is largely reduced in this scheme. IAP can be generated at most of CEPs. Therefore, the experiment requirements become lower.     

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.     

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.     

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     

优化两色组合场参数获得单个阿秒脉冲

基于经典方法优化了组合场中基频场和低频场的强度比,利用优化后的组合场驱动一维氦原子,通过求解含时薛定谔方程研究了氦原子产生的高次谐波,其截止位置得到扩展,再考虑基频场初相位对产生高次谐波的影响,能够得到脉宽为37as的单个阿秒脉冲.     

优化两色组合场参数获得单个阿秒脉冲

基于经典方法优化了组合场中基频场和低频场的强度比,利用优化后的组合场驱动一维氦原子,通过求解含时薛定谔方程研究了氦原子产生的高次谐波,其截止位置得到扩展,再考虑基频场初相位对产生高次谐波的影响,能够得到脉宽为37as的单个阿秒脉冲.     

利用两色组合激光场驱动氦原子产生单个阿秒脉冲

对由波长为1600 nm和800 nm两色激光形成组合场驱动氦原子产生的高次谐波进行了研究,通过改变组合场中主要发射高次谐波期间电场的上升沿和下降沿随时间不同的变化率,使电子长路经和短路径发射谐波的特性发生变化,将由此产生的高次谐波进行叠加时可以得到更短的阿秒脉冲,再调节两束激光场的延迟时间,能够有效地实现抑制电子短路径的贡献,从而得到脉宽为33.7 as的单个阿秒脉冲.     

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     

Use of electron correlation to make attosecond measurements without attosecond pulses

We describe how correlations between electrons can be used to trace the dynamics of correlated two-electron ionization with attosecond precision, without using attosecond pulses. The approach is illustrated using the example of Auger or Coster-Kronig decay triggered by photoionization with an extreme ultraviolet pulse. It requires correlated measurements of angle-resolved energy spectra of both the photo- and Auger electrons in the presence of a laser pulse. To reconstruct the dynamics, we use not only classical time and energy correlation, but also entanglement between the two electrons.     

阿秒激光脉冲的新进展

文章概述了阿秒脉冲产生的基本原理及测量方法，并在此基础上对最近成功报道的产生单个阿秒脉冲的实验进行了详细介绍，指出了阿秒脉冲发展中需要解决的问题及今后的应用前景。     

Self-compression of attosecond high-order harmonic pulses

Self-compression of attosecond high-order harmonic pulses in the harmonic generation medium itself has been demonstrated. The attosecond pulses were generated in an argon-filled gas cell and compressed by exploiting the dispersion characteristics of argon. Since the harmonic generation medium itself was used as the compression medium, continuous chirp control was easily achieved by adjusting the gas pressure. The optimized attosecond pulse was also the most intense, and its duration of 206 as was very close to the transform-limited value of 200 as.     

Probing electron-electron correlation with attosecond pulses

We study two-photon double ionization of helium in its ground state at sufficiently low laser intensities so that three and more photon absorptions are negligible. In the regime where sequential ionization dominates, the two-photon double ionization one-electron energy spectrum exhibits a well defined double peak structure directly related to the electron-electron correlation in the ground state. We demonstrate that when helium is exposed to subfemtosecond or attosecond pulses, both peaks move and their displacement is a signature of the time needed by the He⁺ orbital to relax after the ejection of the first electron. This result rests on the numerical solution of the corresponding non-relativistic time-dependent Schrödinger equation.Received: 17 January 2003, Published online: 18 March 2003PACS: 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 32.80.Dz Autoionization     

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。     

Generating single attosecond pulses via spatial filtering

The first observation of isolated attosecond pulses by Hentschel [Nature 414, 509 (2001)] resulted from an experiment that left the exact mechanism of their generation unresolved. A complete simulation of the experiment reveals the reason for its success: single pulses were efficiently isolated from two or more generated pulses by spatial filtering in the far field. Our explanation suggests a new, simple paradigm for the production of isolated attosecond bursts. We show that this method can be used, in conjunction with carrier-envelope phase stabilization, to select single attosecond pulses by use of 10 fs driving pulses.