表征宽带阿秒软X射线脉冲与提取低电子动能区光电子魏格纳时间延迟

Recent progress in high-order harmonic generation with few-cycle mid-infrared wavelength lasers has pushed light pulses into the water-window region and beyond. These pulses have the bandwidth to support single attosecond pulses down to a few tens of attoseconds. However, the present available techniques for attosecond pulse measurement are not applicable to such pulses. Here in this program we give a new phase-retrieval method using the standard photoelectron streaking technique. The iterative algorithm allows accurate reconstruction of the spectral phase of light pulses, from the extreme-ultraviolet (XUV) to soft x-rays, with pulse durations from hundreds down to a few tens of attoseconds..Meanwhile, in recent years, attosecond streaking experiments have been used to extract the photoionization Wigner delay in atoms, molecules and condensed materials. The traditional retrieval methods assume that the spectrograms can be accurately modeled by the strong filed approximation(SFA). However, the SFA equation does not take into account the interaction between the continuum electron and the ionic core. It is a good approximation only for high-energy photoelectrons with energies higher than 30 or 40 eV. In this program, we will try to add the Coulomb effect to the electron continue state to retrieve the low energy region photoionization Wigner time delay.

最近中红外少周期飞秒激光脉冲诱导高次谐波发射的进展将阿秒脉冲的频率推广到水窗或更短波段。这些具有超宽谱宽度的脉冲脉宽甚至可以达到几十个阿秒。然而，目前存在的所有表征宽带阿秒脉冲的算法都无法准确地重构这样的脉冲。此项目中，申请人拟提出一个新的重构阿秒脉冲谱相位的方法，该方法基于标准阿秒脉冲条纹技术。我们的迭代算法能够准确地重构重构脉宽为几十到几百深紫外或水窗区阿秒脉冲。. 同时，近年来阿秒条纹实验同样可以用来提取原子、分子和凝聚态物质的光电子魏格纳时间延迟。传统提取光电离魏格纳时间延迟的算法都假定光电子谱可以用强场近似准确地描述。然而，强场近似方程并不包含连续态电子与库伦势的相互作用。基于强场近似的算法在电子能量大于30电子伏特或40电子伏特时是十分有效的。在这个项目中，申请人拟在电子连续态平面波上添加库伦效应，从而可以提取低电子动能区光电子魏格纳时间延迟。

处于软X射线到水窗波段能量区间的阿秒量级脉冲为人们研究物质内壳层电子和空穴的电荷控制、能量流的阿秒时间尺度动力学探测提供了有力工具。最近，一系列实验结果报道高次谐波平台展宽已经达到了几百个电子伏特，这些实验无一例外都使用的是少周期长波近红外激光。这些高次谐波可以用来合成脉宽为几十到数个阿秒的激光脉冲。但是由于阿秒脉冲在气体传播过程中很难做到绝对的相位匹配，所以阿秒啁啾很难被实验上完全消除，于是，实验中产生的阿秒脉冲的最短宽度到底是多少始终是困扰人们的一个问题。为了解决这一难题，我们需要在时间域上准确重构阿秒脉冲。最近，三个实验组都声称自己获得了最短可以达到40-50阿秒量级的脉冲，我们依托目前的项目，开发了自相关函数重构阿秒脉冲的代码，并与实验结果做了对比，我们发现，应用我们开发的代码获得了更加准确的结果。结果表明，我们的算法更加准确快速，是重构阿秒脉冲的一个有力工具。