基于正交场的分子高次谐波啁啾特性及其调控研究

The chirp nature of high-order harmonics is an important feature of the high-order harmonic signals which is generated from the interaction of an intense laser field with atomic or molecular gases. It determines to a large extent the pulse duration and shape of the coherent attosecond pulse produced by high-order harmonic generation. The modulation of the harmonic chirp would lead to the pulse-duration control and the shaping of the attosecond laser pulse. This is of great significance to the attosecond science. In this project, we will investigate the chirp nature of the molecular high-order harmonic signals and explore the possible methods to modulate the high-order harmonic chirp. We first extend the pump-probe scheme proposed by D. Shafir et al. to molecular high-order harmonic generation including the effects of multiple ionization channels, molecular structure and dynamics. And then using this extended scheme to theoretically investigate the influence of multiple ionization channels and coulomb potential on the ionization and return time of the recollision electrons. Combining with the Gabor time-frequency analysis, we will discover the new feature of the molecular high-order harmonic chirp due to the effect of multiple ionization channels, molecular structure and dynamics. By the numerical fitting method, the relationship between the electron return time and the harmonic emission time in molecular high-order harmonic generation will be revealed. We will further investigate the roles of multiple ionization channels, molecular structure and dynamics playing in this relationship, to explore the possible methods of modulating the chirp in high-order harmonic signals.

高次谐波啁啾是强激光场与原子分子相互作用辐射的高次谐波谱的重要特性之一。它在很大程度上决定了基于高次谐波合成的相干阿秒脉冲的脉宽和波形。对高次谐波啁啾的调控，可以实现阿秒脉冲脉宽调控和波形整形，这对阿秒科学具有非常重要的意义。本项目拟开展基于正交场的分子高次谐波啁啾特性及其调控研究。推广基于原子高次谐波的正交场泵浦-探测技术，使其包含多电离通道、分子结构和动力学过程等因素的影响。并用于研究多电离通道和库伦势对碰撞电子波包电离回复时间的影响，以及结合Gabor时频分析方法发掘分析由多电离通道、分子结构和动力学过程导致的高次谐波啁啾新特性。通过比较分析以及数值拟合方法，揭示分子高次谐波的碰撞电子回复时间与高次谐波辐射时间的关系，探究多电离通道、分子结构和动力学过程对此关系的影响，挖掘高次谐波啁啾调控的新方法。

强场物理的发展已经能够在阿秒时间尺度和埃空间尺度上探测分子的结构和动力学过程。利用强激光场与物质相互作用产生的高次谐波，强场电离等过程，可以实现超快电子动力学过程的探测。我们首先推广了基于高次谐波的泵浦-探测技术，将其用于研究等离子体中的电子-原子碰撞动力学过程。通过分析多色多光周期组合场和圆偏激光场驱动的原子高次谐波的啁啾特性，我们讨论了由这些高次谐波合成的单阿秒脉冲相对时延的稳定性问题。为了准确地探测原子分子中的电子运动，我们利用中红外场调制的氢原子紫外单光子电离谱，对激光场包络进行了精确地重构，并且该重构方法可推广到结构复杂的激光场。此外，我们还研究了强激光场作用下的原子双电离动力学过程，揭示了非次序双电离过程中的阿秒干涉现象，并发现当增加驱动激光场强度时，双电离机制很明显地从次序双电离向非次序双电离转变。