Investigation on the Characteristics of High-order Harmonic Generation in Molecules

随着超快激光技术的发展，对强激光场与物质相互作用的研究也步入了新的阶段。高次谐波在合成阿秒脉冲以及获得高亮度极紫外相干光源等方面具有重要意义，而且可以利用高次谐波的超快特性对一些动力学过程实现高时间、空间分辨率的观测。对分子高次谐波特性的研究可以帮助人们更好地理解高次谐波的产生过程。分子具有的复杂对称性以及分子轨道构型在人们逐步深入理解强场与物质相互作用机制的过程中，既是挑战，也是机遇。 一方面，本论文对分子产生高次谐波中的一些问题进行了理论研究，主要方法是基于含时密度泛函理论（Time-dependent density functional theory, TDDFT）的数值模拟。TDDFT是一种描述多电子体系的从头计算方法，在分子高次谐波的计算上具有先天的优势，并且正越来越受到关注。另一方面，利用实验室钛宝石飞秒激光放大系统，在实验上探索了高次谐波在气体介质等离子体密度诊断方面的应用方法。主要研究进展与创新成果包括以下几个方面： 1. 利用TDDFT理论对氢分子近电离阈值高次谐波的性质进行了研究。对于过去实验中发现但没有较好解释的反常椭偏依赖现象，TDDFT可以给出较好的细节描述，并与实验报道相符，从而解释了这种反常椭偏依赖出现的原因。计算结果表明近电离阈值附近的高次谐波有可能提供一种可控性良好的椭偏极紫外相干光源。 2. 高次谐波的多轨道问题是目前的研究热点，而隧穿电离是高次谐波产生过程的第一步。利用TDDFT理论研究了在波长为1800纳米的激光脉冲作用下氮分子不同分子轨道的隧穿电离概率及其角度分布。研究表明不同的电场强度会对氮分子各分子轨道的隧穿电离带来一些影响，这些影响可以部分归因于电场对电子云的极化作用。 3. 在实验上实现了利用高次谐波测量气体介质等离子体密度的时间演化规律。采用飞秒强激光来快速电离气体介质，利用高次谐波研究了等离子体在几十皮秒内的绝热演化过程。这种方法的优点在于其具有很高的时间分辨率，以及可以相对比较直观地反映出等离子体密度。Study of light-matter interaction in strong laser fields has entered a new stage with the development of ultrafast laser technology. High-order harmonic generation (HHG) has great significance for generation of single attosecond pulses and high-brightness coherent extreme ultraviolet (XUV) sources, and provides an ultrafast detecting approach for some dynamical processes with a high temporal and spatial resolution. Investigation on the characteristics of HHG in molecules can help people understand its generation mechanism. The complexity of molecular orbital-symmetry and configuration presents both challenges and opportunities in the study of strong-field physics. On one hand, we theoretically investigated some of the issues in HHG of molecules by means of numerical simulation based on time-dependent density functional theory (TDDFT). As an ab initio method for multi-electron system, TDDFT is well suited for study of HHG in molecules, and has become a theoretical tool of increasing concern. On the other hand, using a Ti: sapphire laser amplifier facility, we experimentally quested for the application of HHG on measurements of time-evolution of electron density in plasma. The main innovative results are listed as follows: 1. We study the characteristics of the near-threshold (NT) harmonics of pre-aligned H2 molecules using TDDFT. Our calculation explains the origin of anomalous ellipticity dependence in NT harmonics, and provides detailed descriptions coinciding with experimental observations. It is also revealed that NT harmonics might provide coherent, elliptically polarized XUV sources with good controllability. 2. Multi-orbital characteristic of HHG is a focused issue, where tunnel ionization is the first step. We study the tunneling probability and angular distribution for different molecular orbitals of N2 in response to 1800nm laser pulse by means of TDDFT. It is revealed that strength of the electric field can influence tunneling process of each molecular orbital, which might be attributed to electronic polarization in molecule. 3. We propose a new method of measuring time-evolution of electron density in plasma by HHG. The gas medium is ionized by a strong femtosecond pulse and the adiabatic evolution of electron density is measured by HHG. This method has a very high temporal resolution and is relatively intuitive