多色激光场中分子高次谐波的选择性增强及动力学探测

High-order harmonic generation (HHG), from the interaction between the intensive ultrafast laser and the molecule, is an important tool to understand the molecular microscopic universe. The high-order harmonics usually emit in the form of frequency comb with the similar intensities in the plateau region. However, it is possible to tune the more energy into the target harmonic (or the target channel) and to select the particular harmonic from the harmonic comb by using the multi-color laser fields with both the sub-cycle waveform control and the X-Y polarization control, and such controls provide the more effective means to probe the molecular microscopic structures and their dynamics. This project first theoretically investigates the characteristics of the molecular HHG in such multi-color laser fields and theoretically optimizes the experimental conditions, then experimentally observes the relationship between the molecular HHG characteristics (such as the intensity, the phase, the peak and etc.) and the experimental conditions (such as the molecular structure, the molecular alignment, the laser parameters and etc.) under such multi-color scheme. On the one hand, we seek the more effective enhancement of the single harmonic emission through the macroscopic phase matching and the intra-molecular phase matching, for obtaining the monochromatic and tunable extreme ultraviolet emission with the breakthrough results for the harmonic intensity and the contrast ratio. On the other hand, we selectively improve the particular harmonic or channel of the molecular HHG by using the intra-molecular phase matching conditions and the unique design of such multi-color scheme, for pertinently exploring the molecular microscopic structures and their dynamics.

强场超快激光与分子相互作用产生高次谐波是人们认识分子微观世界的重要手段。高次谐波通常以频率梳的模式产生，且各个级次（不同频率）在平台区具有类似的强度，而亚周期波形可控、X-Y偏振受控的多色激光场可将高次谐波频率梳中的能量集中调谐到某个特定级次或通道，这就为人们选择性探测分子微观结构信息及其动力学过程提供了更有效的手段。本项目首先在理论上探讨多色场驱动的分子高次谐波的辐射特性并优化多色场等条件，然后着重在实验上探寻多色场操控下的分子高次谐波的强度、相位和极值等特性与分子结构、分子排列及激光参数等条件之间的依赖关系：一方面，利用宏观和分子内相位匹配技术，寻求更有效的高次谐波单一级次的选择性增强途径，从而获得强度和对比度皆有突破的极紫外可调谐单色相干光源；另一方面，利用分子内的相位匹配条件和多色场的独特设计，选择性增强高次谐波的特定级次或通道，从而更有针对性地提取分子微观结构信息及其动力学过程。

本项目为了寻求更有效的高次谐波单一级次的选择性增强途径，获得强度和对比度皆有突破的可调谐单色相干XUV 光源，同时开拓更多的多色场操控技术用于不同原子分子的微观结构信息及其动力学过程探测的新方法、新思路。项目组首先对实验装置和实验技术进行了改进和开拓，获得了十项相关发明专利授权。项目组提出了亚周期波形可控、X-Y偏振可控的多色激光驱动场的独特设计，将高次谐波频率梳中的能量集中到某个特定级次，成功实现了特定级次的高次谐波的增强和相邻谐波的极大抑制；利用多色激光场及宏观相位匹配条件实现奇偶次波的选择性增强，也就是高次谐波的奇偶属性是可以通过多色激光场及其宏观相位匹配条件来实现操控的。本项目还在氩气中利用介质激发态实现了高次谐波的增强，并以此研发了一种等离子体状态、寿命及其动力学的诊断装置和方法。此外，项目组还研究了核运动对高次谐波光谱展宽的影响等实验。这些学术成果对XUV光源领域和等离子体诊断领域的发展将起到重要的促进作用。