Microscopic and Macroscopic Aspects of High Harmonic Generation

在极紫外(XUV)光谱区的短脉冲相干光可以用来研究原子、分子和凝聚相过程的超快动力学，这些过程通常发生在几飞秒的时间尺度上，在广泛的科学应用中可能是重要的。由强激光脉冲与非线性介质相互作用产生的高次谐波(HHG)过程已被证明是此类XUV光脉冲的多用途来源。在这个项目中，我们将通过理论建模和计算，同时考虑微观和宏观激光与物质的相互作用，来研究高次谐波产生的机制。其中一些研究将表征HHG产生的XUV光的性质，而另一些研究将探索强场与非线性介质相互作用的内在动力学。大部分工作将与正在进行的与美国和国外几个实验小组的合作直接相关。在与该项目相关的源代码开发研究中，路易斯安那州立大学小组将专注于表征和优化透明晶体的HHG过程。直到最近，这种晶体才被证明能产生谐波，这已经引起了许多人的兴奋，因为它有可能提高产量和容易操纵产生的光。然而，在凝聚相材料中产生谐波的机制仍然不是很清楚。在该项目的这一部分，该小组将扩展他们目前的激光-晶体相互作用的微观、量子力学模型，该模型将发射的光与晶体的能带结构联系起来，并建立一个描述XUV光在宏观介质中的产生、传播和吸收过程的大规模模型。在该项目的另一部分中，路易斯安那州立大学的研究小组将主要通过研究原子气体中激发态附近的HHG来探索强场激光与物质相互作用本身的动力学。特别是，将在发射的XUV光的空间、时间和光谱特性中搜索这些激发态的特征。

Short pulses of coherent light in the extreme ultraviolet (XUV) spectral region can be used to investigate the ultrafast dynamics of atomic, molecular, and condensed-phase processes, which typically occur on the few-femtosecond time scale, and can be important in a wide range of scientific applications. A process known as high harmonic generation (HHG), resulting from the interaction between an intense laser pulse and a nonlinear medium, has proven to be a versatile source of such XUV light pulses. In this project, mechanisms underlying HHG will be studied through theoretical modeling and calculations that consider both the microscopic and macroscopic laser-matter interactions. Some of these studies will characterize the properties of the XUV light generated by HHG, whereas other studies will probe the inherent dynamics of the strong-field interaction with the nonlinear medium. The majority of the work will be directly relevant to ongoing collaborations with several experimental groups in the US and abroad.In the source development studies associated with this project, the LSU group will concentrate on characterizing and optimizing the HHG process from transparent crystals. It is only recently that such crystals have been shown to produce harmonics, and this has generated a lot of excitement due to the potential for increased yield and ease of manipulation of the generated light. However the mechanism by which harmonics are generated in condensed phase materials is still not well understood. In this part of the project, the group will expand on their current microscopic, quantum mechanical model for the laser-crystal interaction, which links the emitted light to the band structure of the crystal, as well as build a large-scale model describing the process of generation, propagation, and absorption of the XUV light in the macroscopic medium. In a separate part of this project, the LSU group will probe the dynamics of the strong-field laser-matter interaction itself, primarily through the investigation of HHG in the vicinity of excited states in atomic gases. In particular, a search will be conducted for signatures of these excited states in the spatial, temporal, and spectral properties of the emitted XUV light.