Theory of High Harmonic Generation in Solids and Gases, at the Microscopic and the Macroscopic Level

在极紫外（XUV）光谱区域使用相干光的超快脉冲有助于我们在几飞秒和亚飞秒时间尺度上理解原子、分子和凝聚相过程的动力学。电子动力学的自然时间尺度是几十或几百阿秒，这些超快脉冲有望捕获和控制这种“运动”。由强激光脉冲和非线性介质相互作用产生的高谐波（HHG）过程已被证明是这种XUV光的通用源。在这个项目中，PI和她的团队将通过考虑微观和宏观激光物质相互作用的计算来研究HHG过程的不同方面。其中一些研究旨在表征XUV光本身的特性，而另一些研究旨在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实验小组的合作直接相关。该项目将通过超快科学的进步，以及通过培养本科、研究生和研究生水平的初级研究人员来开发人力资源，为国家利益服务。PI和她的团队将研究气体和固体中的HHG。对于气相HHG，该小组将继续与一个实验组合作，目标是在增强腔中以MHz重复率产生孤立的阿秒脉冲。对于体相HHG，一个目标将是研究生成过程的基本方面，特别是体相介质中HHG的实空间图像如何从气体介质中演变而来。这将通过一系列模型和从头算来完成。第二个目标将是建立一个灵活的数值工具，用于从一系列散装固体中进行HHG的从头计算。该工具将基于大规模带结构代码的输入和半导体布洛赫方程和麦克斯韦波方程的耦合解。这种方法将有助于处理强激光脉冲在非线性晶体中传播时产生的线性和非线性过程。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The use of ultrafast pulses of coherent light in the extreme ultraviolet (XUV) spectral region helps us understand the dynamics of atomic, molecular, and condensed-phase processes at the few-femtosecond and sub-femtosecond time scale. The natural time scale for electron dynamics is tens or hundreds of attoseconds, and these ultrafast pulses promise the capture and control of this "motion". The process of 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. In this project the PI and her group will study different aspects of the HHG process through calculations that consider both the microscopic and macroscopic laser-matter interactions. Some of these studies aim to characterize the properties of the XUV light itself, whereas other studies aim to 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 experimental groups in the US. The project will serve the national interest through the progress of ultrafast science, as well as through development of human resources via the training of junior researchers at the undergraduate, graduate, and postgraduate levels.The PI and her group will study HHG in both gases and solids. For gas-phase HHG, the group will continue a collaboration with an experimental group with the goal of producing isolated attosecond pulses at MHz repetition rates in an enhancement cavity. For bulk-phase HHG one goal will be to investigate fundamental aspects of the generation process, in particular how the real-space picture of HHG in a bulk medium evolves from that in a gas medium. This will be done through a series of both model and ab-initio calculations. A second goal will be to build a flexible numerical tool for ab-initio calculations of HHG from a range of bulk solids. This tool will be based on inputs from a large-scale band structure code and coupled solutions of the semi-conductor Bloch equations and the Maxwell wave equation. This approach will facilitate the treatment of both linear and nonlinear processes that arise as the intense laser pulse propagates through the nonlinear crystal.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.