Time-Domain Characterization of Macroscopic Quantum Systems via High Harmonic Generation

高谐波产生（HHG）是一个高度非线性的过程，波长在红外波段的激光可以通过频率上转换到极紫外（XUV）甚至软x射线波段。HHG也是产生阿秒脉冲的核心，阿秒脉冲是有史以来最短的光爆发，持续时间比一微秒短10亿倍以上。阿秒脉冲的自然应用是电子动力学的研究，这发生在这个时间尺度上。几十年来，人们一直致力于阿秒光源的开发和应用，提出的工作涉及开发和应用两个方面。其中一个提议的项目涉及研究原子中HHG过程中电子之间的超快相关性。这种相关性预计会在发射的阿秒XUV辐射的时间结构上赋予一个特征。因此，该项目使用HHG过程作为潜在电子动力学的探针。在另一个项目中，将研究透明固体中HHG和阿秒脉冲的产生，它们最近在实验中被发现产生相对高次谐波。虽然HHG在稀原子气体中的过程已经被很好地理解，但在这些透明固体中产生HHG的效率和机制在很大程度上是未知的。因此，该项目是对阿秒源开发的研究。所有提议的项目都将在一个理论框架内通过大规模计算来进行，在这个理论框架中，可以从从头算量子水平到可测量电场的宏观产生来描述HHG过程。拟议的工作将涉及几个新的理论发展以及现有理论框架的一些应用。设想的计算是雄心勃勃的，将代表超快激光-物质相互作用的理论和计算的最新技术。大多数提议的工作与正在进行的与超快强场物理前沿实验组的合作直接相关。这个项目的广泛影响是多方面的，涉及科学和人力资源的发展：(i)超快、相干XUV源的开发和应用进一步加深了我们对原子、分子和生物过程在几飞秒和亚飞秒时间尺度上的动力学的基本理解；（ii）拟议的工作有助于保持美国超快AMO科学的存在，而该领域通常由欧洲、亚洲和加拿大的团体主导；（iii）将通过参与、培训和培训来开发人力资源。并指导初级研究人员，如本科生、研究生和博士后。首席研究员是一名女性，她还积极参与指导妇女学习物理学，特别是在本科阶段。本课题的研究重点是在不同宏观非线性介质中通过强场产生高次谐波和阿秒脉冲来研究超快量子动力学。特别是，提出的工作是研究(i)通过自电离共振附近的高次谐波特征来研究双电子系统中相关的时域效应，更一般地说，共振（在一个或两个电子系统中）对谐波辐射的微观和宏观特性的影响，（ii）最近实验证明的周期性介质（透明固体）中的高谐波产生（HHG）。（iii）由中红外激光脉冲驱动的HHG，就其作为千电子伏阿秒脉冲源的潜力而言。其中一些研究集中在描述与量子系统的强场相互作用的内在动力学，例如相关性，或导致固体中谐波的相干过程的性质。其他研究旨在表征或优化VUV/XUV光本身的特性。这项工作将通过超快激光-物质相互作用的非绝热理论模型的持续发展和应用来完成。这种“数值非线性介质”（NNLM）允许从第一性原理完全模拟最先进的短脉冲激光实验。NNLM基于时变薛定谔方程和麦克斯韦波动方程的耦合解，具有亚周期精度，需要大规模的计算。

High harmonic generation (HHG) is a highly nonlinear process by which laser light with a wavelength in the infrared regime can undergo frequency up-conversion into the extreme ultraviolet (XUV) or even soft X-ray regime. HHG is also at the heart of the production of attosecond pulses, which are the shortest bursts of light ever produced, with durations that are more than a billion times shorter than a microsecond. The natural application of attosecond pulses is in the study of electron dynamics, which occurs on this time scale. Several decades of work have been dedicated to the development and application of attosecond light sources, and the proposed work addresses aspects of both development and applications. One of the proposed projects involves the study of ultrafast correlation between electrons during HHG in an atom. The correlation is expected to impart a signature on the time structure of the emitted attosecond XUV radiation. This project thus uses the process of HHG as a probe of the underlying electron dynamics. In another project HHG and attosecond pulse production in transparent solids, which have recently been experientally found to produce relatively high order harmonics, will be investigated. Although the process of HHG in dilute atomic gases is well understood, the efficiency of and the mechanism by which HHG are produced in these transparent solids is largely unknown. This project is thus a study in attosecond source development. All the proposed projects will be undertaken through large-scale calculations within a theoretical framework in which it is possible to describe the HHG process from the ab-initio quantum level to the macroscopic production of a measurable electric field. The proposed work will involve several new theoretical developments as well as a number of applications of the existing theoretical framework. The calculations envisioned are ambitious and will represent the state-of-the-art for the theory and computation of ultrafast laser-matter interactions. The majority of the proposed work is directly relevant to ongoing collaborations with experimental groups at the forefront of ultrafast strong field physics. The broader impacts of this project are diverse and involve the development of both scientific and human resources: (i) The development and application of ultrafast, coherent XUV sources further our fundamental understanding of the dynamics of atomic, molecular, and biological processes at the few-femtosecond and sub-femtosecond time scale, (ii) the proposed work helps to maintain a presence of ultrafast AMO science in the US while the field is generally dominated by groups in Europe, Asia, and Canada, and (iii) human resources will be developed through involvement, training, and mentoring of junior researchers such as undergraduate and graduate students and postdocs. The PI, who is female, is also actively involved in mentoring women in physics, in particular at the undergraduate level.The research in this proposal centers on the study of ultrafast quantum dynamics through the strong-field production of high order harmonics and attosecond pulses in different macroscopic non-linear media. In particular, the work proposed is to study (i) the time-domain effects of correlation in two-electron systems through the characteristics of high order harmonics in the vicinity of auto-ionizing resonances, and more generally the effects of resonances (in one- or two-electron systems) on the microscopic and macroscopic properties of the harmonic radiation, (ii) high harmonic generation (HHG) in periodic media (transparent solids) which has recently been experimentally demonstrated, and (iii) HHG driven by mid-infrared laser pulses, in terms of their potential as a source of kiloelectron volt attosecond pulses. Some of these studies center on characterizing the inherent dynamics of the strong-field interaction with the quantum system, such as correlation, or the nature of the coherent process that leads to harmonics in solids. Other studies aim to characterize or optimize the properties of the VUV/XUV light itself. The proposed work will be accomplished through the continued development and application of an ab-initio, non-adiabatic theoretical model for ultrafast laser-matter interactions. This "numerical non-linear medium" (NNLM) allows for the complete simulation of the most advanced short pulse laser experiments from first principles. The NNLM is based on the coupled solutions of the time-dependent Schroedinger equation and the Maxwell wave equation with sub-cycle precision, and requires large scale computations.