Coccia Emanuele, Luppi Eleonora
Dipartimento di Scienze Chimiche e Farmaceutiche, University of Trieste, via Giorgieri 1, 34127 Trieste, Italy.
Sorbonne Universités, UPMC Univ Paris 06, UMR 7616, Laboratoire de Chimie Théorique, F-75005 Paris, France.
J Phys Condens Matter. 2021 Nov 23;34(7). doi: 10.1088/1361-648X/ac3608.
High-harmonic generation (HHG) is a nonlinear physical process used for the production of ultrashort pulses in XUV region, which are then used for investigating ultrafast phenomena in time-resolved spectroscopies. Moreover, HHG signal itself encodes information on electronic structure and dynamics of the target, possibly coupled to the nuclear degrees of freedom. Investigating HHG signal leads to HHG spectroscopy, which is applied to atoms, molecules, solids and recently also to liquids. Analysing the number of generated harmonics, their intensity and shape gives a detailed insight of, e.g., ionisation and recombination channels occurring in the strong-field dynamics. A number of valuable theoretical models has been developed over the years to explain and interpret HHG features, with the three-step model being the most known one. Originally, these models neglect the complexity of the propagating electronic wavefunction, by only using an approximated formulation of ground and continuum states. Many effects unravelled by HHG spectroscopy are instead due to electron correlation effects, quantum interference, and Rydberg-state contributions, which are all properly captured by anelectronic-structure approach. In this review we have collected recent advances in modelling HHG by means oftime-dependent approaches relying on the propagation of the time-dependent Schrödinger equation (or derived equations) in presence of a very intense electromagnetic field. We limit ourselves to gas-phase atomic and molecular targets, and to solids. We focus on the various levels of theory employed for describing the electronic structure of the target, coupled with strong-field dynamics and ionisation approaches, and on the basis used to represent electronic states. Selected applications and perspectives for future developments are also given.
高次谐波产生（HHG）是一种非线性物理过程，用于在极紫外（XUV）区域产生超短脉冲，这些脉冲随后被用于在时间分辨光谱学中研究超快现象。此外，HHG信号本身编码了有关目标电子结构和动力学的信息，可能与核自由度耦合。对HHG信号的研究导致了HHG光谱学的产生，该光谱学已应用于原子、分子、固体，最近也应用于液体。分析产生的谐波数量、它们的强度和形状，可以深入了解例如强场动力学中发生的电离和复合通道。多年来已经开发了许多有价值的理论模型来解释和诠释HHG特征，其中三步模型最为著名。最初，这些模型忽略了传播电子波函数的复杂性，仅使用基态和连续态的近似表述。相反，HHG光谱学揭示的许多效应是由于电子关联效应、量子干涉和里德堡态贡献，而这些都可以通过电子结构方法恰当地捕捉到。在这篇综述中，我们收集了近年来通过依赖含时薛定谔方程（或其导出方程）在非常强的电磁场存在下传播的含时方法对HHG进行建模的进展。我们将范围限制在气相原子和分子目标以及固体上。我们关注用于描述目标电子结构、结合强场动力学和电离方法所采用的不同理论水平，以及用于表示电子态的基。还给出了选定的应用和未来发展的前景。
