光场调控二维固体材料产生高次谐波的理论研究

The high-order harmonic generation produced by the nonlinear interaction of laser and materials is the main way to generate coherent attosecond light sources at present. With the development of long-wavelength lasers, the medium for high-order harmonic generation has been extended from gases to solid materials. The solids have the advantages of compactness and high conversion efficiency of harmonics, but their damage threshold is low and the energy range of the harmonics is small, which restricts their applications. The absorption rate of high energy harmonic photons is usually high in bulk materials, while the absorption rate in two dimensional materials is relatively low, and the structure is simple and easy to be analyzed theoretically. This project is taking the two-dimensional materials as the research object, we plan to carry out the researches as follows: 1) optimization scheme of harmonic phase matching in solid systems; 2) controlling the ultrafast dynamics of electrons and holes in solids by the regulation of the external driving fields, thus maximizing the energy range of harmonic photons; 3) ultrafast dynamic imaging method of spacial symmetry and band structure of solids. The implementation of this project can deepen the understanding of the mechanism of solid high harmonic generation, further improve the conversion efficiency of harmonics and expand the energy range of harmonics, and provide theoretical support for related experiments. Besides, by controlling the external laser fields, we can also control the important parameters of solid high harmonic sources, such as ellipticity, angular momentum and phase.

激光与物质非线性相互作用产生高次谐波，是目前实现相干阿秒光源的主要途径。随着长波长激光技术的发展，高次谐波的产生介质已由气体扩展到了固体材料。固体具有紧凑、谐波转换效率高等优点，但其损伤阈值低，产生的谐波的能量范围小，制约了其应用范围。块体材料对高能谐波光子的吸收率高，而二维材料吸收率相对较低，结构简单，易于理论分析。本项目以二维材料为研究对象，开展如下研究：1）固体系统高次谐波相位匹配的优化方案；2）通过光场调控，实现对固体内电子与空穴的超快动力学过程的操控，进而最大限度提高谐波光子能量范围；3）固体空间对称群结构和能带结构的超快动力学成像方法。本项目的实施，可深化对固体高次谐波产生机理的认识，进一步提高谐波转换效率，扩展谐波的能量范围，为相关实验提供理论支持。此外，通过外部激光场的调控，还可实现对高次谐波的椭偏率、角动量和相位等重要参数的调控，促进固体高次谐波光源器件的实用化。

高次谐波是目前产生阿秒光源的主要途径，其光谱还携带了丰富的超快动力学信息。本项目以凝聚态体系为研究对象，通过光场的空间分布特点，提出了一维、二维及三维晶体高次谐波相位匹配的优化方案。通过调控光场的偏振方向，相对相位，相对强度，以及晶体的应变等，实现对晶体内电子与空穴的超快动力学过程的操控，进而优化高次谐波最大光子能量以及椭偏率等。我们还与国内国际同行合作，研究了晶体高次谐波的长短轨道干涉等。在所有团队成员的共同努力下，我们圆满的完成了原科研计划。此外，我们基于不同原子链构型下的电子动力学，提出了统计涨落模型，揭示了液体产生高次谐波的原理。我们还将理论方法推广到了斐波那契准晶体系，揭示了分形能带结构在准晶高次谐波过程的关键作用。在该项目的资助下，我们发表Phys. Rev. Lett. 两篇，ACS Photonics 一篇，PRA、PRB多篇，并多次受邀在国际会议上作学术报告。我们的研究结果可应用于新的极紫外光源、光梳等。