原子和晶体发射高次谐波的动力学研究

High-order harmonic generation(HHG) of matter exposed to laser field is not only a tabletop high-frequency coherent extreme ultraviolet(XUV) source and an important way of getting ultrashort attosecond pulse, but also can be used to detect the internal structures of the matter. So, the study of HHG has important scientific significance and broad application prospects in the future. Recent studies have shown that HHG spectra from crystals in laser field have some similar behaviors, and different characteristics from HHG spectra of gas-phase atom exposed to laser field. In this project, we will theoretically study the kinetics of HHG emission of atoms and crystals in laser fields, by numerical solution of the time-dependent Schrodinger equation of atom and crystal in laser field, and combining with synchrosqueezing transform technology. Through our research, the careful analysis for the dynamic process of the HHG emission of atoms and crystal in laser field can be achieved. By comparing the dynamic differences in each step of the “three-step model” (tunneling, propagation, recobination) during the HHG emissions for the crystals with the covalent bonds, Van der Waals bonds and atoms in laser field, we can understand the dynamic characteristics of the HHG emission from the crystals with the different chemical bonds; Finally, based on the understanding the characteristics of HHG emission from the crystals, we are going to achieve to control the process of HHG radiation of crystals in laser fields by optimizing the laser parameters with genetic algorithm, and trying to enhance intensity of the second plateau(or higher plateau) of HHG from crystals. Our studies in this project will provide theoretical basis for getting stronger HHG from the crystals in laser fields.

物质在激光场中发射的高次谐波不仅是轻便的高频相干XUV光源，是获得超短阿秒脉冲的重要途径，而且还能够用来获得物质内部的结构，因此对高次谐波性质的研究具有重要的科学意义和广阔的应用前景。最近的研究表明，晶体在激光场中发射的高次谐波与气体状态的原子发射高次谐波既有相似的表现，又有不同的特点，本项目拟从理论上研究原子和晶体在激光场中发射高次谐波的动力学过程，通过数值求解含时薛定谔方程方法，结合同步压缩变换方法，实现对原子和晶体在激光场中发射高次谐波的动力学过程进行细致的研究，通过比较共价键、范德瓦斯晶体和原子在“三步模型”中每一步过程(隧穿、演化、复合)的差异，深刻理解不同化学键结合的晶体发射谐波的动力学特征；在理解晶体发射高次谐波特征的基础上，利用遗传算法优化激光参数，实现对晶体发射高次谐波进行控制，并提高第二谐波平台(或更高平台)的发射强度，为从晶体获得强的高次谐波提供理论依据。

原子和晶体在激光场中发射的高次谐波有重要的应用, 高次谐波不仅是台面系统的极紫外光源和获得超短阿秒脉冲的重要途径,而且还可用来探测物质内部的结构等，因此对原子和晶体发射高次谐波性质的研究具有重要意义。本项目研究了原子和晶体在激光驱动下发射高次谐波的特点以及原子和晶体所发射低阶谐波的性质。考虑了高次谐波在介质中的宏观效应，通过遗传算法对多色激光与氖原子相互作用过程中激光参数进行了优化，在高次谐波的中能区能够得到脉宽为47as的单个超短脉冲；提出了利用原子发射低阶谐波的侧峰结构特点，推算出原子的低激发态在激光场中最大Stark移动,并对氢分子离子发射低阶谐波的动力学过程进行了分析,给出了这种离子产生低阶谐波时-频图中出现特殊结构的原因；在晶体发射高次谐波方面，对于零带隙的晶体，通过速度规范和么正变换，提出了一种在狄拉克点避免数值计算发散的方法，以该方法为基础，研究了2D晶体单层石墨烯在激光场中发射高次谐波的特点与激光极化之间的依赖关系，表明单层石墨烯晶体发射的高阶谐波和低阶谐波对激光极化有不同的依赖关系，说明单层石墨烯与原子产生高次谐波对激光极化方向的依赖不同，在产生THz波方面，实现了少周期激光脉冲驱动单层石墨烯能够获得宽频Thz波辐射的方法；通过两束反旋的圆极化和椭圆极化激光驱动氦原子，能使原子发射高次谐波形成具有不同椭偏度的阿秒脉冲输出。