低于电离阈值的谐波产生机制及其相干控制的理论研究

As a promising way to produce coherent extreme-ultraviolet to soft-x-ray radiation and attosecond pulses, high-order harmonic generation has been a leading research field in nonlinear optics. But until now its applications are still limited due to the low conversion efficiency. Therefore, how to increase the conversion efficiency and to extend as much as possible the harmonics to lower wavelength regime are the main goals for high-order harmonic generation. In the past decades, main attention has been paid to the plateau part of the harmonics with photon energies above the ionization threshold of the quantum system (ATH). Recently, lots of novel phenomena that can not be explained with the perturbation theory have been found for the below-threshold harmonics (BTH). These findings bring people new understandings of BTH, while the mechanism of BTH remains to be further studied. The radiation efficiency of BTH is expected to be orders of magnitude higher than ATH, so it would be much easier to obtain ultrafast coherent light source of high brightness from BTH. .In this project, with the combination of quantum trajectory theory and quantum mechanics, theoretical and numerical methods for dealing with the strong field quantum dynamics will be further developed. By solving numerically the Bohmian-Newtonian quantum trajectory equations and the time-dependent Schrödinger/Dirac equations, different kinds of resonance and ionization processes induced by strong laser fields in atomic and molecular systems will be simulated and their effects on the generation of BTH will be investigated. Through the visualization of the quantum trajectories, comprehensive analysis and deep insight will be given into the dynamics of BTH generation, and the mechanism of BTH generation under different conditions will be revealed. By exploring the dependence of the BTH spectra on the parameters of the laser field and the quantum system, coherent control of the generation processes will be achieved and the characteristics of BTH will be optimized. This study will provide theoretical support and guidance for the experiments to obtain tunable coherent radiation of high brightness and attosecond pulse generation.

过去人们对高次谐波产生的研究主要关注的是由光子能量在量子体系电离阈值之上的较高阶次的谐波组成的平台区。近年来，实验上发现低于电离阈值的谐波（BTH）部分具有许多用微扰理论无法解释的新奇现象，使人们对BTH有了新的认识，与此同时BTH的物理机制有待进一步研究。本项目将结合量子轨迹理论与量子力学方法，进一步发展处理强场激光和量子体系相互作用动力学过程的理论和数值计算方法，通过数值求解量子轨迹动力学方程与含时薛定谔方程或狄拉克方程，模拟强场激光作用下原子、分子体系中各种共振及电离过程在BTH产生过程中的作用，利用可视化的量子轨迹综合分析不同外场条件下BTH辐射的动力学过程，全面揭示BTH的产生机制；研究激光场及量子体系各状态参量与BTH频谱结构的关系，实现对BTH辐射过程及其频谱特性的相干控制，为实验获得可调谐的高亮度相干辐射和阿秒脉冲输出提供指导。

由于低于电离阈值的低次谐波的产生具有许多用微扰理论无法解释的新奇现象，使得低阶次谐波的产生过程及其物理机制成为人们研究的热点课题。本项目研究发展了处理强场激光和量子体系相互作用动力学过程的理论和数值计算方法，通过对不同外场条件、不同电离区域内量子体系波包的动力学演化过程和低于电离阈值的谐波辐射过程的研究分析，探索了束缚势场分布、非偶极效应及相对论效应对量子波包演化过程的影响，发现了奇次谐波场与超级拉曼场之间的时空可分辨性以及电场频谱的分辨率对激发脉冲波形具有显著的依赖性；由于非偶极效应的存在，在具有球对称性的原子体系中获得了二次谐波X射线辐射；光电子的相对论效应使得光电子谱的束缚能发生蓝移，同时谱线强度降低；通过对入射光场波形、强度、频率等参量的改变，获得了转换效率、偏振特性等可调控的低阶次谐波场输出，实现对谐波辐射谱及量子态分布的相干控制。.目前，基于本项目研究，我们已正式发表相关学术论文17篇。在项目执行期间，项目组成员参加国际学术会议12人次，参加国内学术会议8人次；项目组还与国外合作研究单位进行了8人次的学术交流访问。

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