高次谐波频谱红移的理论研究

High-order harmonic generation (HHG) is a hot topic in the research field of strong laser pulses interacting with atoms and molecules. HHG is a very important way to generate attosecond ultrashort optical pulses. It has been reported theoretically and experimentally that blue shift of HHG occurs as the intensity of the lasers becomes higher. Recently, the applicant predicted that red shift of HHG will appear in the resonant systems via nonadiabatic process for the first time. It has been confirmed experimentally by Prof. Ozaki's group in Canada. The proposed model can only interpret the phenomena qualitatively, and the physics behind the red shift of HHG is worthy of deep studies. This project intends to introduce the contributions of long-lived excited state and extend the well accepted strong-field approximation (SFA) model in which only the contributions of the ground and continuum states are considered. A four-step transition model will be built which should be suitable to the resonant systems. Based on the proposed model, this project will fully study the influences of molecular dissociation and the propagation effects of HHG in mediums. Quantitative agreement between experimental measurements and theoretical simulations is expected. The control of spectral shift of HHG will be studied. The project will expand the previous SFA model, which is widely accepted in HHG, by introducing the contribution of a resonant state. The implementation of this project can reveal the non-adiabatic electron dynamics in strong-laser fields, and promote the development of theoretical and experimental research of HHG and the attosecond pulse generation.

高次谐波是强激光与原子分子相互作用领域里一个非常重要的研究课题，是产生阿秒超短光脉冲的重要途径。此前有很多实验及理论报道，随着入射激光场强度的增加，高次谐波频谱会出现蓝移现象。申请人首次理论预言了共振体系非绝热高次谐波频谱会出现红移，随后该现象被加拿大Ozaki实验组所证实。申请人此前的理论模型只能定性解释实验现象，因此高次谐波频谱红移背后的深层物理机制十分值得研究。本项目拟引入长寿命共振激发态对高次谐波过程的贡献，拓展此前广为接受的只考虑基态和连续态的强场近似模型，建立适合共振体系高次谐波产生的四步跃迁模型。本项目将基于该四步模型，定量研究分子解离过程及高次谐波在介质中的传输效应对频谱红移的影响，以期定量解释最近实验上观测到的高次谐波红移现象，并提出能定量控制高次谐波频谱移动的方案。本项目的实施能够揭示电子的非绝热动力学过程，推动高次谐波的理论和实验研究，并可应用于超短阿秒光脉冲的产生。

高次谐波是目前产生阿秒相干光源的重要手段，研究对象已经从激光与气体相互作用拓展到激光与固体相互作用。我们围绕高次谐波在激光强度增大时频谱反常红移的现象做了系统研究。我们的研究结果表明，高次谐波的频谱红移是个普遍现象，广泛存在于激光与气体原子分子，等离子体和固体的相互作用中。该反常红移既存在于多光子电离区，又存在于隧穿电离区。导致高次谐波频谱红移的原因如下：1）原子分子及等离子体系统长寿命共振激发态的影响；2）分子解离过程导致的电离几率的不对称；3）固体系统不同能带的弛豫。在系统共振激发的研究上，我们的研究成果已经得到了国际同行的实验证实（Phys. Rev. Lett. 117, 203001 (2016)）。在分子解离导致的高次谐波频谱红移上，我们已经与华中科技大学开展合作，在实验上证实了我们的理论，相关结果已经投稿。高次谐波的频谱移动记录了原子分子内的超快动力学信息，通过我们的理论方案进行解码，可得到系统内阿秒时间尺度的动力学信息。利用高次谐波频谱红移，可以通过改变驱动激光的光强，连续调节高次谐波的光子能量，操控相干阿秒光源。在该项目的支持下，我们已经在Phys. Rev. A等期刊上发表相关科学论文6篇，在国际会议上作邀请报告两次。