优化激光波形产生高强度X射线高次谐波的理论研究

High harmonics generated by the interaction of an ultrafast femtosecond laser with atomic or molecular gases could provide with a new light source from the extreme ultraviolet to X-rays. However, the present intensities of X-ray high harmonics are low, which have hindered their wide applications in science and technology. New attempts are desired both theoretically and experimentally to resolve this issue. In this project, at first, we would like to develop the optimization model of laser waveform to enhance the intensity of single-atom X-ray high harmonics based on B-spline functions and genetic algorithm, and improve the optimization model of two- or three-color laser waveform to produce the single attosecond pulse in the spectral range of X-rays. Secondly, we will design a theoretical model to optimize the distributions of the intensity and phase of multicolor laser beams in the local space for enhancing the intensity of macroscopic high harmonics. Then, we will combine technologies of the multicolor laser waveform optimization and waveguide, and adjust the macroscopic conditions to improve the intensity and spatial coherence of X-ray high harmonics. Finally, we would like to further improve the intensities of X-ray high harmonics by adjusting the gas pressure in the gas jet interacting with a multicolor laser waveform. By these studies, we could understand the physical mechanisms of the phase matching for X-ray high harmonics under different circumstances and provide some hints for experimental and theoretical research studies, thus paving a way for employing the X-ray high harmonic as a new light source.

超快飞秒激光与原子、分子气体相互作用产生的高次谐波可以提供从极紫外到X 射线波段的新光源，然而，目前X 射线高次谐波的强度弱，阻碍了它在科学研究和工程技术上的广泛应用，迫切需要在理论和实验上来解决这个难题。本项目中，首先，发展基于B样条函数和遗传基因算法的激光波形优化模型，提高单原子X射线高次谐波的强度，完善两色或三色激光波形优化模型，产生X射线波段内的单个阿秒脉冲，其次，设计优化多色激光束在空间局域内的强度和相位分布的理论模型，提高宏观高次谐波的强度，再次，结合多色激光波形优化和波导管技术，调节宏观条件，改善X射线高次谐波的强度和空间相干性，最后，调节气体压强，进一步改善多色激光波形与气体靶作用产生X射线高次谐波的强度。通过这些研究，理解X射线高次谐波在不同条件下相位匹配的物理机制，为实验和理论研究提供一些思路，推动X射线高次谐波向着成为可用光源的目标迈进。

超快飞秒激光与原子、分子气体介质相互作用产生的高次谐波可以提供从极紫外到X射线波段的新光源，可以用来产生孤立阿秒脉冲。改变激光波形来提高单个原子的发射效率和改变宏观条件来调整相位匹配的机制是改善高次谐波产生效率的有效方法，亟需发展优化多色激光波形的模型和揭示不同宏观条件下高次谐波相位匹配的机制。. 本项目研究了多色激光波形或中红外激光与气体介质相互作用产生的高强度软X射线高次谐波、孤立阿秒脉冲以及相关的相位匹配机制，发展了基于基因算法的多色激光波形优化模型，探究了驱动激光的宏观传播效应对高次谐波产生过程的影响，初步提出了测量孤立阿秒脉冲的全光学方案和调控阿秒脉冲时间结构的方法。. 通过本项目的研究发现，两色优化激光波形在特定宏观条件下可以控制X射线高次谐波的光谱结构，三色优化激光波形在波导管内的宏观传播效应有利于产生短脉冲的软X射线孤立阿秒脉冲，优化两色啁啾激光脉冲形成的“时间门”结构可以有效产生软X射线孤立阿秒脉冲，利用散焦辅助的相位匹配可以拓展高次谐波的截止能量，在过电离条件下通过驱动光的波前旋转可以产生空间分离的孤立阿秒脉冲，借助环形光束可以在远场分离高次谐波场和驱动红外激光。. 本项目的研究为提高软X射线高次谐波的发射效率提供了可行的途径，为有效产生孤立阿秒脉冲提供了新的方法，为理解高次谐波和孤立阿秒脉冲产生的物理机制提供了新的思路，有利于推进软X射线高次谐波向着可用光源的目标迈进。