单纯气体和混合气体中原子发射高次谐波的宏观效应

High-order harmonic generation (HHG) is an extreme nonlinear optical process occuring when atoms are exposed to an intense infrared laser field, and the wavelength of HHG has been extended into the extreme ultraviolet (XUV) or soft x-ray regime. It is an ideal light source to use the continueous harmonics in the cutoff region to produce the attosecond pulses. Thus it has the important scientific meaning and application prospect to study the HHG, especially the macroscopic HHG in a gas medium for producing the attosecond pulses. In this project, we will establish the relation between the macroscopic HHG emitted from a (inert)gas medium (or mixed gases) and the characteristic of the gas drived by combined fields. By relying on the numerical simulation, we will study the phase-matching conditions of high harmonics in a gas, and the interference effect of high harmonics in mixed gases. Specifically, by changing the characteristics of the gas, such as the pressure of the gas, width of a gas jet, the ratio between two gases, the form of the combined fields and so on, we will investigate how the harmonics behave after macroscopic nonlinear propagation at the exit of a gas medium or at the far field (detecting system). These studies will allow one to optimize the harmonic efficiency by changing the characteristics of the gas, achieve the goal of producing an isolated attosecond pulse (IAP) with good coherence and high brightness, and provide the theoretical support for implementing this sort of high-quality IAP experimentally..Our major goal is to extend the Quantitative rescattering theory(QRS) to investigate HHG of complex system(i.e. atoms in a gas or mixed gas drived by combined fields). Based on this theoretical tool, we can then discuss some specific issues, such as the phase-matching conditions of harmonics, interference effect of HHG from mixed gases, and so on. In order to make the theoretically proposed methods for producing an IAP applicable experimentally, we also need to consider the further propagation (in the vacuum) of harmonics emitted at the exit of a gas medium. All these together allow one to completely describe an experimentally measured HHG spectrum. On the other hand, it needs to optimize the spatial and temporal emission of harmonics to obtain an IAP with good coherence and high brightness. This can be achieved by optimizing the macroscopic conditions, which are involved in the generation, propagation, and detection processes of HHG. So another goal of this project is to produce a high-quality IAP towards the attosecond nonlinear interaction by nicely taking into account all the experimental realities. Our studies in this project will provide with the completely theoretical understanding of HHG from a gas or mixed gases and applicable approaches to produce an IAP.

原子在强激光场中产生高次谐波的波长可以延伸到极紫外（XUV）乃至软X射线范围，在截断位置附近所具有的超连续性是构造超短阿秒脉冲的理想光源，对高次谐波性质的研究，特别是研究高次谐波在气体中的宏观效应对阿秒脉冲的产生具有重要的科学意义和应用前景。本项目我们将研究在单纯(惰性)气体和混合气体中原子在组合场驱动下发射高次谐波的宏观效应与气体状态之间的关系。利用数值模拟方法研究高次谐波在单纯气体中的相位匹配问题以及在混合气体中的干涉效应等特点，通过改变单纯气体的状态（如压强、厚度）和混合气体的状态（如压强、不同气体的比例等参数）以及组合场的形状等，了解高次谐波经介质非线性传播后的宏观性质以及在气体表面和远场的变化特点，优化出气体状态与高次谐波发射效率之间的关系，来达到产生相干程度强、亮度高的单个阿秒脉冲的目标，为能在实验中实现这类优质的单个阿秒脉冲提供理论依据。

研究了单纯气体氦、氖、氩和一氧化碳等在激光场中发射高次谐波的性质，通过对激光波形的控制，提出了利用氦原子发射高次谐波获得单个超短阿秒脉冲的方法，最短可得到25阿秒的超短脉冲；在考虑到气体的折射、吸收、Kerr效应以及等离子体效应的基础上，通过求解三维Maxwell方程，对单纯气体氖、氩和混合气体（由氦和氖原子组成）在激光驱动下发射高次谐波的宏观效应进行了研究，对于单纯气体（如氖原子）, 结果表明气体压强和气体靶长度对发射的高次谐波及其合成的阿秒脉冲都有影响，阿秒脉冲的强度不能单纯地依靠增加气体压强和靶的长度得到提高, 随着激光波长的增加，实现高次谐波的相位匹配会变得较为困难; 对于激光驱动下的由氦和氖组成的混合气体来说，结果表明激光参数以及气体混合比例对高次谐波干涉效应都有影响，从理论上发现在截止位置附近发生干涉相长时混合气体的最佳比例为85%；还研究了由同一束激光脉冲与两个气体靶作用发射高次谐波的干涉效应. 对于激光场中原子和分子的电离，我们证实了无论是在多光子电离区域还是在隧穿电离区域，由PPT模型计算的电离几率优于相应的ADK模型给出的结果。