【作者】 成春芝；

【导师】 周效信；

【作者基本信息】 西北师范大学 ， 原子与分子物理， 2011， 硕士

【摘要】 阿秒(10-18s)脉冲可以探测原子内电子的超快动力学过程,从而揭示物质的结构和动力学特征。因此,研究阿秒脉冲的实现有重要意义。当高强度的激光场与原子或分子相互作用时可以产生高次谐波,人们可以利用高次谐波获得相干极紫外高频段辐射源XUV和X射线源。高次谐波平台区的末端会出现超连续的特点,利用这些超连续性的谐波是实现阿秒脉冲的重要手段之一。理论上研究原子在强激光场中产生的高次谐波可以利用强场近似和数值求解原子在强激光场中含时薛定谔方程。本论文首先是在强场近似下,利用Lewenstein模型来处理强激光场中原子产生高次谐波的过程,介绍了原子的Lewenstein模型的理论推导过程,接着研究了在长波红外激光场(波长800nm～2000nm)驱动下氢原子产生的高次谐波,分析了在截止位置附近高次谐波的转换效率随激光波长的变化规律。与大多数理论者研究的在平台区域原子发射高次谐波的转换效率随波长的变化不同的是,发现在截止位置附近原子发射高次谐波的转换效率比平台区域的转换效率低,但获得的阿秒脉冲的宽度会随波长的增大而缩短。本论文还数值求解了一维模型原子在强激光场中的含时薛定谔方程,利用分裂算符方法研究了一维模型He~+离子在中心波长为1064nm的线性啁啾激光和高频波脉冲形成的组合场中产生的高次谐波,分析了由高次谐波产生的阿秒脉冲的特征。发现在组合场中,由于啁啾脉冲的作用和在适当的时刻加入了高频脉冲,不仅使高次谐波谱的平台区域能得到很大的扩展,而且谐波转化效率也得到有效的提高,当对第二平台区域的不同范围内高次谐波迭加都可得到单个阿秒脉冲,最短可达21阿秒。为了更好的理解高次谐波展宽与阿秒脉冲发射的物理过程,我们还对该过程进行了经典分析和时频分析。更多还原

【Abstract】 The attosecond pulse can probe ultrafast electron dynamics in atoms and reveal public material structure and dynamics characteristic . Therefore, it is very important to research the realization of attosecond pulse. It is well-known that the interaction of intense laser field with atoms and molecules can lead to high-order harmonic generation (HHG). Coherent sources of radiation in XUV and x-ray regions can be obtained by HHG. The end of the plateau of the harmonic generation will appear a supercontinuous characteristics, and that HHG is one of the important means by these supercontinuum to synthesize attosecond pulses.Theoretically, HHG can be well studied for atoms by the strong field approximation and solving the time-dependent Schr?dinger equation in intense laser field. Firstly, within the strong field approximation,we study the high-order harmonic behavior of atoms exposed to an intense laser field using Lewenstein model. The Lewenstein model for atoms is introduced, then we study high harmonic generation of hydrogen atom exposed to the long wavelength infrared laser field(800nm～2000nm), and analyze the wavelength dependence of conversion efficiency of high-order harmonic generation near the cut-off position. It is different from the wavelength dependence of high-order harmonic yield studied by most theorists in the plateau of the harmonic generation, and it is found that high-order harmonic yield is much lower near cut-off than in the beginning of the plateau of the harmonic generation, but the width of the attosecond pulse is shorter with increasing wavelength.Moreover, we study the generation of isolated attosecond pulses from a one dimensional(1D) model He~+ ion in an intense few-cycle chirped laser and its high frequence pulses by solving the time-dependentSchr equation with split-operator method, and analyze the characteristics of the attosecond pulse produced by HHG. Due to the presence of the chirped pulse and the high frequence pulse in the case of the combined fields, it is shown that the plateau of HHG is extended largely and the conversion efficiency of HHG is enhanced effectively. By superimposing some orders harmonics on different region for the second plateau, all of the pulses obtained are single attosecond ones, minimum pulse achieves 21as. To better understand the physical origin of HHG enhancement and attosecond pulse emission, we perform classical analysis and the time-frequency analysis.更多还原