原子在激光场中的激发和电离
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摘要
强激光与原子分子相互作用中产生的一系列非线性现象越来越受到了人们的关注。如今强场物理快速发展,已经不再是一个只存在于实验室或者计算机模拟里面的“空中楼阁”,在很多与人们生活息息相关的领域都发挥了至关重要的作用:医学领域和生物工程方面的全息立体成像技术、利用超紫外射线在半导体器件上的光刻技术、生命科学里面的电子探针X射线显微分析乃至人工激光诱导核聚变等等。在这些领域中都能找到强场物理的影子,可以说“旧时王谢堂前燕,飞入寻常百姓家”。但是受限于超紫外光源的成本、计算模拟的资源开销,很多问题还没有得到解决。作者理论研究了强场物理中的三个热点问题,为这几个问题的解决提供了新的方案和思路。这三部分的具体内容包括:
     第一,我们基于Lewenstein理论计算了氮气分子的高次谐波谱。我们通过在x轴添加弱的线性激光改进了传统的“极化门”方案。通过调节脉宽等激光场参数成功地将阿秒脉冲的强度提高了两倍,同时阿秒脉冲的持续时间缩短到了105阿秒。时频分析的结果表明,改进的方案提高了氮气分子的电离几率从而提高了谐波的效率。此外我们发现当分子轴与激光场的夹角越小谐波的效率越高。
     第二,我们研究了激光场作用下原子束缚态跃迁产生的载波相位效应。通过求解三维含时薛定谔方程得到了原子束缚态在激光场结束之后的布居数。我们发现即使激光脉冲持续时间达到了几十个光周期,原子束缚态的布居数仍然能够展示出明显的载波相位效应。利用含时微绕理论我们发现双光子跃迁可以分解成两个振幅相同、携带相位信息不同的跃迁子路径。这两个跃迁子路径之间的干涉造成了长脉冲下的载波相位效应。另外我们利用相邻高里德堡态之间能级差小这一性质,得到了低频场的载波相位效应。我们的结果为实验物理学家提供了一个探测激光脉冲的绝对相位效应的新方案。
     第三,基于B样条函数和球坐标系我们开发了一套求解He原子与激光场作用的三维含时薛定谔方程的方法。作者详细介绍了在球坐标中如何把含时薛定谔方程转化为一组耦合微分方程组,通过求解这个微分方程组得到He原子在激光场作用后的波函数。通过对比其他文献中的He原子基态能级和激光场结束之后的电离几率,验证了程序的精确性。另外我们分别计算了不同初态电离几率同脉宽的变化曲线,我们发现尽管这两个态的能级非常相似,但是初态在1s2p态的初态在1s2s态的情况电离几率要大两倍左右。为了节省计算时间,我们成功地将程序改写为适用于GPU计算的并行程序,使得计算速度照比CPU提高了近一百五十倍。
Nonlinear phenomena in the interaction between strong field and atom or moleculehave attracted lots of attention. Recently, the strong field has developed fast. Thestrong field plays an important role in many areas, holographic imaging and stereoimaging in medicine and bioengineering, electron microprobe X-ray micro-analysis inbiological science, laser fusion, and so on. However, lots of problems in strong fieldremain unsolved, limited by the costs of the XUV laser source and the speed of CPU.Three hot topics of strong field were studied theoretically in this thesis. We providednew schemes and ideas to solve the problems. This thesis is divided into three parts:
     First, we present a new scheme to control the harmonic generation process fromaN2molecule with a modulated polarization gating. The polarization gating ismodulated by a weaker linearly polarized pulse. We shorten the attosecond pulseduration by31attoseconds, and enhance the efficiency by one time. Moreover, weinvestigate the dependence of (HHG) on orientation ofN2molecule and find theHHG intensity is enhanced when the angle between the axis of the molecule and theapplied electric field decreases.
     Second, we provide a path to investigate the carrier envelope phase (CEP) effectin atomic bound-bound transitions. An obvious CEP effect on a two-photonthree-level transition can be observed for a long laser pulse which contains severaltens of optical cycles. The CEP effect originates from the interference between thesum-frequency and the difference-frequency within the two-photon transition. Andthis interference is independent of the pulse duration. Therefore the CEP effect canstay robust even for a long laser pulse. Besides, we theoretically investigate the CEPeffect in a bound-bound transition of an atom in a low-frequency long laser pulseusing the characteristic of small energy difference between two high Rydberg states.We find that the population changes dramatically with the CEP of the low-frequency pulse. This CEP effect is attributed to the interference between the positive-frequencyand negative-frequency components in one-photon transition. These results mayprovide a new method to measure the CEP value of a long laser pulse with lowfrequency.
     Finally, we develop a program to solve the time dependent Schrodinger equation(TDSE) based on the B-spline and spherical coordinate. We demonstrate how totransform the two electrons TDSE into a coupled differential equations. Using thegraph method we get the transition matrix. We apply the Adams method to solve thesedifferential equations. We compare our results with the literatures related to this workby computing the energy level of He and ionization rate. Then, we calculate theionization yield as a function of the full width at half-maximum (FWHM) durationwith different initial states, finding the ionization yield originated from the initial state1s2p is higher than the initial state1s2s. We successfully convert the programinto Graphic Processing Unit (GPU) parallel code, for saving the time. The computingspeed of GPU is nearly150times faster than CPU.
引文
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