【作者】 冯亚辉；

【导师】 曾和平；

【作者基本信息】 华东师范大学 ， 光学， 2014， 博士

【摘要】 激光场作用于气体分子,诱导的旋转拉曼效应促使分子沿主轴平行于激发光场偏振的方向排列。连续激光或纳秒量级光场激发的绝热分子排列与激发光场脉冲同步,激光脉冲结束后,分子重新恢复到原本自由杂乱的随机排列。飞秒激光诱导的非绝热分子排列,激发脉冲结束后,受激的旋转波包在空间自由演化,周期性的呈现出重相与退相,使分子在波包相干性存在的很长一段时间内周期性的排列与反排列。分子排列诱导的时空交叉相位调制,对介质的非线性特性有显著影响,使后续传输的光场频谱及脉宽等特性发生变化。分子排列被广泛的应用于诸如分子轨道成像,高次谐波产生,M-XFROG脉冲检测及超快光学存储与成像等方面,而且对超快化学反应,介质非线性过程等方向的研究有极大的推动作用。实际上,分子排列过程中总是不可避免同时存在光克尔效应及等离子体散焦作用,因此,在对分子排列与其应用进行研究时,明确三者在时空调制过程中的作用时间,作用程度及作用方式就极为重要。本论文重点研究了分子排列机理,分子排列探测及排列过程中伴随的光克尔效应及等离子体散焦作用,主要内容包括以下几个方面：1.论证了分子排列机理。激光电场作用于气体分子,诱导出感生偶极矩,该偶极矩与激光电场作用产生一个方向性扭矩,使分子主轴排列到平行于激发光场偏振方向。根据激发脉冲宽度与分子排列周期的可比拟性,可以分为分子绝热排列及非绝热排列。两种排列方式的区别在于,排列能否周期性再现,无论哪种方式,分子排列程度都可以用《os2θ》的取值表示。依据分子排列机理,给出了多种提高分子排列的方法,并分析了每种方法的适用条件。2.证明了当使用空间强度呈高斯分布的激光脉冲激发气体分子时,分子排列诱导的介质折射率变化为阶梯分布,预排列的分子等效于一个周期性变化的气相正/负透镜,使后续传输的探测光束发生显著的空间聚/散焦效应。变化周期与分子排列周期同步。3.利用该空间聚散焦效应,发展了一种全新的分子排列探测方法,该方法操作简单,测量精确。采用本探测方法,能区分出分子的平行与垂直排列,同时可以清晰的看出光克尔效应及等离子体散焦效应对分子排列信号的影响。裸眼观察探测光斑形貌,即可对分子排列状态进行准确区分,并能判断出分子排列程度的相对大小。4.研究了光克尔效应对分子排列的影响,讨论了两者的合作及竞争关系。指出平行排列情况下,光克尔效应与分子排列为合作关系,使探测信号的峰值强度增加,在时间上使其向零延时时刻前移；垂直排列下,光克尔效应与分子排列为竞争关系,使探测信号在零延时附近出现一个小的信号峰。以分子排列诱导的介质折射率变化为基准,拟合了光克尔效应及等离子体效应诱导介质折射率变化的曲线,计算了介质的非线性系数n2。5.探讨了等离子体散焦对分子排列的影响。在时间和强度上还原了等离子体散焦信号。指出了等离子体产生机理的多样性,讨论了等离子体产生速率跟激发光强的指数对应关系。论证了等离子体散焦效应使实验探测到信号峰/信号谷的时刻随光强增加而后移现象,讨论了等离子体散焦与光克尔效应的竞争关系。提供了一种新的等离子体密度测量方法。更多还原

【Abstract】 The impulsive rotational Raman excitation induced by laser pulse will force gas molecules align parallel with the laser pulse polarization direction. When CW laser pulse or nanosecond laser pulse are used, the induced molecular alignment stars almost at the time that the pulse stars, and become random align as long as the pulse off. While the femtosecond laser induced molecular alignment will repeat itself at a long time, until the rotational wave packet loses coherence. Molecular alignment caused spatiotemporal modulation can greatly change the nonlinear character of the gaseous media, which influences the frequency spectrum and pulse width of subsequent laser pulse. These could be used at molecular-orbital reconstruction, high-order harmonic generation, M-XFROG laser pulse diagnosis, as well as revivable ultrafast optical buffer and imaging with molecular rotational wave-packets. Meanwhile, molecular alignment facilitates the ultra-fast chemical reaction and the nonlinear progress in gaseous media. In fact, there are inevitable Kerr effect and plasma defocusing accompanying with molecular alignment, so when talk about molecular alignment and its applications, it is important to distingue the act time, degree and mode of the three different effects.In this paper, we mainly introduce molecular alignment mechanism and the companied Kerr effect and plasma contribution, our main content includes the following parts:1. Every non-spherical polarizable molecular placed in a laser electric field will generate an induced dipole moment, which interacts with the laser field to form a torque, which forces the molecular axis to rotate toward parallel with the direction of the field polarization. According to the comparability of impulsive pulse width with alignment period, molecular alignment could be divided into two groups: adiabatic alignment and non-adiabatic alignment. The statistic metric《cos2θ》 could be used to measure the molecular alignment in both of the two generalities.2. The pre-aligned molecules exerts orientation-dependent refractive index changes, we proved that a Gaussian-shaped spatial distribution pump pulse induced pre-aligned molecules acted as a newly established gas-phase nonlinear lens, which caused the subsequent pulse experiences spatical (de)focusing. Based on the (de)focusing effect, we developed a direct measurement of molecular alignment, where the parallel and perpendicular alignments were clearly characterized, and the intensity depended Kerr effect and plasma contribution were distinguished from measured signals.3. Based on the (de)focusing effect, we illustrated the electronic Kerr effect in measured alignment signal, explained the cooperated or competed relationship of them. When the probe pulse polarization parallel with the pump, Kerr effect could increase the signal peak strength and push the peak forward to zero time delay. At the perpendicular case, Kerr effect caused a small peak near zero time delay. Meanwhile, the nonlinear refractive index was extracted straightforwardly.4. Referring to the methods used in researching Kerr effect, we studied plasma influence. Direct retrieved plasma effect based on alignment-induced spatiotemporal modulation. Pointed out the diversity of plasma mechanism, and understood the rate of plasma producing grow exponentially depend on pulse intensity. At this experiment, we offered a reliable measurement of plasma density.更多还原