【作者】 李昊；

【导师】 曾和平；

【作者基本信息】 华东师范大学 ， 光学， 2014， 博士

【摘要】 飞秒强激光脉冲在气体分子介质中传输时,脉冲与分子的相互作用将引起分子的取向或定向行为。通过调节飞秒脉冲的各个参量,可以实现对分子取向或定向行为的超快操纵与控制。与随机排列的各项同性的气体分子相比,被取向的分子介质呈现出各向异性的特点,具有许多特殊的光学性质,可以诱导出许多一般情况下难以观察到的物理效应,如分子取向诱导的双折射效应,空间聚焦-散焦效应等。所有这些基于分子取向的效应都有许多重要的应用价值,如基于分子取向的超快光学偏振门,基于分子取向的电离增强与高次谐波产生效率的提高,以及基于分子取向的脉冲传输的调制等。通过各种手段实现对分子取向与定向的操控就是为了实现各种与分子取向有关的应用。本论文围绕着利用超快光学技术操控分子的取向与定向展开讨论,同时从实验研究和理论计算两个方面展开研究,着眼于基于分子取向与定向效应的若干应用,具体工作可以分为以下几个部分：一、基于分子取向的双折射效应,分子取向可以作为超快频率分辨的光学门(molecular alignment based cross-correlation frequency resolved optical gating, MXFROG)加以使用。利用MXFROG技术,首次完整地测量了中心波长约为200nm的深紫外四次谐波；通过MXFROG技术,现实了对偏振相同的基波和三次谐波的同时测量；发展了MXFROG技术,将对椭圆偏振脉冲的诊断转化为对线偏振脉冲的诊断；借助斯托克斯参量,实现了对椭圆偏振飞秒脉冲的完整诊断,将MXFROG技术的使用范围从之前的线偏振超短脉冲拓展到任意偏振的超短脉冲。二、飞秒脉冲激发的分子取向与激发光强密切相关,空间分布为高斯型的取向光激发的分子取向将会产生空间聚焦-散焦效应。利用这种效应,在完全相同的实验条件下,实现了对乙炔分子C2H2和氧气分子02的分子取向信号的测量；同时,通过对比氧气和乙炔的分子取向信号,并根据已知的氧气分子的非线性参数(如极化率分量差),还原得出乙炔分子对应的未知非线性参数。该测量方法同样适用于其他线形分子的极化率分量差的测量。三、研究了飞秒双色场激发分子定向的机制与特点,以及双色场各个参数对分子定向度的影响,包括双色场的基波与二次谐波的相对强度、相对相位和双色场的光谱宽度。理论上证明了在双色场单脉冲能量一定的条件下,当双色场中基波与二次谐波强度比为2：1时,双色场所能激发的分子定向度达到最大；理论上证明了飞秒双色场激发的分子定向度与双色场的光谱宽度密切相关,而与双色场基波和二次谐波的中心波长无关。四、研究了飞秒双色场与少周期太赫兹场各自在激发分子定向的优势与不足,提出了将两者优势互补的操控分子定向的方案。理论计算证明,我们的方案能有效实现一氧化碳分子CO分子定向度的2倍增强,优于目前其他的双脉冲方案。通过适当调节飞秒双色场与太赫兹场的延时与太赫兹场的载波包络相位,本方案同样适用于转动周期较短的分子,实现对分子定向度的增强与控制。更多还原

【Abstract】 The impulsive rotational Raman interaction between femtosecond laser pulse and molecules induces the molecular alignment or orientation phenomena when the laser field propagates in gaseous media. The ultrafast manipulation of molecular alignment or orientation can be realized via adjusting and controlling the parameters of femtosecond pulse. Comparing with random oriented molecules, aligned molecules show a lot of special optical properties and be able to induce many unusual physical phenomena, which are unobservable in ordinary condition, such as the alignment based birefringence, alignment induced spatial focusing and defocusing effect and so on. All these oriented-molecule based effects have tremenduous application potential, such as the alignment based inoinzation enhancement, the promotion of high harmonic generation efficiency, and the modulation of propagation of ultrashort laser pulse. The manipulation of molecular alignment and orieantion by various approaches is to realize all kinds of oriented-molecule based applications.This dissertation mainly focus on the ultrafast manupation of molecular alignment and orientation, with the aim of realizing some oriented-molecule based applications. We carry out research through experiments as well as throretical calculation, mainly including the following:1. The characterization of fourth harmonic wave is performed via the technique of molecular alignment based cross-correlation frequency resolved optical gating (MXFROG). Meanwhile, The measurement of fundermental wave and third harmonic wave is realized simultaneously by MXFROG. Moreover, we managed to realized the complete characterization of elliptically polarized feotosecond laser pulse by introducing the Stokes parameters in the experiments, and therefore extend the scope of application of MXFROG.2. The molecular alignment singal is closely related to the intensity of incidence field so that the alignment singal created by Gaussian-shaped femtosecond laser field would induce spatial focusing and defocusing effect. By making use of such effect, we measure the alignment singals of oxygen and acetylene under the identical experimental conditions. With the available nonlinear parameters of oxygen, we managed to retrieve the corresponding unknown parameters of acetylene by comparing the molecular alignment singals of oxygen and acetylene.3. Theoretical investigation on the mechanism and properties of the interaction between femtosecond dual-color field and polar molecules has been performed. The influence of the parameters of the dual-color field upon the molecular orientation degree is also discussed. It is also theoretically proved that if the energy of dual-color pulse is given, when the intensity ratio between fundamental wave and second harmonic wave equals2, the dual-color field can induced maximum molecular orientation degree.4. Comparison investigation between dual-color field and terahertz field in creating molecular orientation has been performed. It is theoretically proposed that the combination of dual-color field and terahertz field can make good use of their respective advantages, and the calculation shows that the combination fields can enhance the molecular orientation degree by a factor of2. The proposal is suitable both to the molecules with longer and the shorter revival periods.更多还原