【作者】 陆培芬；

【导师】 曾和平；

【作者基本信息】 华东师范大学 ， 光学， 2014， 博士

【摘要】 超短强激光脉冲在空气中传输时,会引发一系列的光学非线性效应,例如,光学克尔自聚焦、多光子电离、等离子体散焦、自相位调制、自陡峭效应、群速度色散等。当克尔自聚焦和等离子体散焦达到动态平衡时,介质中将形成一条传输距离远远大于瑞利长度的自引导的光丝通道。线偏振的超短脉冲激光在空气中传输时,将改变空气中的N2和O2分子的排列方向,泵浦脉冲结束之后,被激发的空气分子表现出无外场条件下的周期性排列回复的行为。本论文以空气分子的无外场排列控制超短脉冲的传输为基础,主要研究了分子排列对于光丝中的三次谐波产生和等离子体光栅的调制作用,提出了一种基于分子排列的超快光学偏振门,并且应用这种偏振门的特性,从原理性实验上实现了基于分子排列的超快光学成像。具体工作可以分为以下几个部分：一、利用空气介质中的无外场分子排列,提出一种基于分子排列的超快光学偏振门。分析此偏振门的基本特性,包括双折射效应,瞬时开关时间,周期性回复,空间上的类透镜作用,以及频谱调制。二、原理性实验上实现了基于空气分子排列的超快光学成像方法。实验中,通过泵浦探测技术,写脉冲将图像信息加载到分子排列中,预排列分子作为缓冲存储器,读脉冲在特定延时下经过预排列分子后,读取图像信息,实现了二维单色成像,全息成像以及彩色成像。三、研究了空气分子排列对于光丝中的三次谐波产生过程的影响。通过分析分子排列对介质的线性以及三阶非线性极化率的改变,发现当分子的排列方向与探测光丝的偏振方向相同时,光丝中的三次谐波增强。同时,还分析了分子排列对三次谐波的频谱调制,比较了探测光丝中基波和三次谐波光谱受分子排列影响的异同。四、研究了空气分子排列对等离子体光栅的线性和非线性方面的影响。在预排列分子中形成的等离子体光栅,由于分子排列相关的电离率导致了等离子体光栅的线性衍射效率的变化。同时,分子排列通过对三阶非线性极化率的调制,操控等离子体光栅增强的三次谐波的转换效率。五、研究了泵浦光丝对于后续共线传输的探测光丝的影响。通过测量探测光丝产生的等离子密度的变化,分别研究了泵浦光丝产生的等离子体和尾波中的分子排列对于探测光丝的钳制强度和电子密度的调制。同时,还分析比较了两光丝零延时附近的克尔效应,等离子效应,以及分子排列的作用。更多还原

【Abstract】 Intense ultrashort laser pulses propagating in air include several nonlinear optical effects, such as optical Kerr self-focusing, multi-photon ionization, plasma defocusing, self-phase modulation, self-steepening and group velocity dispersion. The dynamical balance between Kerr self-focusing and plasma defocusing leads to a self-guide filament with a propagating range longer than Rayleigh length. Molecules in air can be aligned by a linearly polarized ultrashort pulse and the excited molecules present periodic revivals of alignment after the pump pulse.In this dissertation, based on the manipulation of the ultrashort pulse propagation in air by molecular alignment, we investigate the modulation of the plasma grating and the third-harmonic generation in the pre-aligned molecules and propose an ultrafast optical gating by molecular alignment and explored an application of this gating in ultrafast optical imaging. Mainly includes the following:1. A novel ultrafast optical gating is proposed based on the molecular alignment in air. The optical gating has unique features including birefringence, femtosecond-scale switching time, periodic revivals, spatial quasi-lens effect and spectral modulation.2. A proof-of-principle experiment of ultrafast optical imaging by using impulsive alignment of diatomic molecules in air is carried out. By employing the pump-probe technique, the image information is written into the molecular alignment by the writing pulse and then can be read out at periodic molecular alignment revivals by a reading pulse. The capacities of raised and intagliated monochromatic imaging, as well as colorful and even holographic-like imaging are experimentally demonstrated.3. Experimentally investigate the modulation of the third harmonic generation (THG) in a filament by molecular alignment. Due to the perturbations of the effective linear and third-order susceptibility induced by the molecular alignment, the THG is enhanced or suppressed by following the impulsive molecular alignment. And the spectral modulation of the third harmonic and is also studied by comparing with the case of the fundamental wave from the same filament.4. Experimentally demonstrate the precise manipulation of the multiphoton-ionization-induced plasma grating by impulsive molecular alignment. The plasma grating induced in the pre-aligned molecules is modulated due to the alignment-dependent-ionization, leading to the modulation of the linear diffraction efficiency. And the dependence of the plasma-grating-enhanced nonlinear THG on the impulsive molecular alignment is also investigated.5. Experimentally investigate the role of the pump filament on the filamentation dynamics of succeeding collinearly-propagating probe pulse. The clamping intensity and electron density of the probe filament are modulated by the plasma and impulsive molecular alignment created by the pump filament. Also, the roles of the Kerr effect, plasma effect, and molecular alignment are indentified in the interaction between the two collinearly-propagating filaments around zero time delay.更多还原