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题名	飞秒激光成丝非线性光学中的若干新效应研究
作者	倪洁蕾
学位类别	博士
答辩日期	2013
授予单位	中国科学院上海光学精密机械研究所
导师	徐至展
关键词	飞秒激光成丝、远程空气激光、高阶克尔效应、时空聚焦
其他题名	New Nonlinear Optics Phenomena in Femtosecond laser filamentation
中文摘要	在大约 1014W/cm2 至1015W/cm2的光强量级，强场激光与原子、分子的相互作用导致了一系列新奇的效应。一方面，这时的光场已经足够强，能够显著扭曲原子分子中的库仑势场，从而使基于微扰论的非线性光学失效；另一方面，在超短脉宽条件下，上述光场强度也还不足以导致原子分子体系的完全电离，因此与光场相互作用的主体仍然是中性的原子分子而非等离子体。在这一较为狭窄的光强范围，人们已经观测到诸如高次谐波产生、阈上电离、非序列双电离等具有高度非线性、非微扰特征的激光-原子相互作用新现象。同时，在这一光强范围中，激光脉冲自身在传输过程中也出现了非线性新特性。1995年，密歇根大学的Gerard Mourou小组将飞秒激光聚焦至空气中，无意中发现超强超短激光脉冲能够在大气中诱导产生远长于衍射距离的等离子通道。这一有趣的新发现导致了飞秒激光成丝这一全新研究领域的诞生。 经过十几年来的广泛研究，飞秒激光成丝的主题物理图像已逐渐明晰，即成丝过程中飞秒激光脉冲同时受到到克尔自聚焦和等离子体散焦的作用，并在两者的动态平衡中实现长距离的自导传输。飞秒激光成丝的研究也已产生了一系列新现象，如长距离稳定传输、超连续白光产生、锥光辐射等，并且被广泛应用于光学前沿领域如远程监控、高次谐波产生、分子准直、天气控制等。 然而，作为一个年轻的前沿研究领域，飞秒激光成丝过程的基本物理至今仍未被完全理解和揭示。例如，最近实验报导的在成丝过程中的高阶克尔效应饱和的行为令人费解，直接质疑了长期被认可的飞秒激光成丝的基本物理图象。此外，最近在空气中发现的在飞秒激光成丝过程中产生的基于超快粒子数反转的自发辐射光放大（即远程空气激光）新现象，也已备受关注。与此同时，利用和发展新的光场操控方法和技术，实现对飞秒光丝的形态以及光丝中的光场时空分布的有效调控正在逐渐成为当前该领域的热点。 基于上述背景，本论文着眼于若干飞秒成丝非线性光学中的新效应进行了细致研究，期望在微观尺度上获得飞秒光丝中原子分子与超快光场相互作用的更为全面和清晰的物理图象。本论文主要工作和创新性成果如下： 1. 针对近期颇有争议的飞秒激光成丝的传统模型和高阶克尔模型，通过测量三、五次谐波的产生效率比值，证明了成丝过程中具有非微扰的高阶克尔效应，为促进对成丝过程的新认识提供了实验信息。 2. 首次利用中红外可调谐激光脉冲在成丝过程中实现了高亮度、多波长的远程空气激光，包括远程氮气激光和二氧化碳激光，并进一步研究了该激光的增益特性、时间特性、种子效应，该空气激光在远程探测方面有重要的应用。 3. 首次提出利用混合气体对飞秒激光成丝的钳制光强进行操控，研究不同钳制光强下远程空气激光的产生效率，揭示了远程空气激光产生的两大重要因素：粒子数反转和种子光，为进一步揭示空气激光的产生机制提供了有效信息。 4. 首次从实验上确定了飞秒激光成丝中的远程空气激光的产生机制，证明了该空气激光的产生来自于飞秒激光成丝过程中飞秒尺度内建立的粒子数反转，而非来自四波混频、受激拉曼放大等非线性频率转换。 5. 首次将时空聚焦技术引入飞秒激光成丝领域，有效地缩短了光丝的长度，并且提高了光丝内部的钳制光强。与传统聚焦技术相比，光丝的长度缩短了~25倍，钳制光强提高了~30%。
英文摘要	At the peak intensities ranging from ~1014W/cm2 to ~1015W/cm2, interaction of intense laser fields with atoms and molecules can lead to many novel effects. On the one hand, the strength of the electric field of electromagnetic wave approaches that of the Coulomb field in atoms and molecules, resulting in the breakdown of perturbative theory. On the other hand, when irradiated by the ultrashort laser pulses at peak intensities within such a range, most neutral atoms can still survive because of the limited ionization yield. In this narrow intensity range, highly nonlinear and nonperturbative phenomena such as high-order harmonic generation, above-threshold ionization, and non-sequential double ionization have been discovered. In addition, it has also been found that propagation of these pulses in transparent media shows unique new characteristics. In 1995, a team led by Gerard Mourou from University of Michigan reported that self-focusing of femtosecond laser pulses with a peak power more than a few GW could form a long plasma channel with a length much greater than that is allowed by diffraction. This interesting observation is recognized as the birth of femtosecond laser filamentation science. Over the past years, extensive studies on femtosecond laser filamentation has led to a seemingly complete picture of the fundamental process of filamentation, which can be briefly described by a dynamic balance of Kerr self-focusing and plasma defocusing. A variety of new phenomena have been discovered, such as long-distance self-guided propagation, supercontinuum generation, conical emission generation, etc. The discoveries have in turn triggered a broad range of applications, such as remote sensing, high-order harmonic generation, molecular alignment, weather control, and so on. However, we should point out here that so far a complete picture of femtosecond laser filamentation has not been established, as evidenced by the facts listed as following. First of all, the recent measurement of negative higher-order Kerr effect (HOKE) terms in gases has given rise to a controversial debate, directly challenging the long-standing paradigm of femtosecond laser filamentation. Second, the generation of remote lasing based on ultrafast generation of population inversion during femtosecond laser filamentation has been attracting increasing attention. Lastly, the emergence of new methods to manipulate both femtosecond laser filament and the optical field inside the filament shows that this field is still full of opportunities for innovation. Based on the points mentioned above, in this thesis we focus on several new phenomena in filamentation nonlinear optics, aiming at creating a more complete microscopic picture of the interaction of ultrafast laser field and atoms/molecules in a femtosecond laser filament. The main results and innovations of this thesis are listed as follows: 1. We show experimental evidence of non-perturbative high-order Kerr effect in the process of femtosecond laser filamentation by measuring the 5th/3rd harmonic generation efficiency ratio, providing useful information on the explanation of femtosecond filamentation. 2. We demonstrate a harmonic-seeded remote laser in nitrogen and carbon dioxide driven by intense midinfrared femtosecond laser pulses, in which population inversion occurs at an ultrafast time scale (i.e., less than ∼200 fs). The bright remote laser opens the perspective for remote detection of multiple pollutants based on nonlinear optical spectroscopy. 3. For the first time, we propose to manipulate the clamped intensity of a filament by gas mixture. Using this method, harmonic-seeded remote laser emissions at 391 nm wavelength from strong-field ionized nitrogen molecules is investigated in three different gas mixtures, i.e., N2-Ar, N2-Xe and N2-Ne, revealing two key factors, population inversion and seed, in the generation of remote laser. 4. For the first time, we provide decisive experimental evidence of ultrafast population inversion for the generation of remote air lasing, ruling out the possibilities of nonlinear frequency conversion such as four wave mixing and stimulated Raman scattering, using pump-probe spectroscopy. 5. For the first time, we demonstrate that the implementation of spatio-temporal focusing in femtosecond laser filamentation enables great enhancement of clamped intensity by 30%, and distinct reduction of filament length by 25 times, in comparison of traditional focusing.
语种	中文
内容类型	学位论文
源URL	[http://ir.siom.ac.cn/handle/181231/15757]
专题	上海光学精密机械研究所_学位论文
推荐引用方式 GB/T 7714	倪洁蕾. 飞秒激光成丝非线性光学中的若干新效应研究[D]. 中国科学院上海光学精密机械研究所. 2013.

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