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题名	强场光电离诱导的远程氮气激光机理研究
作者	李紫婷
文献子类	博士
导师	程亚
关键词	飞秒激光脉冲，远程空气激光，相干激射，激光诱导击穿 femtosecond laser filamentation, coherent emission, air laser, laser induced breakdown
其他题名	Mechanism of Remote Nitrogen Air Laser Induced by Optical Ionization of Intense Ultrashort Pulse
英文摘要	超强超短激光脉冲的迅猛发展推动了激光与物质相互作用领域的研究。当光强达到1014W/cm2至1015W/cm2量级时，原子分子中的库伦势场被光场显著扭曲，此时微扰论失效，激光与物质相互作用从微扰区进入非微扰区，从线性范畴步入高度非线性范畴。基于强场超快激光的极端非线性光学，是当今物理学、光学以及光子学领域中的诸多前沿分支（如光学频率梳、阿秒科学、强场原子分子物理、三维微纳制备、非线性光学成像等）的重要科学基础。 激光脉冲在克尔自聚焦和等离子体散焦相互作用下形成的自导光通道，具有承载光强高（可达到1013W/cm2）、指向性好(直径为百微米量级)、传输距离远（可达几千米）等特点，在该通道中诱导了原子分子多光子电离、隧穿电离等电离效应。隧穿电离作为其中最基础又最重要的量子过程，是当前强场物理研究热点之一，产生了很多有趣的物理现象，例如飞秒激光成丝、空气激光、高次谐波等等。其中实验发现的远程空气激光是基于中红外可调谐超快激光与空气分子的极端非线性相互作用，由此带来的一系列崭新的物理图像和技术进展引起国内外的广泛关注。空气激光作为一种无谐振腔相干光源，同时具有超高的频谱分辨和超快的时间分辨能力，不仅为军事和民用领域中大气遥感、爆炸物的远程探测提供了一种全新的可能实现途径，在显著提高远程探测的信噪比和灵敏度的同时提高远程探测的稳定性，而且为分子动力学研究提供了一种“全光学”的新手段，用于获取分子内层轨道的信息以及超精细的能级结构。 然而，空气激光在飞速发展的同时也面临重大挑战。目前研究的气体介质集中在氮气（N2）、氧气（O2）等空气分子原子中，从产生机制上划分主要有三类空气激光，分别是电子碰撞激发诱导的中性氮分子激光、多光子共振激发的原子激光和强场电离诱导的氮分子离子激光。其中氮分子离子激射超快粒子数反转的建立机制尚未澄清，另外实现远距离后向传输的高信噪比激光信号是空气激光应用于远程探测亟待克服的技术瓶颈。 基于以上研究背景，本论文着眼于中红外强场激光诱导的远程空气激光研究：在物理方面，期望在微观尺度上获得强场诱导成丝中分子原子与超快光场作用的更为全面和清晰的物理图像。在技术方面，将着重解决影响空气激光背向运转的关键问题，发掘激光成丝的实际应用。论文的主要工作及创新研究结果如下： 1. 首次通过调节双色场中泵浦光脉冲和探测光（种子光）脉冲的偏振方向夹角实现对N2+激光偏振特性的相干操控。其中发现并且详细研究了波长为391nm的N2+激光一些较为反常的实验现象，当泵浦光和种子光的偏振方向夹角在 之间，产生的N2+激光呈椭圆偏振，当泵浦光和种子光偏振夹角发生变化，其椭偏度也会随之变化。泵浦光的偏振方向不变的情况下，随着种子光脉冲偏振方向的变化，N2+激光391nm 的P支和R支的偏振变化呈相反的趋势。这个实验结果与之前提出的种子放大机制不符。另外从实验上获得了N2+的高振动态跃迁光谱，拓展了空气激光的研究内容，为空气激光应用于远程探测奠定了基础。 2. 首次从实验上研究分别用线偏振和圆偏振光产生的自发辐射和受激辐射之间的关系，以及自发辐射和受激辐射强度随着等离子通道长度的变化信息。这些实验结果为中性氮分子激光的进一步研究奠定了理论基础，不仅使我们更深入地理解圆偏振飞秒激光脉冲在氮分子中产生粒子数反转的机制，为产生背向自发辐射放大提供相应的信息，而且通过对这些数据的测量获得提高氮分子激发效率的方法，指引我们通过调节泵浦激光脉冲参数和优化聚焦几何参数，产生可以应用到远程空气探测的背向中性氮分子激光。 3. 首次结合纳秒激光诱导击穿（nano-LIBS）和飞秒激光诱导击穿（femto-LIBS）技术，用飞秒-纳秒双光束激光系统实现激光诱导击穿作用距离的扩展。我们在结合这两种技术的同时，使得存在于这两种技术中的固有缺点得到有效的缓和。为了获得最远的探测距离，系统地研究了探测距离对纳秒和飞秒激光脉冲参数的依赖。; With the rapid development of ultra-intense and ultra-short laser, interaction of ultrafast intense laser fields with matter has become the main impetus in contemporary physics. With the peak intensity above 1014W/cm2～1015W/cm2, the strength of electric field of electromagnetic wave approaches that of the Coulomb field in atoms and molecules, causing the failure of linear and perturbation theory. The extreme nonlinear optics in a strong ultrafast laser field has become the important scientific basis of many advanced branches in the field of physics, optics and photonics, such as optical frequency comb, attosecond science, strong field physics of atoms and molecules, three-dimensional micro-nano fabrication, nonlinear optical imaging, etc. Femtosecond laser induced filamentation is attracting growing interest due to its high load intensity (1013W/cm2), good directivity (hundreds of micrometer in diameter), long transmission distance (up to several kilometers). In laser-induced plasma channel, which can be described by a dynamic balance of Kerr self-focusing and plasma defocusing, many highly nolinear and nonperturbative phenomena can be induced, such as high-order harmonic generation, tunneling ionization, multiphoton ionization and air lasing. The remote air laser induced by femtosecond laser, which is based on the nonlinear interaction of the strong infrared ultrafast laser with nitrogen from the air molecules, was found in the experiment, resulting in a series of new physical picture. As a kind of no cavity laser with high spectral resolution and ultrafast time-resolved ability, the remote air laser not only can be used in atmospheric remote sensing, remote detection of explosives with high stability and SNR, but also provides a new tool to acquire molecular inner orbit information and hyperfine energy level structure. Currently, there are three types of air lasers which are generated based on different pumping mechanisms. In the first type of air lasers, population inversion is achieved by dissociation of molecular oxygen or nitrogen followed by excitation of the atomic fragments using high-peak-intensity picosecond ultraviolet (UV) lasers, which gives rise to bidirectional amplified spontaneous emissions (ASEs) from air atoms and molecules. The second type of air lasers rely on generation of the population inversion by electron collisional excitation of the neutral N2 molecules, in which the energetic electrons are produced in the plasma channel with ultrafast intense laser pulses. The third type of air lasers are realized by focusing intense ultrafast laser pulses in air which generates population inversion conditions in molecular nitrogen ions ( ). Recently, several mechanisms have been proposed to comprehend the physics behind this intriguing strong field phenomenon, whereas a full understanding is still lacking. Based on the research background mentioned above, the work of this thesis focus on was based on the novel phenomena and mechanism involved in generation of air lasers generation. The main results and innovative points are listed as following: 1. We experimentally investigate generation of nitrogen molecular ion ( ) lasers with two femtosecond laser pulses at different wavelengths. The first pulse serves as the pump which ionizes the nitrogen molecules and excites the molecular ions to excited electronic states. The second pulse serves as the probe which leads to stimulated emission from the excited molecular ions. We observe that changing the angle between the polarization directions of the two pulses gives rise to elliptically polarized laser fields, which is interpreted as a result of strong birefringence of the gain medium near the wavelengths of the laser. In this work, we report on another unusual behavior of the laser at the wavelength of 391 nm. Previous results show that the lasers induced by tunnel ionization possess the same polarization direction as that of the seed pulses when the pump and seed pulses are either parallelly or perpendicularly polarized to each other. This can be well understood from a seed-amplification point of view. In such a case, the laser signal generated by the seed-amplification mechanism typically inherits all the characteristics of the seed pulses. Interestingly, when the angle between the polarization directions of the two linearly polarized pump and seed pulses is variable in the range of 0°-90°, we find that the laser field becomes elliptically polarized with a variable ellipticity depending on the angle between the polarization directions of the two pulses. Moreover, the P-branch and R-branch lines in the laser show dramatically different behaviors with the varying polarization direction of the seed pulses (i.e., the polarization direction of the pump is always fixed). We attempt to provide a plausible explanation to qualitatively understand this unexpected observation. 2. We report on experimental investigations on the spontaneous and stimulated emissions from excited nitrogen molecules generated in both linearly and circularly polarized intense laser fields. The spontaneous emission is measured from the side direction of the laser induced filament whereas the stimulated emission generated by seed amplification is measured in the forward direction of the laser propagation. The comparison between the signal intensities of the spontaneous fluorescence emission and the seed-amplified stimulated emission provides insight into the population inversion generated in nitrogen molecules with the circularly polarized femtosecond laser pulses. 3. We extend the detection range of laser-induced breakdown spectroscopy by combining high-intensity femtosecond laser pulses with high-energy nanosecond CO2 laser pulses. The femtosecond laser pulses ionize the molecules and generate filament in air. The free electrons generated in the self-confined plasma channel by the femtosecond laser serve as the seed electrons which cause efficient avalanche ionization in the nanosecond CO2 laser field. We show that the detection distance has been extended by three times with the assistance of femtosecond laser filamentation.
学科主题	光学
内容类型	学位论文
源URL	[http://ir.siom.ac.cn/handle/181231/31008]
专题	中国科学院上海光学精密机械研究所
作者单位	中国科学院上海光学精密机械研究所
推荐引用方式 GB/T 7714	李紫婷. 强场光电离诱导的远程氮气激光机理研究[D].

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