【作者】 石理平；

【导师】 丁良恩； 曾和平；

【作者基本信息】 华东师范大学 ， 光学， 2014， 博士

【摘要】 中心波长800nm的钛宝石飞秒激光在透明介质成丝过程中具有非常丰富的物理效应,如频谱展宽,脉冲压缩,锥状辐射,谐波输出,化学反应,布居反转等。结合这些效应,有望将飞秒光丝应用于高能量白光的产生,孤立阿秒脉冲的产生与应用,大气环境科学的研究,全光光学器件的制备,无谐振腔超快激光脉冲的输出,诸如此类。紫外波段的激光具有成丝阈值低,聚焦性能好,临界电子密度高,多光子电离截面大等特点,然而由于缺乏高能量的紫外飞秒激光,对该波段成丝过程的研究则不是特别完善。本文详细研究和讨论了中心波长267nm,峰值功率高达20GW的深紫外飞秒激光脉冲在各种气体介质中的成丝现象、成丝机制及其潜在的应用价值。主要包括以下内容：首先,以荧光成像法结合全息成像法和瞬态电流法对光丝进行诊断后发现,相对于近红外飞秒光丝,紫外光丝具有自更高的电子密度、更小的几何尺寸和更长的等离子体通道等特性。结合这些特性,本文探索了267nm深紫外飞秒光丝在全光光学器件——等离子体光栅、光丝诱导荧光光谱技术、光学频率上转换等方面的应用价值。其次,以两束近乎平行的267nm紫外光丝的非线性相互作用在空气中诱导形成了周期为波长量级、自由电子密度调制度达1018cm-3的等离子体光栅,并观察到其对800nm探测激光的非线性布拉格衍射效应。周期性的等离子体通道形成了类似于“墙壁”的效果,将激光束缚在狭小的等离子体波导中,并有效地抑制因光束发散引起的能量损耗。功率密度高达1014W/cm2的探测光照射等离子体光栅后非但未破坏其结构,反而通过驱动电子碰撞电离促使其电子密度增加至1019cm-3,折射率调制度提升一个数量级,性能得到明显改善。损坏阈值高出传统固体光学器件三个数量级,奠定了等离子体光栅在操控超快超强激光方面应用的基础,如文中展示的啁啾脉冲的压缩与展宽、谐波转换过程的准位相匹配。再者,自由电子密度高、等离子体通道长、几何尺寸小、等离子体温度低的特点使紫外光丝在远程荧光光谱技术中具有得天独厚的优势。文中详细论证了基于267nm紫外光丝的荧光光谱技术,并提出了利用缓冲气体提供种子电子并结合紫外+红外双色场光丝增强等离子体荧光强度的方法以提高光丝诱导荧光光谱技术灵敏度的提高。紫外光丝提供种子电子,红外光丝加热种子电子并引起周围原子分子的碰撞电离和激发。通过双色场光丝诱导的三步激发,能有效地将荧光光谱信噪比和灵敏度提升1-2个数量级。最后,利用惰性气体中高强度光丝的四波混频效应,本文研究了利用267nm+400nm双色场光丝输出脉冲能量达μJ量级的多波长真空紫外超快脉冲,如133nm、114nm、100nm和89nm。这是目前利用飞秒光丝四波混频的方法获得的最短波长的极紫外微焦量级飞秒超快脉冲。本文提出了通过控制气压以平衡原子色散、等离子体色散和古伊相移的方法,成功地实现了紫外光丝谐波转换过程的位相匹配操控,并将最高谐波转换效率提升至0.8%。同时在这些多色场光丝的非线性相互作用过程中观察到了脉冲分裂、脉冲压缩、光斑自修复等一系列有趣的物理现象。今后将深紫外飞秒成丝的研究拓展至固体和液体中,则有望将其应用于光丝诱导化学反应,微纳尺度的材料精密加工,周期量级极紫外超快脉冲的产生,飞秒时间分辨原子分子超快动力学的研究,乃至激光武器等研究方向。更多还原

【Abstract】 The propagation of ultra-short intense800nm Ti:sapphire laser pulses in the transparent media is rich in nonlinear physics and may have a broad range of applications, induced supercontinuum genereation, pulse self-compression, conical emission, harmonics generation, chemical reaction, population inversion and trapping, remote sensing of chemical and biological agents, electric discharge, remote lasing action, single attosecond pulse generation, etc. Ultraviolet femtosecond pulses show the advantages of lower filamentation threshold power, more excellent focusing ability, and higher critical plasma density; however, the investigations of ultraviolet filamentation in transparent media are limited by the lack of intense ultraviolet femtosecond laser.This paper introduces and discusses the main aspects and potential applications of267nm deep-ultraviolet femtosecond laser filamentation in gaseous media including air, argon, neon and helium. The nonlinear phenomena of femtosecond filamentation such as spectral modulation, intensity clamping, mode self-healing, pulse splitting are described. Self-guiding is shown to subsist in low gas pressure. The various techniques consist of fluorescence imaging, external electric field induced transient current and in-line holographic imaging provide a comprehensive image of the ultraviolet femtosecond filamentation process and its characteristics. It is shown that higher free electrons density, longer plasma channels, and smaller beam profile accompany the ultraviolet filamentation, allowing for a better application in all-optical devices, fluorescent spectroscopy, and frequency up-conversion.The interference between two almost parallel267nm ultraviolet filamentary pulses produced periodic wavelength-scale plasma channels surrounding with air molecules. Such a periodic refractive index modulation can be functioned as effective plasma grating exhibits a nonuniform photonic band gap that support the efficient Bragg diffraction of probe laser pulse. As a result of the nonlinear interaction between these two ultraviolet filaments, enhanced ionization is produced, which represent waveguide walls for laser beams that can suppress their divergence, resulting in a dramatic increase of the ionization rate. It is shown that the modulation depth of plasma grating was enhanced by one order of magnitude under the irradiation of intense800nm laser pulses with the peak intensity of1014W/cm2. This is ascribed to laser driven electrons impact ionization, which increased the peak plasma density up to1019cm-3. The unique feature of plasma grating such as ultrahigh damage threshold and low cost enables its application in compression of ultrashort intense positively or negatively chirped pulses.Due to the higher multi-photon ionization rate and lower electron temperature inside the ultraviolet filaments, its potential application in remotely nonlinear fluorescence spectroscopy is proposed. The method of ultraviolet+infrared dual-color filamentation in mixed gaseous media was introduced to enhance the plasma fluorescence intensity and signal-to-noise ratio. A semiclasscal three-step-excitation model was verified by the fluorescence emission from the impact excitation of high potential atoms such as neon and helium. Firstly, some free electrons were liberated from buffer gases through multi-photon absorption of ultraviolet photons due to its low ionization potential. Secondly, as a subsequent intense infrared pulse was tightly coupled and guided into the preformed plasma channels, the well-confined free electrons were driven by the laser electric field along its polarization direction. A host of free electrons were further peeled through these energetic electrons collision ionization. Thirdly, some neutral neon or helium atoms in excited states approaching ionization potential were kicked into ionic states through the collisions of these hot electrons, facilitating the impact ionization of neon atoms. As a result, more high-lying excited states were populated through ion conversion and dissociative recombination. Generation of ultrashort vacuum ultraviolet pulses by using nonlinear frequency conversion through filamentation was experimentally investigated. Gently focusing267nm and400nm pulses into argon efficiently generated multicolor vacuum ultraviolet pulses including133nm,114nm,100nm, and89nm generated by sum-frequency four-wave mixing, which are the shortest wavelength generated through filamentation in gaseous media. The pressure dependence of the conversion efficiency can be explained by phase matching induced by dynamical balance among dispersion originated from neutrally atomic atoms, plasma and Gouy phase shift. The pulse breakup effect was observed during ultraviolet filamentation. The generated multicolor vacuum ultraviolet pulses with the energies exceeding10μJ are available for time-resolved studies of atomic and molecular ultrafast dynamics.In the following, the propagation of ultraviolet femtosecond pulses in solid and liquid media will stimulate the studies of underlying filamentation physics and potential allpications, such as filamentation induced chemical reaction, filamentation induced micro/nano precision machining, few-cycle extreme ultraviolet pulses generation, femtosecond resolved dynamics research of atomes and molecules, remote sensing by filamentation induced lasing action, and even long distance laser weapon, etc.更多还原