紫外光丝
UV filaments
AI总结：
本研究分析了不同波长下单根光丝的特性差异，建立了紫外长脉冲光丝的准稳态传输理论，通过266 nm实验验证了相关结论，并展示了紫外光丝在同位素选择性LIBS和激光诱导放电领域的应用。
摘要：
本章首先讨论了紫外（UV）光丝与中红外光丝的主要定性差异：从紫外波段的多光子电离，到近红外至中红外波段的隧穿电离。随后，针对单根光丝的特性随波长的变化规律进行了通用定性分析。由于这类光丝的脉冲持续时间较长，因此可以采用准稳态理论来描述其传输过程。通过本征值方法可得到稳态场包络，并将其与Townes孤子进行了对比。不过，该解与高斯形状足够接近，因此有理由采用参数演化方法进行分析。\n在理论介绍之后，本文开展了266 nm波长下的实验验证。飞秒紫外光丝通过三倍频钛宝石（Ti:sapphire）光源和KrF放大器产生。长脉冲光丝的光源为振荡器-放大器式Nd-YAG调Q系统，经二倍频、脉冲压缩后再次二倍频，最终得到能量300 mJ、脉宽170 ps的脉冲。紫外脉冲的亚纳秒持续时间可能会重新引发关于光丝本质是移动焦点还是自诱导波导的讨论。\n最后两节介绍了紫外光丝的两项应用。研究表明，利用发射光谱中观测到的窄凹陷，可实现同位素选择性激光诱导击穿光谱（LIBS，laser-induced breakdown spectroscopy）。这些凹陷源于光丝撞击固体产生的羽流中物质的重吸收作用。这些吸收线宽度仅几皮米，精确位于物质基态跃迁的波长中心，不存在任何斯塔克位移或展宽。最后一项应用是激光诱导放电，这是一种严格沿诱导紫外光丝路径传播的导向放电。激光诱导放电有望实现闪电控制，这也是欧洲的一个热门研究课题。
英文摘要：
This chapter starts with a discussion of the main qualitative differences between UV and mid-IR filaments: from multiphoton ionization in the UV to tunnel ionization in the near- to mid-IR. A general qualitative analysis of the properties of single filaments versus wavelength follows. Because of their long pulse duration, a quasi-steady-state theory of their propagation is possible. An eigenvalue approach leads to a steady-state field envelope that is compared to the Townes soliton. However, that solution is close enough to a Gaussian shape to justify a parametric evolution approach. After this theoretical introduction, an experimental verification at 266 nm follows. Femtosecond UV filaments were generated with frequency-tripled Ti:sapphire sources and KrF amplifiers. The source for long-pulse filaments is an oscillator-amplifier Nd-YAG Q-switched system, frequency doubled, compressed, and frequency doubled again to reach 170 ps pulses of 300 mJ energy. The sub-nanosecond duration of the UV pulse may revive the debate as whether the filament is a moving focus or self-induced waveguide. Two applications of UV filaments are presented in the last two sections. It is shown that isotopically selective laser-induced breakdown spectroscopy (LIBS) is possible by exploiting the narrow dips observed in the emission spectrum. These dips are due to reabsorption by the material in the plume created by the impact of the filament on a solid. These absorption lines are only a few-pm wide and are exactly centered at the wavelength of a transition from ground state of the material, without any Stark shift or broadening. A final application is laser-induced discharge, which is a guided discharge that follows exactly the path of the inducing UV filament. Laser-induced discharge may lead to the control of lightning, which is a topic of intense research in Europe.
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