飞秒激光成丝产生太赫兹波及其与介质相互作用的机理研究

In the past three decades, terahertz (THz) science and technology has enjoyed a flourishing development, especially in the field of THz source research, attracting intense attention from all over the world. Femtosecond laser filamentation is one of the most competitive THz sources for its capability of remotely emitting THz wave with broad bandwidth and high intensity. Up to now, the underlying physical mechanism of THz wave generation during filamentation has been widely studied. However, major physical models still remain controversial, and contradicting reports lie on the experimental observations, which are new challenges facing the community. Recently, the discovery of strong spatial confinement of THz wave inside the filament might shed light on this issue. It has been revealed that the interaction between THz wave and plasma during filamentation cannot be negligible. Therefore, this proposal is aimed at investigating the fundamental properties of THz wave confined propagation inside the plasma filament, and revisiting the dynamics of THz pulse generation. The main contents of this project are as follows. With THz wave generation and imaging system, we plan to experimentally explore the characteristics of the THz beam spatial profile within the plasma region, e.g. THz wave energy distribution, length of THz wave confined propagation, and THz beam divergence angle in far field, etc. On the other hand, by establishing physical models of THz wave interaction with the plasma filament, we plan to theoretically reproduce the above THz beam spatial properties, and optimize the simulation outcomes according to the experimental results. In this way, the relationship between major physical models can be built, and the mechanism of THz wave generation during filamentation can be well understood. This will help to resolve the aforementioned controversies and contradictions, providing a significant theoretical basis on developing THz source and its applications.

在近年蓬勃发展的太赫兹科技领域，飞秒激光成丝是一种极具竞争力的太赫兹源，因其可远程产生频谱宽、强度高的太赫兹波而受到众多学者关注。目前，成丝辐射太赫兹波的基础研究虽已取得一定进展，但仍存在主流物理模型争议和重要实验结论矛盾等问题，这是当前亟待解决的新挑战。最近，太赫兹波空间强束缚现象揭示出，成丝过程中太赫兹波被限制在光丝中进行传输，其与介质（空气等离子体）的相互作用不容忽视，这为重新审视太赫兹波产生的动力学机制提供了新的契机。因此，本项目拟采用近场成像和有限元模拟等实验与理论研究手段，探究与等离子体相互作用过程中独特的太赫兹波空间分布特征，以阐明其内在机理；另一方面，通过提炼相互作用规律与核心物理量，建立主流物理模型间的联系，旨在澄清太赫兹波产生的根本物理机制。综上，本项目研究将有助于完善飞秒激光成丝辐射太赫兹波的基本理论，解决现存分歧，为太赫兹源的基础研究和技术应用提供重要科学依据。

在近年蓬勃发展的太赫兹科技领域，飞秒激光成丝是一种极具竞争力的太赫兹源，因其可远程产生频谱宽、强度高的太赫兹波而受到众多学者关注。然而，阐述飞秒激光成丝辐射太赫兹波物理机制的两个主流模型，即“四波混频”和“光电流”，在解释太赫兹波二维偏振特性时都存在困难，理论结果与实验结果仍有一定偏差。..最近，“太赫兹波空间强束缚现象”揭示出，成丝过程中太赫兹波被限制在光丝中进行传输，其与介质（空气等离子体）的相互作用不容忽视，这为重新审视太赫兹波产生的动力学机制提供了新的契机。..首先，本项目发现在单色飞秒激光成丝过程中，太赫兹偏振态会从椭偏到线偏演变。进一步分析表明，上述现象是由于在“太赫兹波空间强束缚效应”作用下，等离子体光丝通道内的太赫兹传输模式由轴上逐渐转变为离轴传输。此时，原有的太赫兹双折射受到削弱，使其偏振态发生变化。这为全面理解飞秒激光成丝辐射太赫兹波繁杂的偏振特性提供了新思路。..其次，本项目探究了“太赫兹波空间束缚传输效应”的探测与表征方法。采取了三种实验操作：1）纵向截丝，测量太赫兹时域波形，2）横向截丝，测量太赫兹光束直径，3）近场小孔二维扫描对太赫兹光斑进行成像。之后，通过相应的实验现象：1）太赫兹脉冲在时域发生超前偏移，2）太赫兹光束直径小于其峰值频率对应的波长，来判断是否存在太赫兹波空间束缚传输效应。此工作为探测太赫兹波与空气等离子体介质相互作用提供了实验基础。..最后，本项目探索了太赫兹波与空气等离子体介质相互作用下的“太赫兹波空间强束缚效应”对太赫兹波二维正交偏振特性的影响。通过将“太赫兹波空间强束缚效应”作为“第三阶段”引入到双色飞秒激光成丝辐射太赫兹波的动力学过程中去，补充“四波混频”和“光电流”支配下的第一、二阶段，获得了对太赫兹波二维偏振态更好的复现，从而完善了双色飞秒激光成丝辐射太赫兹波的基本理论。