涡旋强飞秒激光脉冲在空气中非线性传输特性研究

Femtosecond laser filamentation has attracted considerable interest because of its wide range of applications (e.g., remote sensing), and its special fundamental science perspective to study the laser-matter interaction motivated by the nonlinear phenomena(e.g., supercontinnum generation) induced in filamentation. The key to achieving these applications is to establish the complete picture of filamentation. Unfortunately, it encounters controversial debates (e.g., the defocusing of negative higher-order Kerr effect), and needs breakthrough on controlling the formation and temporal-spatial distribution of filaments before it is brought into practical applications. The femtosecond vortex beam, which has a screw-type phase distribution and carries orbital angular momentum, provides a special channel to modulate the laser-matter interaction and control the characteristics of filaments. In contrast to the past researches which focused on the stability of vortex structures during highly nonlinear processes, this project will systemically investigate the propagation of vortex beams in air and supercontinnum spectra taking place during filamentation by adjusting the parameters of the vortex light field(e.g., power and topological charge). It holds promise for lengthening the propagation distance and broadening the supercontinnum spectra of the focused vortex beam as compared with the vortex-free beam(i.e., traditional Gaussian beams), and developing a new method to manipulate filaments. Furthermore, from the perspective of the molecular strong-field ionization , the dynamics of molecule in the optical field inside the filaments will be explored for revealing the physical mechanism of plasma and its temporal-spatial distribution, and deepening the understanding of the physical picture of the femtosecond laser filamentation.

飞秒激光脉冲在空气中非线性传输所成的光丝，具有远程探测等方面的应用价值，也是研究强光场与原子分子相互作用的独特通道。目前，成丝基本物理图像虽已逐渐清晰，但仍存在高阶克尔效应是否可以忽略等争议，而有效调控光丝的形态和时空分布则是其应用价值得以实现的关键。在本项目中，利用具有特殊相位分布和携带特定光子轨道角动量的涡旋飞秒光束，突破以往通常只研究涡旋光束在非线性传输过程中结构稳定性的局限，我们拟系统研究涡旋飞秒光束在空气中的丝化现象及其诱导的超连续谱随功率、拓扑荷等光场参数变化的演化规律，有望获得比普通飞秒光束更远距离的激光成丝和更宽的超连续谱，并通过驱动光场实现对光丝形态和时空分布的操控，进而基于强场分子电离理论，研究丝中强光场下的分子电离动力学，揭示等离子体的产生机制和影响其时空演化的因素，获得对飞秒激光成丝物理图像更为深入而全面的认识。

本项目拟研究涡旋飞秒光束在空气中的传输特性。在本项目资助下，我们首先从理论上获得了涡旋光束传输至远场端的相关特性（如光强、偏振度和相干度等），发现了在传输距离足够远时其光强分布会演变成类高斯分布。.在项目执行过程中，受限于涡旋飞秒光束的光强，我们将实验重点集中于如何提高涡旋飞秒光束光强以及更好理解强场电离动力学这两方面。所取得的主要进展如下：.1)我们发现在激光系统内用非对称拉曼晶体可使激光线性偏振化得到优化统一，而采用紧凑平凹腔结构可提高晶体的光强转化效率。同时，利用反馈波前技术对光束进行整形，可以有效控制光经过介质后的偏振分布。.2)我们系统研究了中红外波段Xe原子电离产率随光强的变化规律。结合理论计算，我们获得了可有效且精确描述强场电离的理论模型，并确认可利用该模型更好地标定出长波长激光脉冲绝对光强。.3)利用量子化修正的半经典模型，我们确认了在中红外波段强场电离实验中发现的呈现出特别波长依赖关系的低能结构与电子隧穿过程中的非绝热效应有关。