【摘要】 通过结合等离子体衍射测量与数值模拟，研究了飞秒激光成丝过程中空间非对称分布的产生机制、调控方法及其对超连续辐射特性的影响。激光光斑模式不理想是实际应用中普遍存在的问题。证明了激光模式的非对称性会造成光丝内部光强和等离子体的非对称分布。利用外部聚焦条件调制这种非对称性，当数值孔径降低至1/1000时，所产生的弯曲光丝的运动轨迹曲率降低至0.1 mm/m~2。此外，通过调控光丝的非对称性分布，实现了超连续辐射空间分布的调控。阐明了飞秒激光成丝过程中非对称性空间分布的产生机制，为基于激光成丝的超连续光源调控、太赫兹波调控等应用提供了重要的理论依据。更多还原

【Abstract】 Objective Ultrafast laser filamentation finds applications in remote sensing, lightning control, free-space communications, air lasing, terahertz(THz) wave generation, and pulse compression, owing to its unique characteristics. The spatial distributions of intensity and plasma density inside the filaments critically determine the key properties of filament-induced emissions(e. g., THz waves and supercontinuum sources), including spatial profile, polarization, divergence, and conversion efficiency. Consequently, beam shaping techniques, such as Gaussian beam, super-Gaussian beam, flat-top beam, Bessel beam, and Airy beam, are employed to control the spatial distributions of filaments as well as their emissions. Practical implementations result from beam perturbations caused by optical aberrations, thermal distortions, diffraction, turbulence, and nonlinear effects. These deviations from ideal beam profiles induce asymmetric intensity/plasma distributions within filaments, detrimentally affecting secondary processes(e. g., THz emission, supercontinuum generation, and air lasing). Quantifying these non-ideal effects is thus essential for optimizing applications. This work specifically investigates how the asymmetric laser beam profile governs filament asymmetry and what is the impact on the supercontinuum profile. This work provides fundamental insights for controlling filament-derived radiation sources.Methods The plasma distribution inside femtosecond laser filaments is characterized experimentally by a pump-probe plasma diffraction technique. A second harmonic laser beam at 400 nm is chosen as the probe beam to propagate perpendicularly through the plasma channel. Refractive index modulation induced by plasma imprints a spatially varying phase shift onto the probe wavefront. This phase modulation is converted into a measurable diffraction pattern. To explain and prove the experimental results, numerical simulations based on the nonlinear propagation equation and Fresnel diffraction theory are performed.Results and Discussions The experimental and simulation results give contributions to the generation mechanisms, the modulation methods, and the impact on supercontinuum generation with spatial asymmetry during femtosecond laser filamentation. Experimental measurements reveal that non-ideal beam profiles fundamentally alter filamentation dynamics. Plasma diffraction patterns demonstrate that input beam asymmetry induces characteristic curved diffraction fringes(Fig. 2), directly mapping asymmetric plasma distributions. Critically, this asymmetry is focusing-condition dependent. High-numerical aperture(NA) focusing configurations show obvious transverse density contrast in filaments [Fig. 4(a)], while low-NA(1/2000) configurations suppress asymmetry. Besides, beam displacement at the focusing lens provides an active control on the filament asymmetry. By this method, the spatial profile of supercontinuum generated from filaments is effectively controlled.Conclusions Imperfections in laser beam spatial profiles represent a pervasive challenge in practical applications. We demonstrate that asymmetries in the laser mode profile induce corresponding asymmetries in the laser intensity distribution and plasma density distribution inside the filament. Furthermore, the asymmetry degree is modulated by external focusing conditions, exhibiting significant attenuation under low NA configurations. Additionally, by actively manipulating the asymmetric distribution of the filament, the spatial distribution of supercontinuum is controlled. This research elucidates the fundamental mechanisms governing asymmetric spatial distributions in femtosecond laser filamentation, providing crucial theoretical foundations for applications reliant on filament-derived radiation, such as supercontinuum sources and terahertz waves.更多还原