Spatial characteristics and modulation of terahertz pulse from femtosecond laser air filament at high repetition rates

Terahertz source based on laser air plasma filamentation in air has a broad spectral bandwidth and strong electric field, no damage threshold limitations, which finds the applications in remote sensing, communications, biomedical and so on. The spatial properties of filament-based THz beams, particularly beam pointing stability, remain largely unexplored. The beam pointing of the THz generated from two-color laser filament in air was measured at the repetition rates up to kHz. Beam pointing jitter of the air filament-based THz increased by about four-fold when the laser repetition rate increased from 200 Hz to 1000 Hz. By applying an external DC electric field to the air filament, the beam pointing jitter was reduced by more than two-fold. Based on the fluid dynamics evolution induced by filamentation, the strong THz beam pointing jitter at the high repetition rates and its suppression by the external electric fields are both explained. The theoretical results indicate that the turbulence induced by high repetition rate gives rise to beam pointing jitter, and present a quantitative analysis of the correlation between the magnitude of turbulence and the intensity of jitter. This technique provides a way to generate a stable, high-repetition-rate THz source with broad bandwidth and strong field, which opens more opportunities for THz applications.

基于空气中激光空气等离子体丝化（laser air plasma filamentation）产生的太赫兹（Terahertz，THz）源具备光谱带宽宽广、电场强度优异且无损伤阈值限制的特性，可应用于遥感、通信、生物医学等诸多领域。然而，基于激光丝结构的太赫兹光束的空间特性，尤其是光束指向稳定性，目前仍未得到充分研究。本研究针对空气中双色激光丝产生的太赫兹光束指向性，在最高可达千赫兹（kHz）的重复频率下开展了测量实验。当激光重复频率从200赫兹（Hz）提升至1000赫兹时，空气丝基太赫兹源的光束指向抖动量提升了约四倍。通过向空气丝施加外部直流（DC）电场，光束指向抖动量可被抑制两倍以上。基于丝化过程诱导的流体动力学演化机制，本研究对高重复频率下显著的太赫兹光束指向抖动现象，以及外部电场对其的抑制作用给出了合理解释。理论结果表明，高重复频率诱导的湍流是导致光束指向抖动的根本原因，并对湍流强度与抖动幅度之间的相关性进行了定量分析。该技术为生成带宽宽广、场强优异且稳定的高重复频率太赫兹源提供了可行路径，为太赫兹领域的应用拓展了更多可能性。