飞秒激光泵浦的背向空气激光的研究进展

Recent progress of the backward air laser pumped by femtosecond laser pulses

以空气作为介质的无腔式激光在近几年迅速发展成为超快光学领域的一个热点研究内容。一方面因为空气激光为光学远程探测提供了全新概念的光源，有望极大提高光学远程探测的灵敏度。另一方面，空气激射现象中蕴含着丰富的物理过程，诸多基础性问题亟待回答。在本文中，我们对800nm飞秒激光泵浦的中性氮气分子的激射现象进行系统性研宄报道。实验中发现，圆偏振800nm飞秒激光泵浦的情况下在传播的前向和后向都能够观察到波长为357nm的受激辐射。在外加种子光的情况下，前向和后向辐射的能量都能得到约2个数量级的增强。基于圆偏振激光可以有效激发该辐射的实验观测，我们提出高能电子碰撞泵浦氮气分子是实现粒子数反转的物理机制。基于Maxwell-Bloch模型，我们对该等离子体内的受激辐射过程进行了数值模拟，模拟结果与实验测量的脉冲时域波形较好地符合。

Cavity-free lasing of air molecules has attracted much research interest in recent years and has become a hot research topic in the community of ultrafast optics. On one hand, such an air laser may provide a concept new light source for optical remote sensing, promising substantial improvement of detection sensitivity. On the other hand, there are very rich physics involved in the lasing process, with several fundamental questions still open. In this study, we investigated the lasing action of neutral nitrogen molecules in a plasma filament pumped by 800 nm femtosecond laser pulses. It was observed that both backward and forward coherent emission at 337 nm can be achieved when circularly polarized 800 nm pulses was employed as driven laser. In the presence of external seeding laser, the energy of emission can be enhanced by 2 orders of magnitude. Base on our observations, we proposed that the collision excitation of nitrogen molecules by energetic electron is responsible for population inversion. Based on the Maxwell-Bloch equations, we performed numerical simulations of this lasing process and reproduced well the experimental observations.