Opt Lett. 2023 Feb 1;48(3):664-667. doi: 10.1364/OL.478025.
We report on a spectral splitting effect of the cavity-less lasing emission of nitrogen ions at 391.4 nm pumped by 800-nm femtosecond laser pulses. It was found that with the increase of the nitrogen gas pressure and pump pulse energy, both R and P branches experience spectral splitting. With an external injected seeding pulse, a similar split spectral line is observed for the amplified emission. In contrast, for the fluorescence radiation, no such spectral splitting phenomenon is observed with much more abundant R branch structures. Our theoretical model considers gas ionization by the pump pulse, the competition of excitation of all relevant electronic and vibrational states, and an amplification of the seeding pulse in the plasma with a population inversion. Our simulation reproduces this spectral splitting effect, which is attributed to the gain saturation resulting in the oscillation of the amplitude of the amplified signal.
我们报告了在无腔条件下，氮离子在 800nm 飞秒激光脉冲激发下产生的 391.4nm 激光发射的光谱分裂效应。研究发现，随着氮气气压和泵浦脉冲能量的增加，R 和 P 支都经历了光谱分裂。在外注入种子脉冲的情况下，放大发射也观察到了类似的分裂光谱线。相比之下，对于荧光辐射，尽管 R 支结构丰富得多，但没有观察到这种光谱分裂现象。我们的理论模型考虑了泵浦脉冲引起的气体电离、所有相关电子和振动态的激发竞争，以及等离子体中种子脉冲的放大和粒子数反转。我们的模拟再现了这种光谱分裂效应，这归因于增益饱和导致放大信号的振幅振荡。
