Ni Jielei, Chu Wei, Jing Chenrui, Zhang Haisu, Zeng Bin, Yao Jinping, Li Guihua, Xie Hongqiang, Zhang Chaojin, Xu Huailiang, Chin See-Leang, Cheng Ya, Xu Zhizhan
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, China.
Opt Express. 2013 Apr 8;21(7):8746-52. doi: 10.1364/OE.21.008746.
Recently, amplification of harmonic-seeded radiation generated through femtosecond laser filamentation in air has been observed, giving rise to coherent emissions at wavelengths corresponding to transitions between different vibrational levels of the electronic B(2)Σ(u)(+) and X(2)Σ(g)(+) states of molecular nitrogen ions [Phys. Rev. A. 84, 051802(R) (2011)]. Here, we carry out systematic investigations on its physical mechanism. Our experimental results do not support the speculation that such excellent coherent emissions could originate from nonlinear optical processes such as four-wave mixing or stimulated Raman scattering, leaving stimulated amplification of harmonic seed due to the population inversion generated in molecular nitrogen ions the most likely mechanism.
最近，人们观察到通过飞秒激光在空气中的丝状化产生的谐波种子辐射的放大，从而在与分子氮离子的电子B(2)Σ(u)(+)和X(2)Σ(g)(+)态的不同振动能级之间的跃迁相对应的波长处产生了相干发射[《物理评论A》84, 051802(R) (2011)]。在此，我们对其物理机制进行了系统研究。我们的实验结果不支持这样的推测，即如此优异的相干发射可能源自诸如四波混频或受激拉曼散射等非线性光学过程，这使得由于分子氮离子中产生的粒子数反转而导致的谐波种子的受激放大成为最有可能的机制。
