title: Identification of the physical mechanism of generation of coherent N2(+) emissions in air by femtosecond laser excitation.
abstract: 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.
authors.affiliation: 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.
authors.forename: Jielei
authors.initials: J
authors.lastname: Ni
authors.forename: Wei
authors.initials: W
authors.lastname: Chu
authors.forename: Chenrui
authors.initials: C
authors.lastname: Jing
authors.forename: Haisu
authors.initials: H
authors.lastname: Zhang
authors.forename: Bin
authors.initials: B
authors.lastname: Zeng
authors.forename: Jinping
authors.initials: J
authors.lastname: Yao
authors.forename: Guihua
authors.initials: G
authors.lastname: Li
authors.forename: Hongqiang
authors.initials: H
authors.lastname: Xie
authors.forename: Chaojin
authors.initials: C
authors.lastname: Zhang
authors.forename: Huailiang
authors.initials: H
authors.lastname: Xu
authors.forename: See-Leang
authors.initials: SL
authors.lastname: Chin
authors.forename: Ya
authors.initials: Y
authors.lastname: Cheng
authors.forename: Zhizhan
authors.initials: Z
authors.lastname: Xu
mesh_terms: D000388:Air / Q000032:analysis*; D000393:Air Pollutants / Q000032:analysis*; D004784:Environmental Monitoring / Q000295:instrumentation*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D007834:Lasers; D009391:Nephelometry and Turbidimetry / Q000295:instrumentation*; D009584:Nitrogen / Q000032:analysis*
chemical_list: D000393:Air Pollutants; D009584:Nitrogen