Population Redistribution Among Multiple Electronic States of Molecular Nitrogen Ions in Strong Laser Fields.
We carry out a combined theoretical and experimental investigation on the population distributions in the ground and excited states of tunnel-ionized nitrogen molecules at various driver wavelengths in the near- and midinfrared range. Our results reveal that efficient couplings (i.e., population exchanges) between the ground N_{2}^{+}(X^{2}Σ_{g}^{+}) state and the excited N_{2}^{+}(A^{2}Π_{u}) and N_{2}^{+}(B^{2}Σ_{u}^{+}) states occur in strong laser fields. The couplings result in a population inversion between the N_{2}^{+}(X^{2}Σ_{g}^{+}) and N_{2}^{+}(B^{2}Σ_{u}^{+}) states at wavelengths near 800 nm, which is verified by our experimental observation of the amplification of a seed at ∼391  nm. The result provides insight into the mechanism of free-space nitrogen ion lasers generated in remote air with strong femtosecond laser pulses.
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China.
Jinping
J
Yao
Department of Applied Physics, Nanjing University of Science and Technology, Nanjing 210094, China.
Shicheng
S
Jiang
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China.
Wei
W
Chu
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China.
Bin
B
Zeng
State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, China.|Collaborative Innovation Center of Quantum Matter, Beijing 100871, China.
Chengyin
C
Wu
Department of Applied Physics, Nanjing University of Science and Technology, Nanjing 210094, China.|State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China.
Ruifeng
R
Lu
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China.|School of Physics Science and Engineering, Tongji University, Shanghai 200092, China.
Ziting
Z
Li
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China.|University of Chinese Academy of Sciences, Beijing 100049, China.
Hongqiang
H
Xie
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China.
Guihua
G
Li
Department of Applied Physics, Nanjing University of Science and Technology, Nanjing 210094, China.
Chao
C
Yu
School of Physics Science and Engineering, Tongji University, Shanghai 200092, China.
Zhanshan
Z
Wang
State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, China.
Hongbing
H
Jiang
State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, China.|Collaborative Innovation Center of Quantum Matter, Beijing 100871, China.
Qihuang
Q
Gong
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China.
Ya
Y
Cheng