Fig. 1. Schematic experimental setup for measurement of the forward coherent emission of nitrogen ions pumped by UV femtosecond laser pulses, and energy diagram of nitrogen ions

Fig. 2. The spectrum and polarization measurement of the ultraviolet femtosecond pump laser

Fig. 3. The spectrum of the forward emission from nitrogen ions, and the 428 nm signal intensity as a function of nitrogen gas pressure and pulse energy of the pump laser

Fig. 4. The results of the polarization measurement for the signals

Fig. 5. The population density of the relevant energy levels depending on the pump laser intensity and the time evolution of the pump laser field from numerical simulation