Fig. 1. Backward mid-infrared femtosecond filament-induced emission spectrum from N₂ and Ar mixture^[44] (inset: energy diagram for N₂ lasing by Ar collision excitation. The laser energy was set to 7 mJ, partial pressures of N₂ and Ar were 0.1 MPa and 0.5 MPa, respectively)

Fig. 2. Backward emission spectra induced by circularly (red) and linearly (black) polarized femtosecond laser in pure nitrogen gas^[45] and calculated electron energy distributions in the circularly and linearly polarized femtosecond laser fields^[54]. (a) Emission spectra; (b) calculated electron energy distribution in the circularly polarized femtosecond laser field; (c) calculated electron energy distri

Fig. 3. Lasing action of N2+ by the pump of few-cycle laser pulses^[80]. (a) Few-cycle laser-induced emission spectrum of

Fig. 4. Experimental setup of the laser polarization shape by polarization gating technique and spectra^[94]. (a) Experimental set up, where HWP indicates a half wave plate, and OA is the optical axis of the multiple order quarter wave plate (MQW) and that of the zero-order quarter wave plate (ZQW), the angle θ is the angle between the OAs of MQW and ZQW; the inset shows schematic of light-induced coupling; (b) spectra of the forward probe pulses c

Fig. 5. Coherent emission of N2+ induced by a pair of near-infrared few-cycle laser pulses^[97]. (a) Schematic of the experimental set

Fig. 6. N2+ lasing actions induced by the femtosecond laser pulses modulated by the quarter-wavelength plates with different orders^[98]