Transition from triggered super-radiance to seed amplification in N₂ ⁺ lasing

Air lasing induced by laser filamentation opens a new route for research on atmospheric molecular physics and remote sensing. The generation of air lasing is composed of two processes, i.e., building up optical gain of air molecules in femtosecond time scale and emitting coherent radiation in picosecond time scale. Here, we focus on the emission mechanisms of N₂ ⁺ air lasing and reveal, by examining the intensities and temporal profiles of N₂ ⁺ lasing at 391 nm generated respectively in a time-varying polarization-modulated and a linearly polarized pump laser field under different nitrogen gas pressures, that the N₂ ⁺ lasing can emit through either triggered super-radiance or seed amplification. We find that the two pressure-sensitive factors, i.e., the dipole dephasing time T₂ and the population inversion density n, determine which of these two mechanisms dominates the N₂ ⁺ lasing emission process, enabling manipulation of the transition from triggered super-radiance to seed amplification or vice versa. Our findings clarify the emission mechanism of N₂ ⁺ lasing under different pressures and provide a deeper understanding of N₂ ⁺ air lasing not only in the establishment of optical gain but also in the lasing emission process.