Observation of the optical X²Σ_g⁺-A²Π_u coupling in N₂⁺ lasing induced by intense laser field

We propose a simple pump-coupling-seed scheme to examine the optical X²Σ_g⁺-A²Π_u coupling in N₂⁺ lasing. We produce the N₂⁺ lasing at 391 nm, corresponding to the B²Σ_u⁺(v'=0)-X²Σ_g⁺(v=0) transition, by externally seeding the N₂⁺ gain medium prepared by irradiation of N₂ with an intense pump pulse. We then adopt a weak coupling pulse in between the pump and seed pulses, and show that the intensity of the 391-nm lasing can be efficiently modulated by varying the polarization direction of the coupling pulse with respect to that of the pump pulse. It is found that when the polarization directions of the pump and coupling pulses are perpendicular, the 391-nm lasing intensity is more sensitive to the coupling laser energy, which reflects the inherent nature of the perpendicular X²Σ_g⁺-A²Π_u transition.