Strong-Field-Induced N₂⁺Air Lasing in Nitrogen Glow Discharge Plasma

We investigate N2+ air lasing at 391 nm, induced by strong laser fields in a nitrogen glow discharge plasma. We generate forward N2+ air lasing on the B2 Σu+ (v′ = 0) X2 Σg+(v′ = 0 transition at 391 nm by irradiating an intense 35-fs, 800-nm laser in a pure nitrogen gas, finding that the 391-nm lasing quenches when the nitrogen gas is electrically discharged. In contrast, the 391-nm fluorescence measured from the side of the laser beam is strongly enhanced, demonstrating that this discharge promotes the population in the B2 Σu+ (v′ = 0) state. By comparing the lasing and fluorescence spectra of the nitrogen gas obtained in the discharged and laser-induced plasma, we show that the quenching of N2+ lasing is caused by the efficient suppression of population inversion between the B2 Σu+ and X2 Σg+ states of N2+, in which a much higher population occurs in the X2 Σg+ state in the discharge plasma. Our results clarify the important role of population inversion in generating N2+ air lasing, and also indicate the potential for the enhancement of N2+ lasing via further manipulation of the population in the X2 Σg+ state in the discharged medium.