Strong-Field-Induced N_2^+ Air Lasing in Nitrogen Glow Discharge Plasma

We investigate N_2^+ air lasing at 391 nm, induced by strong laser fields in a nitrogen glow discharge plasma. We generate forward N_2^+ air lasing on the B^2\!\varSigma_\rm u^+(v'=0)–X^2\!\varSigma_\rm g^+ (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 B^2\!\varSigma_\rm u^+(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 N_2^+ lasing is caused by the efficient suppression of population inversion between the B^2\!\varSigma_\rm u^+ and X^2\!\varSigma_\rm g^+ states of N_2^+, in which a much higher population occurs in the X^2\!\varSigma_\rm g^+ state in the discharge plasma. Our results clarify the important role of population inversion in generating N_2^+ air lasing, and also indicate the potential for the enhancement of N_2^+ lasing via further manipulation of the population in the X^2\!\varSigma_\rm g^+ state in the discharged medium.