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Cavity-free air lasing offers a promising route towards the realization of atmospheric lasers for various applications such as remote sensing and standoff spectroscopy; however, achieving efficient generation and control of air lasing in ambient air is still a challenge. Here we show the experimental realization of a giant lasing enhancement by three to four orders of magnitude in ambient air for the self-seeded N 2 + lasing at 428 nm, assigned to the B 2 Σ u + ( ν ′ = 0 ) and X 2 Σ g + ( ν ′ ′ = 1 ) emission, by modulating the spatiotemporal overlap of ultrashort near-infrared control-pump pulses in a filamentary plasma grating; meanwhile, the spontaneous emission from the same transition is only enhanced by three to four times. We find that this enhancement is sensitive to the relative polarization and interference time of the two pulses, and reveal that the formation of the plasma grating induces different population variations in the B 2 Σ u + ( ν ′ = 0 ) and X 2 Σ g + ( ν ′ ′ = 1 ) levels, resulting in an enormous population inversion between the two levels, thereby a higher gain for the giant enhancement of N 2 + lasing in ambient air.

We experimentally demonstrate an asymmetric enhancement of the N2+ lasing at 391 nm for the transition between the B2Σ u + (v = 0) and X2Σ g + (v” = 0) states in an intense laser field with the ellipticity, ε , modulated by a 7-order quarter-wave plate (7-QWP). It is found that when the 7-QWP is rotated from α = 0 to 90°, where α is the angle between the polarization direction of the input laser and the slow axis of the 7-QWP, the intensity of the 391-nm lasing is optimized at ε ∼ 0.3 with α ∼ 10°-20° and 70°-80° respectively, but the optimization intensity at α ∼ 10°-20° is about 4 times smaller than that at α ∼ 70°-80°. We interpret the asymmetric enhancement based on a post-ionization coupling model, in which the birefringence of the 7-QWP induces an opposite change in the relative amplitudes of the ordinary ( Eo ) and extraordinary ( Ee ) electric components under the two conditions, so that the same temporal separation of Eo and Ee leads to a remarkably different coupling strength for the population transfer from the X2Σ g + (v ”=0) to A2Πu (v ’=2) states.