Asymmetric enhancement of N₂⁺ lasing in intense, birefringence-modulating elliptical laser fields

We experimentally demonstrate an asymmetric enhancement of the N₂⁺ lasing at 391 nm for the transition between the B²Σu⁺ (v = 0) and X²Σg⁺ (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 X²Σg⁺ (v "=0) to A²Π_u (v '=2) states.