Observation of the Post-Ionization Optical Coupling in N2+ Lasing in Intense Laser Fields

In this chapter, we introduce our recent studies on the mechanisms responsible for the optical amplification in N2+ induced by irradiating nitrogen molecules N2 with intense laser fields. We demonstrate that the lasing intensities of N2+ at 391.4 nm, corresponding to the B2Σu+ (ʋ = 0)-X2Σg+ (ʋ″ = 0) transition, can be strongly modulated by manipulating the polarization state of the driven laser field, which is ascribed to the post-ionization coupling between the ground X2Σg+ state and the first excited A2Πu state of N2+. By using pump-probe methods, we show direct evidences for the optical transition between the X2Σg+ and A2Πu states of N2+ in intense laser field, based on which we show the optimization of N2+ lasing by designing intense laser fields.