Polarization ellipticity dependence of N⁺ ₂ air lasing: The role of coupling between the ground state and a photo-excited intermediate state

We experimentally investigate the dependence of N⁺ ₂ lasing radiation on the ellipticity of an 800 nm pump laser. The results show that 391 and 428 nm lasing signals exhibit different behaviors with the change of ellipticity. The 391 nm lasing signals show abnormal ellipticity dependence regardless of the pump intensity, whereas for the 428 nm lasing signals, the abnormal characteristic only occurs at a high laser intensity. The qualitative analysis reveals that the physical origin of the phenomenon is the postionization coupling between the ground X²6_g ⁺ state and the intermediate A²5_u state of N⁺ ₂ ions.