Abnormal dependence of strong-field-ionization-induced nitrogen lasing on polarization ellipticity of the driving field

We experimentally investigate lasing behaviors of population-inverted N2⁺ ions for the transitions between the vibrational level (v′ = 0) of excited electronic B ²Σu+ state and the two lowest vibrational levels (v = 0, 1) of the ground X ²Σg+ state in an elliptically polarized laser field. It is found that, as the polarization of the pump laser evolves from linear to circular, the lasing signal for the 0 → 0 transition at 391 nm first increases with a maximum enhancement of ∼40% at the ellipticity of 0.3 and then decreases, whereas for the 0 → 1 transition at 428 nm, the lasing signal decreases monotonically. This difference between the 391- and 428-nm lasing signals is ascribed to the high sensitivity of the strong-field response of the molecular ion to molecular vibrations, which indicates the possibility to control the vibrational state distribution of molecules by tuning the ellipticity of the laser pulse.