Influence of polarization ellipticity on N2+lasing in a strong laser field

A near-infrared femtosecond laser pulse produces laserlike coherent radiation from singly ionized N2+ molecules when it experiences filamentation in air or pure nitrogen, and the lasing radiation reaches maximal intensity when the driving pulse is slightly polarized elliptically. Based on a strong-field transient ionization model that considers ionization-induced quantum coherence, we reproduced the general feature of this counterintuitive dependence of the lasing intensity on pump laser ellipticity by numerically solving the Maxwell-Bloch equations of the lasing amplification process. The simulation results suggest that the competition effect between the increase of B 2 u + & LARR; X 2Eu+ inversion probability and the decrease of overall N2+ ion production with the increasing ellipticity is mainly at the origin of observed ellipticity dependence. Additionally, the simulation predicts that ionization-induced quantum coherence also exhibits a nonzero ellipticity dependence and plays a nontrivial role in the weakly seeded and self-seeded scenarios. Our findings suggest that controlling the ionization-induced coherence could be a promising approach to improve the N2+ lasing generation.