Optical gain in the P branch of N2+ lasing by polarization-modulated laser pulses

Abstract

A near-infrared laser pulse generates lasing of $\mathrm{N}{{}_{2}}^{+}$ during filamentation, while the underlying mechanism of optical gain is still under debate. Here, we study the rotational lasing spectra by manipulating the polarization properties of the pump laser with the polarization gating (PG) method. The independent enhancement of lasing emission was observed in the $P$ branch when the pump laser was modulated by linear polarization gating (LPG). The increased gain is attributed to the promoted population inversion degree between rotational states located on the ${B}^{2}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}(v=0)$ and ${X}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}(v=0)$ states. Based on a stimulated emission mechanism, we simulated the lasing spectra and population distributions on the rotational states. The results suggest that the rotational inversion-induced lasing emission mainly contributes to the $P$ branch, which agrees well with the experimental observations.