Optimization of N_{2}^{+} lasing through population depletion in the X^{2}Σ_{g}^{+} state using elliptically modulated ultrashort intense laser fields

Abstract

We generate unidirectional coherent radiations at 391 and 428 nm corresponding to the B^{2}Σ_{u}^{+}-X^{2}Σ_{g}^{+} (0,0) and (0,1) emission of N_{2}^{+} by irradiating neutral N_{2} with an elliptically modulated near-IR ultrashort laser field generated using a quarter-wave plate (QWP) with the different orders, and reveal that the lasing intensity at 391 nm becomes maximum at a specific orientation of the fast axis of QWP with respect to the polarization direction of the input laser field. We reproduce this observation by numerical calculations and show that the lasing intensity can be optimized by adjusting the temporal separation between the two polarization components of the elliptically modulated laser pulse and their relative amplitudes so that the inverted population between the B^{2}Σ_{u}^{+} and X^{2}Σ_{g}^{+} states can be achieved efficiently and the population in the X^{2}Σ_{g}^{+}(v^{″}=0) level is further depleted in the latter part of the laser pulse by the A^{2}Π_{u}-X^{2}Σ_{g}^{+} transition.