Modulation of population inversion in N2+ by a pump-control-seed scheme

Abstract

We experimentally demonstrate an externally seeded ${\mathrm{N}}_{2}{}^{+}$ lasing action on the transition between the $B{\phantom{\rule{4pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{u}{}^{+}(v=0)$ and $X{\phantom{\rule{4pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}{}^{+}({v}^{\ensuremath{'}}=0)$ states at 391.4 nm by irradiating a ${\mathrm{N}}_{2}$ gas with an intense 400-nm pump laser pulse and reveal that the populations in the rotational levels of ${J}^{\ensuremath{'}}=7--17$ in the $B{\phantom{\rule{4pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{u}{}^{+}(v=0)$ state are responsible for the lasing based on the rotational revival structure in the lasing intensity recorded by the pump-probe measurements. By introducing additionally an 800-nm control pulse, we find that the lasing intensity is suppressed when the timing of the control pulse is set between the 400-nm pump and 400-nm seed laser pulses while it can be enhanced when the control pulse overlaps temporally the seed pulse. By solving the time-dependent Schr\"odinger equation including the continuous ionization of ${\mathrm{N}}_{2}$ and multistate coupling among the ${B}^{\phantom{\rule{4pt}{0ex}}2}{\mathrm{\ensuremath{\Sigma}}}_{u}{}^{+}$, ${A}^{\phantom{\rule{4pt}{0ex}}2}{\mathrm{\ensuremath{\Pi}}}_{u}$, and $X{\phantom{\rule{4pt}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}{}^{+}$ states in ${\mathrm{N}}_{2}{}^{+}$ induced by the control pulse, we show that the population inversion in ${\mathrm{N}}_{2}{}^{+}$ can be achieved by the 400-nm pump laser pulse and can be further modulated by the control pulse. We show also that the ${\mathrm{N}}_{2}{}^{+}$ lasing intensity can be suppressed or enhanced depending on the timing and intensity of the control pulse originating from the competition between the two dynamical processes, that is, the ionization of ${\mathrm{N}}_{2}$ and the population transfer among the three electronic states of ${\mathrm{N}}_{2}{}^{+}$.