Control of $\text{N}_2^+$ Air Lasing

A near-infrared laser generates gain on transitions between the $\text{B}^{\text{2}} \Sigma_{\text{u}}^{\text{+}}$ and $\text{X}^{\text{2}} \Sigma_{\text{g}}^{\text{+}}$ states of the nitrogen molecular cation in part by coupling the $\text{X}^{\text{2}} \Sigma_{\text{g}}^{\text{+}}$ and $\text{A}^{\text{2}} \Pi_{\text{u}}$ states in the V-system. Traditional time resolved pump-probe measurements rely on post-ionization coupling by the pump pulse to initialize dynamics in the $\text{A}^{\text{2}} \Pi_{\text{u}}$ state. Here we show that a weak second excitation pulse reduces ambiguity because it acts only on the ion independent of ionization. The additional control pulse can increase gain by moving population to the $\text{A}^{\text{2}} \Pi_{\text{u}}$ state, which modifies the lasing emission in two distinct ways. The presence of fast decoherence on $\text{X}^{\text{2}} \Sigma_{\text{g}}^{\text{+}}$ to $\text{A}^{\text{2}} \Pi_{\text{u}}$ transitions may prevent the formation of a coherent rotational wave packet in the ground state in our experiment, but the control pulse can reverse impulsive alignment by the pump pulse to remove rotational wave packets in the $\text{B}^{\text{2}} \Sigma_{\text{u}}^{\text{+}}$ state.