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

Abstract

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.

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