Lasing without population inversion in N 2⁺

We experimentally demonstrate the existence of long-lived coherent polarizations coupling simultaneously ground state X (X2ςg+) to excited states A (A2Πu) and B (B2ςu+) of N2+ inside a plasma created by a short intense laser pulse at 800 nm. This three-level V-scheme arrangement is responsible for a strong optical gain without population inversion at the B-X transition at 391.4 nm. Simulations based on Maxwell-Bloch equations reproduce well the kinetics and the pressure dependence of the gain.