Strong-field-induced N₂⁺ lasing by phase control of free induction decay

We prepare N-2(+) mostly in the vibrational ground X-2 Sigma(+)(g) state by an ultraviolet 267-nm pulse and create the free induction decay (FID) of the B-2 Sigma(+)(u) - X-2 Sigma(+)(g)(0, 0) transition at 391 nm by a resonant excitation pulse. By controlling the phase shift of the FID using a near-IR control pulse through the dynamical Stark effect and rotational coherence, we reveal that the absorption spectrum of this transition, exhibiting a Lorentzian absorption profile, can be changed into a Fano-type profile, and then, a Lorentzian emission profile, resulting in N-2(+) lasing. Our results shed light on the long-term controversy over the mechanism of strong-field-induced N-2(+) lasing and reveal a general route to achieving amplification of light resonantly interacting with molecular ions without population inversion.