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A cavity-free laser in the sky could lead to revolutionary improvements inoptical remote sensing for atmospheric science. Abundant in air, nitrogenmolecules are prime candidates as an active medium for such a laser. Nitrogenmolecules, either neutral or singly ionized, exhibit a significant optical gainin the UV regime when pumped by an intense femtosecond laser. Despite numerousworks, the physical origin of this lasing is still not understood. Here weoffer a consistent interpretation of all experimental facts by introducing aconcept drawn from the field of quantum optics: transient lasing withoutinversion. Our experiments and simulations identify the crucial role ofmacroscopic coherence, instead of population inversion, in the nitrogen ionslasing system. Manipulating the macroscopic coherence provides a new level ofcontrol over nitrogen air lasing and offers new perspectives for the generationof very short wavelength laser where population inversion is difficult toachieve.

Lasing without population inversion in air