Lasing without population inversion in air

TL;DR

This paper demonstrates that nitrogen air lasing can occur without population inversion, driven by macroscopic coherence, which could revolutionize atmospheric optical sensing and short-wavelength laser generation.

Contribution

It introduces the concept of transient lasing without inversion in nitrogen air, supported by experiments and simulations, highlighting the role of macroscopic coherence.

Findings

Lasing in nitrogen air occurs without population inversion.

Macroscopic coherence is crucial for nitrogen lasing.

Control of coherence enables new laser applications.

Abstract

A cavity-free laser in the sky could lead to revolutionary improvements in optical remote sensing for atmospheric science. Abundant in air, nitrogen molecules are prime candidates as an active medium for such a laser. Nitrogen molecules, either neutral or singly ionized, exhibit a significant optical gain in the UV regime when pumped by an intense femtosecond laser. Despite numerous works, the physical origin of this lasing is still not understood. Here we offer a consistent interpretation of all experimental facts by introducing a concept drawn from the field of quantum optics: transient lasing without inversion. Our experiments and simulations identify the crucial role of macroscopic coherence, instead of population inversion, in the nitrogen ions lasing system. Manipulating the macroscopic coherence provides a new level of control over nitrogen air lasing and offers new perspectives for the generation of very short wavelength laser where population inversion is difficult to achieve.