Lasing without population inversion in air

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.

References in corpus (8)

• Lorentz meets Fano spectral line shapes: A universal phase and its laser control
• High gain backward lasing in air
• High-brightness switchable multi-wavelength remote laser in air
• Population Redistribution among Multiple Electronic States of Molecular Nitrogen Ions in Strong Laser Fields
• Recollision induced superradiance of ionized nitrogen molecules
• Remote creation of strong and coherent emissions in air with two-color ultrafast laser pulses
• Signature of superradiance from a nitrogen gas plasma channel produced by strong field ionization
• Near-resonant Raman amplification in the rotational quantum wavepackets of nitrogen molecular ions generated by strong field ionization