Optical gain via impulsive rotational excitation of nitrogen molecular ions

Studies on the generation of optical gain in the constituents of air are motivated by potential applications in air lasing [1], a concept, the realization of which would enable efficient single-ended remote sensing in the atmosphere. Various schemes of turning air into an active laser medium are being investigated [2-4]. One of the most promising approaches is based on pumping atmospheric nitrogen by ultra-intense femtosecond laser pulses propagating in the filamentation regime in air [5]. It has been suggested that air filamentation may result in the optical gain on the electronic transitions of N2
⁺
at 391 nm and 428 nm emission wavelengths [6]. However, the gain mechanism responsible for those emissions has been controversial.