Air lasing is a concept that is based on the utilization of the constituents of air as a gain medium in a standoff, impulsive, laser-like optical source. While both forward-propagating and backward-propagating laser emissions could be generated, the backward-propagating emission is of the most practical significance for its potential to enable single-ended remote sensing schemes. I will review recent results on air lasing from singly ionized nitrogen molecular ions N2+, pumped through femtosecond laser filamentation in air. So far, lasing has been demonstrated only in the forward direction, and the mechanisms that enable population inversion have been highly controversial.
Pavel Polynkin, Pavel Polynkin,
"Air lasing through femtosecond filamentation (Conference Presentation)", Proc. SPIE 10193, Ultrafast Bandgap Photonics II, 101930O (7 June 2017); doi: 10.1117/12.2263264; https://doi.org/10.1117/12.2263264