Lasing of ambient air with microjoule pulse energy pumped by a multi   terawatt IR femtosecond laser
  We report on the lasing action of atmospheric air pumped by an 800 nm femtosecond laser pulse with peak power up to 4 TW. Lasing emission at 428 nm increases rapidly over a small range of pump laser power, followed by saturation above ~ 1.5 TW. The maximum lasing pulse energy is measured to be 2.6 uJ corresponding to an emission power in the MW range, while a maximum conversion efficiency of is measured at moderate pump pulse energy. The optical gain inside the filament plasma is estimated to be excess of 0.7/cm. The lasing emission shows a doughnut profile, reflecting the spatial distribution of the pump-generated white-light continuum that acts as a seed for the lasing. We attribute the pronounced saturation to the defocusing of the seed in the plasma amplifying region and to the saturation of the seed intensity. 
Guillaume Point, Yi Liu, Yohann Brelet, Sergey Mitryukovskiy, Pengji   Ding, Aur\'elien Houard, and Andr\'e Mysyrowicz
physics.optics