Backward lasing of femtosecond plasma filaments

Stimulated emissions in both backward and forward directions from a plasma filament in ambient air or pure nitrogen have been observed in recent years. In this article, we present our recent experimental results concerning the backward stimulated emission. We first demonstrate that backward stimulated emission from neutral N₂ molecules can be effectively generated with a circularly polarized 800nm femtosecond laser pulse in pure nitrogen. Then, we show that the presence of oxygen is detrimental to the laser gain. To further confirm the presence of population inversion, we send a counter-propagating seeding pulse into the plasma filament. This leads to an amplification of the seeding pulse by two orders of magnitude. The crucial role of pump laser polarization indicates that the inelastic collisions between the energetic electrons and the neutral N₂ molecules are at the origin of population inversion between the relevant states.