A self-induced white light seeding laser in a femtosecond laser filament

We report on the generation of remote self-seeding laser amplification by using only one 800 nm Ti:sapphire femtosecond laser pulse. The laser pulse (∼40 fs) is first used to generate a filament either in pure nitrogen or in ambient air in which population inversion between the B²Σ_u⁺ and X²Σ_g⁺ states of N₂⁺ is realized. Self-induced white light inside the filament covering the B²Σ_u⁺−X²Σ_g⁺ transition is then serving as the seed to be amplified. The self-induced narrow-band laser at 428 nm has a pulse duration of ∼2.6 ps with perfect linear polarization properties. This finding opens new possibilities for remote detection in the atmosphere. (letter)