Theory of a filament initiated nitrogen laser.

We present the theoretical model for a single-pass, discharge-type standoff nitrogen laser initiated by a femtosecond filament in nitrogen gas. The model is based on the numerical solution of the kinetic equation for the electron energy distribution function self-consistently with balance equations for nitrogen species and laser equations. We identify the kinetic mechanisms responsible for a buildup of population inversion in the filament afterglow plasma and determine the dependence of population inversion density and the parameters of nitrogen lasing at a 337 nm wavelength corresponding to the transition between the C³Π_u (v = 0) excited and the X¹Σ_g (v = 0) ground electronic states in a nitrogen molecule on the polarization and wavelength of the driver laser pulse used to produce the filament. We show that population inversion is achieved on an ultrafast time scale of ≈10 ps and decays within the time: <100 ps. We derive the low-signal gain 2.2 cm⁻¹ for lasing from a circularly polarized 0.8 μm near-IR filament and 0.16 cm⁻¹ for a linearly polarized 4 μm mid-IR filament. The results of the numerical simulations demonstrate good quantitative agreement with the experimental measurements. [ABSTRACT FROM AUTHOR]