Externally seeded backward lasing radiation from femtosecond laser   filament in nitrogen gas
  Recently, S. Mitryukovskiy et al. presented experimental evidence showing that backward stimulated radiation at 337 nm can be obtained from plasma filaments in nitrogen gas pumped by circularly polarized 800 nm femtosecond pulses (Opt. Express, 22, 12750 (2014)). Here, we report that this backward stimulated radiation is enhanced by a factor of ~ 16 in the presence of a seed pulse. This enhanced stimulated radiation can be either linearly or circularly polarized, dictated by the seeding pulse, which is distinct from the non-polarized nature of the ASE without seeding pulse. We also measured the spatial profile and estimated the energy of the radiation. This seeding effect confirms unambiguously the existence of population inversion between the C3{\Pi}u and B3{\Pi}g state of nitrogen molecules inside plasma filament and provides a possible solution to control the properties of this backward stimulated radiation. 
Pengji Ding, Sergey Mitryukovskiy, Aur\'elien Houard, Arnaud Couairon,   Andr\'e Mysyrowicz, and Yi Liu
physics.optics