  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.
