  We experimentally investigate generation of backward 357 nm N2 laser in a gas
mixture of N2/Ar using 800-nm femtosecond laser pulses, and examine the
involved gain dynamics based on pump-probe measurements. Our findings show that
a minimum lifetime of population inversion in the excited N2 molecules is
required for generating intense backward nitrogen lasers, which is ~0.8 ns
under our experimental conditions. The results shed new light on the mechanism
for generating intense backward lasers from ambient air, which are highly in
demand for high sensitivity remote atmospheric sensing application.
