  We experimentally demonstrate generation of strong narrow-bandwidth emissions
with excellent coherent properties at ~391 nm and ~428 nm from molecular ions
of nitrogen inside a femtosecond filament in air by an orthogonally polarized
two-color driver field (i. e., 800 nm laser pulse and its second harmonic). The
durations of the coherent emissions at 391 nm and 428 nm are measured to be
~2.4 ps and ~7.8 ps respectively, both of which are much longer than the
duration of the pump and its second harmonic pulses. Furthermore, the measured
temporal decay characteristics of the excited molecular systems suggest an
"instantaneous" population inversion mechanism that may be achieved in
molecular nitrogen ions at an ultrafast time scale comparable to the 800 nm
pump pulse.
