  Generation of laser-like narrow bandwidth emissions from nitrogen molecular
ions generated in intense near- and mid-infrared femtosecond laser fields has
aroused much interest because of the mysterious physics underlying such a
phenomenon as well as the potential application of such an effect in
atmospheric spectroscopic sensing. Here, we perform a pump-probe measurement on
the nonlinear interaction of rotational quantum wavepackets of nitrogen
molecular ions generated in mid-infrared (e.g., at a wavelength centered at
1580 nm) femtosecond laser fields with an ultrashort probe pulse whose broad
spectrum overlaps both P- and R-branch rotational transition lines between the
upper and lower electronic states. The results show that in the near-resonant
conditions, stimulated Raman amplification can efficiently occur which converts
the broad bandwidth ultrashort probe pulse into the narrow bandwidth laser-like
beam. Our finding provides an insight into the physical mechanism of strong
field induced lasing actions in atmospheric environment.
