Harmonic-seeded resonant Raman amplification in strong-field ionized   nitrogen molecules
  Generation of free-space laser-like emissions of high spatiotemporal coherence and narrow bandwidth in atmosphere opens promising opportunities for remote spectroscopic sensing. Here, we report on generation of such laser-like emissions, which results from the combined contributions of perturbative and non-perturbative nonlinear optical effects in nitrogen molecules exposed to intense mid-infrared laser fields. We systematically investigate the dependence of the generated free-space laser spectrum on wavelength and power of the driver laser. It is revealed that the free-space laser is produced by resonant Raman amplification of the fifth harmonic of the driver pulses in rotational wavepacket of the molecular nitrogen ions. 
Jinping Yao, Wei Chu, Chuanshan Tian, Ziting Li, Zhanshan Wang, and Ya   Cheng
physics.optics