Nonperturbative nonlinear optics underpinned by strong field tunnel ionization has opened the door to high-order harmonic generation and attosecond science. Previous studies have observed that the nonperturbative interaction of intense laser fields with molecules can generate laserlike narrow-bandwidth coherent emissions instantly after the photoionization.  

The generation of laserlike narrow bandwidth emissions from nitrogen molecular ions (N_2^+) in intense near- and mid infrared femtosecond laser fields has aroused much interest because of the mysterious physics underlying such a phenomenon. Several mechanisms have been proposed for understanding the origin of the population inversion generated in the 800 nm laser field, but a reliable mechanism is yet to be identified.  

Recently, a research group led by Prof. CHENG Ya at State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, performed a pump-probe measurement on the nonlinear interaction of strong-field-excited rotational quantum wave packets of N_2^+ with a weak ultrashort probe pulse.  

The results indicate the occurrence of highly efficient near-resonant stimulated Raman scattering in the quantum wave packets of N_2^+ ions generated in strong laser fields in the midinfrared region, of which the underlying mechanism was different from that of the air lasers generated in atmospheric environment when pumping with 800 nm intense pulses.  

This unambiguous evidence on the near-resonant coherent Raman scattering provides an insight on the mechanism of laserlike emissions generated in air with midinfrared pump pulses.  

Last but not least, they note that the phenomenon should not be unique to N_2^+ ions and similar behavior may also be found in neutral molecules such as NO and CO, which will be studied in future investigation.  

Their findings, entitled "Near-Resonant Raman Amplification in the Rotational Quantum Wave Packets of Nitrogen Molecular Ions Generated by Strong Field Ionization", were published in Physical Review Letters.  

This work was supported by the National Basic Research Program of China, National Natural Science Foundation of China, and the Strategic Priority Research Program of Chinese Academy of Sciences (CAS). 

(a) Schematic diagram of resonant and the first nearresonant Raman scattering in nitrogen molecular ions. (b) Schematic diagram of resonant and the second near-resonant Raman scattering in nitrogen molecular ions. (c) Calculated Stokes wavelengths for the near-resonant Raman processes in (a) and (b). (Image by LIU Zhaoxiang)