An anatomy of strong-field ionization-induced air lasing

It is known that in an intense laser field of a sufficiently high strength combined with a sufficiently long wavelength, i.e., in the regime of the Keldysh parameter γ < 1, photoionization of atoms and molecules can be realized through a quantum tunnel process. The tunnel ionization preferentially occurs from the orbital with the lowest ionization energy, thus the majority of the generated ions will stay on the ground state. It is surprising that tunnel ionization of nitrogen molecules with mid- and near-infrared intense laser fields can initiate strong laser-like emissions, indicating generation of stimulated emissions in molecular nitrogen ions. The physical mechanism behind the observation is still under debate. Here, we review the major progresses we made in the past a few years. The focus is placed on investigations on the lasing action at 391 nm wavelength initiated by either mid-infrared strong laser fields in the wavelength range from 1.2 to 2 µm or near-infrared intense laser fields around 800 nm wavelength. We reveal that the mechanisms of lasing actions are different for the pump lasers in the above two spectral regions. We also show that the coherent wavepackets of molecular nitrogen ions generated in the intense laser fields uniquely allow for efficient nonlinear interaction with light at resonance frequencies.