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Nitrogen molecular ions (N₂ ⁺) in air plasma pumped by femtosecond laser pulses give rise to superradiant emission at 391.4 nm in the presence of an external seed pulse at proper wavelength. Due to the transient alignment of the nitrogen molecular ions, the superradiance signal presents a strong modulation as a function of the temporal delay between the pump and the seed pulses. Through Fourier transformation with high frequency resolution, we distinguished the contribution of the finely separated rotation levels of the upper and lower states. It was found that the population density of certain rotational levels in the upper state is higher than that in the lower one, indicating that population inversion of the rotation levels of the two involved states is a key enabling factor for this superradiant emission.

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Singly ionized nitrogen molecules in ambient air pumped by near-infrared femtosecond laser give rise to superradiant emission. Here we demonstrate coherent control of this superradiance by injecting a pair of resonant seeding pulses inside the nitrogen gas plasma. Strong modulation of the 391.4 nm superradiance with a period of 1.3 fs is observed when the delay between the two seeding pulses are finely tuned, pinpointing the essential role of macroscopic coherence in this lasing process. Based on this time-resolved method, the complex temporal evolution of the macroscopic coherence between two involved energy levels has been experimentally revealed, which is found to last for around 10 picoseconds in the low gas pressure range. These observations provide a new level of control on the "air lasing" based on nitrogen ions, which can find potential applications in optical remote sensing.

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