Unexpected Sensitivity of Nitrogen Ions Superradiant Emission on Pump Laser Wavelength and Duration

Nitrogen molecules in ambient air exposed to an intense near-infrared femtosecond laser pulse give rise to cavity-free superradiant emission at 391.4 and 427.8 nm. An unexpected pulse duration-dependent cyclic variation of the superradiance intensity is observed when the central wavelength of the femtosecond pump laser pulse is finely tuned between 780 and 820 nm, and no signal occurs at the resonant wavelength of 782.8 nm ($2{\omega }_{782.8\mathrm{nm}}={\omega }_{391.4\mathrm{nm}}$). On the basis of a semiclassical recollision model, we show that an interference of dipolar moments of excited ions created by electron recollisions explains this behavior.

Figure 1

(a) Forward 391.4 nm emission observed on a yellow fluorescent paper, after the pump laser around 800 nm was filtered out by the color glass filter (BG 40). (b) Experimental spectrum of the forward emission. The doublet structure corresponds to the $R$ and $P$ bands of the nitrogen ions. The experiments were performed with a nitrogen pressure of 5 mbar and a pump laser energy of 1.1 mJ.

Figure 2

(a)–(c) Measured forward 391.4 nm superradiance signal as a function of the pump laser central wavelength for pulse durations of 35, 40, and 50 fs. (d) Sideways fluorescence at 391.4 nm in the case of a 40 fs incident pump laser pulse. The gas pressure in (b) was 2.5 and 5 mbar for (a), (c), and (d).

Figure 3

Experimentally measured 391.4 nm lasing signal for different pulse durations with a constant central wavelength at 800 nm. The nitrogen gas pressure was 5 mbar. The dots present the experimental measurements.

Figure 4

(a) Schematic illustration of electron recollision excitation. (b) Trajectories of the electrons (solid lines) in the laser field (blue dashed line), which lead to the periodic recollision excitation of the parent ion. The recollision occurs from opposite sides of the parent ions, leading to interference of the dipole moments, with the direction of the recolliding electrons indicated by the arrow (green).

Figure 5

Calculated average dipolar momentum of the state after the laser-molecule interaction: (a) as a function of the laser wavelength for the pulse durations 35, 40, and 50 fs; (b) as a function of the pulse duration for the laser wavelength of 800 nm.