Population Redistribution Among Multiple Electronic States of Molecular Nitrogen Ions in Strong Laser Fields

Abstract

We carry out a combined theoretical and experimental investigation on the population distributions in the ground and excited states of tunnel-ionized nitrogen molecules at various driver wavelengths in the near- and midinfrared range. Our results reveal that efficient couplings (i.e., population exchanges) between the ground N_{2}^{+}(X^{2}Σ_{g}^{+}) state and the excited N_{2}^{+}(A^{2}Π_{u}) and N_{2}^{+}(B^{2}Σ_{u}^{+}) states occur in strong laser fields. The couplings result in a population inversion between the N_{2}^{+}(X^{2}Σ_{g}^{+}) and N_{2}^{+}(B^{2}Σ_{u}^{+}) states at wavelengths near 800 nm, which is verified by our experimental observation of the amplification of a seed at ∼391 nm. The result provides insight into the mechanism of free-space nitrogen ion lasers generated in remote air with strong femtosecond laser pulses.

Backward nitrogen lasing actions induced by femtosecond laser filamentation: influence of duration of gain

“…In the first type of air lasers, population inversion is achieved by dissociation of molecular oxygen or nitrogen followed by excitation of the atomic fragments using high-peak-intensity picosecond ultraviolet (UV) lasers, which gives rise to bidirectional amplified spontaneous emissions (ASEs) at either 845 nm from oxygen atoms or 870 nm from nitrogen atoms [8][9][10]. The second type of air lasers are realized by focusing intense ultrafast laser pulses in air which created population inversion conditions either in neutral N 2 molecules or in N 2 ions [4][5][6][7][11][12][13][14][15]. Although the pump mechanism behind the air laser from neutral N 2 molecules has been clarified which can be attributed to electron collisional excitation, the mechanism of the air laser from N 2 ions is still far from being fully understood and under intensive investigation.…”

Optical gain in rotationally excited nitrogen molecular ions

“…1). In the case of 800 nm pumping, it has been argued that X 2 + g to A 2 u coupling enables stimulated emission in N + 2 via population transfer [10,11]. If A 2 u -state population transfer were key, our use of the 1500 nm driver would inhibit lasing.…”

“…Indeed, it is counterintuitive that strong-field ionization of the nitrogen molecule appears to preferentially create a molecular ion in the upper, not the lower, emission state. Several alternative gain scenarios have been recently suggested [9][10][11]. The lack of understanding of the gain mechanism hinders the optimization of the lasing process that could lead to the unseeded lasing in the backward direction, which would be of the most practical significance.…”

Recent Advances in Air Lasing: A Perspective from Quantum Coherence

“…Based on the fact that a picosecond‐delayed external seed is significantly amplified for both the co‐propagation and counter‐propagation cases of the pump and seed pulses, this kind of lasing action is attributed to the seed amplification in N ions with the population inversion . Some scenarios have been proposed to explain its ultrafast gain, including the collisional excitation of rescattering electrons, multiple‐states couplings, the alignment‐induced transient inversion, etc. For details of this specific subject, one can refer to a recent review .…”