Sub-10-fs population inversion in N2+ in air lasing through multiple state coupling

Abstract

Laser filamentation generated when intense laser pulses propagate in air has been an attractive phenomenon having a variety of potential applications such as detection and spectroscopy of gases at far distant places. It was discovered recently that the filamentation in air induces ‘lasing', showing that electronically excited N2+ is population-inverted, exhibiting marked contrast to the common understanding that molecular ions generated by intense laser fields are prepared mostly in their electronic ground states. Here, to clarify the mechanism of the population inversion, we adopt few-cycle laser pulses, and experimentally demonstrate that the lasing at 391 nm occurs instantaneously after N2+ is produced. Numerical simulations clarify that the population inversion is realized by the post-ionization couplings among the lowest three electronic states of N2+. Our results shed light on the controversy over the mechanism of the air lasing, and show that this post-ionization coupling can be a general mechanism of the atmospheric lasing.

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

“…Molecular nitrogen ion (N) lasing is the third type of air lasing, which originates from the transition from the excited state to the ground state . Lasing actions from N ions have been experimentally observed by several research groups since the first report in 2011 . Besides its promising application in the remote sensing similar to the other two types of air lasing, the understanding of the physical procedure in N lasing is also attracting great interests.…”

“…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 .…”

Spectroscopic analysis of high electric field enhanced ionization in laser filaments in air for corona guiding

“…During femtosecond laser FIL in air, many nonlinear effects occur simultaneously, such as intensity clamping, molecular alignment, harmonic generation, self-phase modulation and so on [3][4][5] . These complex and interesting nonlinear phenomena accompanying femtosecond laser FIL lead to promising applications in different fields, such as remote sensing [6,7] , few-cycle pulse compression [8] , atmospheric condensation and precipitation [9][10][11] , secondary light source generation ranging from air lasing to terahertz emission [12][13][14][15][16][17][18] , and triggering and guiding high-voltage discharge with possible applications in lightning control [19][20][21][22][23][24] .…”