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

TLDR

Laser filamentation in air, produced by intense laser pulses, enables remote gas detection and spectroscopy, and recent observations show that it induces lasing via population inversion of electronically excited N₂⁺ ions, contrary to the usual ground‑state preparation of molecular ions. The study aims to clarify the mechanism of N₂⁺ population inversion by using few‑cycle laser pulses and showing that 391‑nm lasing occurs immediately after ionization. Numerical simulations reveal that post‑ionization coupling among the lowest three electronic states of N₂⁺ creates the population inversion. The findings resolve the debate on air lasing mechanisms and demonstrate that post‑ionization coupling among N₂⁺ states is a general atmospheric lasing mechanism.

Abstract

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 N 2 + 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 N 2 + is produced. Numerical simulations clarify that the population inversion is realized by the post-ionization couplings among the lowest three electronic states of N 2 + . 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.