Population inversion and transfer in N₂⁺ air laser

Meng-Cheng Xie^1,2*, YU-HUNG KUAN¹, WEN-TE LIAO¹, CHENGYIN WU³, ZHENG LI³

¹Department of Physics, National Central University, Taoyuan, Taiwan
²College of Science Joint Science Program, National Central Univercity, Taoyuan, Taiwan
³State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing, China

* Presenter:Meng-Cheng Xie, email:jjes40511@gmail.com

Motivated by the hot debate on the mechanism of the laserlike emission at 391nm and 428nm from N₂ gas irradiated by a strong 800nm pump laser and a weak 400nm seed laser, we theoretically study the temporal profile, optical gain and periodic modulation of the 391nm and 428nm signal from N₂⁺ . Our calculation sheds light on the long standing controversy on whether population inversion is indispensable for the optical gain, and demonstrate the Ramsey fringes of the emission intensity at 391nm and 428nm formed by additionally injecting another 800nm pump or 400nm seed, which provides strong evidence for the coherence driven modulation of transition dipole moment and population transfer between the A²Π_u(ν = 2)-X²Σ_g⁺ states and the B²Σ_u⁺ (ν = 0)- X²Σ_g⁺ states. Our results show that the 391nm and 428nm optical gain is susceptible to the degree of population inversion within N₂⁺ states manipulated by the Ramsey technique, and thus clearly reveal their symbiosis. This study provides with not only the physical picture of producing population inversion in N₂⁺ but also versatile methods for coherent control of N₂⁺ air laser [1].
[1] to be published in Opt. Lett. arXiv:2006.08200 (2020).

Keywords: air laser