We investigate lasing of a nitrogen gas induced by intense femtosecond laser pulses at around 400 nm. By examining both the self-induced and externally seeded forward emission spectra, we unambiguously identify the ${{\mathrm{N}}_2}^{+}$ lasing actions at 427.8 and 423.6 nm assigned respectively to the $B{}^2\mathrm{\Sigma }{{}_u}^{+}-X{}^2\mathrm{\Sigma }_g^{+}$ (0, 1) and (1, 2) emissions and show that the lasing mechanism is totally different from the lasing induced by near-infrared (800 nm) laser pulses, in which the population transfer from $X{}^2\mathrm{\Sigma }_g^{+}$ to $A{}^2\mathrm{\Pi }_u$ proceeds through the resonant $X{}^2\mathrm{\Sigma }_g^{+}-A{}^2\mathrm{\Pi }_u$ transition, facilitating the population inversion between the $B{}^2\mathrm{\Sigma }{{}_u}^{+}-X{}^2\mathrm{\Sigma }_g^{+}$ states [Phys. Rev. Lett. 123, 203201 (2019)]. We simulate the population distributions among the vibrational levels in the three lowest electronic states of ${{\mathrm{N}}_2}^{+}$ by the 400-nm laser pumping and find that the population is efficiently transferred between the $X{}^2\mathrm{\Sigma }_g^{+}$ state and the $B{}^2\mathrm{\Sigma }{{}_u}^{+}$ state by a Rabi oscillation combined with a Raman-type transition, leading to the population inversion so that the lasing occurs at the $B{}^2\mathrm{\Sigma }{{}_u}^{+}-X{}^2\mathrm{\Sigma }_g^{+}$ (0, 1) and (1, 2) emissions.

中文翻译：

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通过在 400 nm 振动介导的 Rabi 振荡在 N2+ 中进行种群反转

我们研究了由大约 400 nm 的强飞秒激光脉冲引起的氮气激光。通过检查自诱导和外部种子前向发射光谱，我们明确地确定了${{\mathrm{N}}_2}^{+}$ 427.8 和 423.6 nm 处的激光作用分别分配给 $乙{}^2\mathrm{\Sigma }{{}_{你}}^{+}-X{}^2\mathrm{\Sigma }_G^{+}$ (0, 1) 和 (1, 2) 发射并表明激光发射机制与近红外 (800 nm) 激光脉冲引起的激光发射完全不同，其中粒子数从 $X{}^2\mathrm{\Sigma }_G^{+}$ 到 $\mathrm{一种}{}^2\mathrm{\Pi }_{你}$ 通过谐振进行 $X{}^2\mathrm{\Sigma }_G^{+}-\mathrm{一种}{}^2\mathrm{\Pi }_{你}$ 过渡，促进人口倒置 $乙{}^2\mathrm{\Sigma }{{}_{你}}^{+}-X{}^2\mathrm{\Sigma }_G^{+}$状态[物理。牧师莱特。 123 , 203201 (2019)]。我们模拟了三个最低电子态的振动能级之间的种群分布${{\mathrm{N}}_2}^{+}$ 通过 400 nm 激光泵浦，发现种群在 $X{}^2\mathrm{\Sigma }_G^{+}$ 国家和 $乙{}^2\mathrm{\Sigma }{{}_{你}}^{+}$ 通过拉比振荡结合拉曼型跃迁，导致粒子数反转，从而使激光发生在 $乙{}^2\mathrm{\Sigma }{{}_{你}}^{+}-X{}^2\mathrm{\Sigma }_G^{+}$ (0, 1) 和 (1, 2) 排放。