We investigate lasing of a ${\mathrm{N}}_2$ gas induced by intense few-cycle near-IR laser pulses. By the pump-probe measurements, we reveal that the intensity of the $B{{}^2\mathrm{\Sigma }}_u^{+}-X{{}^2\mathrm{\Sigma }}_g^{+}$ lasing emission of ${\mathrm{N}}_2^{+}$ oscillates at high (0.3–0.5 PHz), medium (50–75 THz), and low ($\sim 3\mathrm{THz}$) frequencies, corresponding to the energy separations between the rovibrational levels of the $A{{}^2\mathrm{\Pi }}_u$ and $X{{}^2\mathrm{\Sigma }}_g^{+}$ states. By solving the time-dependent Schrödinger equation, we reproduce the oscillations in the three different frequency ranges and show that the coherent population transfer among the three electronic states of ${\mathrm{N}}_2^{+}$ creates the population inversion between the $B{{}^2\mathrm{\Sigma }}_u^{+}$ and $X{{}^2\mathrm{\Sigma }}_g^{+}$ states, resulting in the lasing at 391 nm.

• Received 13 May 2019
• Revised 23 August 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.203201