Rotational, Vibrational, and Electronic Modulations in N2+ Lasing at 391 nm: Evidence of Coherent BΣ2u+−XΣ2g+−AΠ2u Coupling

We investigate lasing of a N2 gas induced by intense few-cycle near-IR laser pulses. By the pump-probe measurements, we reveal that the intensity of the BΣ2u+−XΣ2g+ lasing emission of N2+ oscillates at high (0.3–0.5 PHz), medium (50–75 THz), and low (∼3 THz) frequencies, corresponding to the energy separations between the rovibrational levels of the AΠ2u and XΣ2g+ 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 N2+ creates the population inversion between the BΣ2u+ and XΣ2g+ states, resulting in the lasing at 391 nm.