Mechanism and control of rotational coherence in femtosecond laser-driven N2

We investigate the formation of rotational coherence of N 2 + resonantly interacting with an intense femtosecond laser field by numerical simulations based on a strong-field ionization-coupling model described with the density matrix formalism. The created N 2 + system is unique in many aspects: the variable total population within the pump duration due to the intensity-dependent ionization injection, the readily accessible resonance owing to the effect of Stark shift, and the involvement of a few dozen of quantum states. By regarding the N 2 + system as an open and non-stationary Λ-type cascaded multi-level system, we quantitatively studied the dependence of rotational coherence in different electronic-vibrational states of N 2 + on the alignment angle θ and the pumping intensity. Our simulation results indicate that the quantum coherence between the neighbouring rotational states of J , J +2 in the vibrational state ν =0, 1 of the ground state of N 2 + can be changed from a negative to a positive. The significant contribution of rotational coherence to inducing an extra gain or absorption of N 2 + air lasing is further verified by solving the Maxwell’s propagating equation. The finding provides crucial clues on how to manipulate N 2 + lasing by controlling the rotational coherence and paves the way to studying strong-field quantum optics effects such as lasing without inversion and electromagnetically induced transparency in molecular ionic systems.