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 Lambda-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 theta 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 nu=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. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement