We consider a duplicated two-level system where each two-level subsystem is driven by a strong resonant femtosecond pulse and a weak resonant femtosecond pulse connects cross transitions. Strong interference effects are induced in such a configuration leading to a modulation of the medium gain as a function of the relative phase phi between the pulses [J. C. Delagnes and M. A. Bouchene, Phys. Rev. Lett. 98, 053602 (2007)]. We study here the influence of the temporal overlap between the pulses on the dynamics of the system and the medium gain. We show that the dynamics is dominated by two competing phenomena : Light shifts (LS) induced by the driving pulse that prevails for small delays and free induction decay (FID) that prevails for large delays. LS enhance the medium gain for 
and reduce it for 
, whereas FID always leads to a decrease of the medium gain.
Voir en ligne : Phys. Rev. A 76, 053809 (2007)