A sequence of two femtosecond pulses - a strong driving 
-polarized pulse and a weak propagating 
-polarized pulse - excites resonantly the 
transition of an atomic system. Strong interference effects take place in the system between absorption and emission paths leading to a substantial amplification of the sigma pulse. We study the influence of the fine structure on the medium gain when the contribution of the off-resonant 
level is taken into account. A drastic reduction of the medium gain is obtained. This effect is explained within the bright-state–dark-state formalism where the strong driving pulse creates a wave packet that can be trapped in a state - the bright state - leading to a significant reduction of the gain for the sigma pulse. Finally, we also show that periodical gain dependence with the driving pulse energy exhibits a significant change in its period value (compared with expected Rabi oscillations).
Voir en ligne : Phys. Rev. A 76, 045805 (2007)