Influence of wave-packet dynamics on the medium gain of an atomic system

A sequence of two femtosecond pulses-a strong driving pi-polarized pulse and a weak propagating sigma-polarized pulse-excites resonantly the S-1/2 -> P-1/2 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 P-3/2 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).