Propagation of short laser pulses in a medium of Lambda-atoms in the intuitive sequence and under conditions of adiabatic following is studied. The regime of superluminal propagation is obtained and the conditions of efficient population transfer in the medium are analyzed.
We study the propagation of frequency-chirped laser pulses in optically thick media. We consider a medium of atoms with a $\ensuremath{\Lambda}$ level-scheme (Lambda atoms) and also, for comparison, a medium of two-level atoms. Frequency-chirped laser pulses that induce adiabatic population transfer between the atomic levels are considered. They induce transitions between the two lower (metastable) levels of the $\ensuremath{\Lambda}$-atoms and between the ground and excited states of the two-level atoms. We show that associated with this adiabatic population transfer in $\ensuremath{\Lambda}$-atoms, there is a regime of enhanced transparency of the medium---the pulses are distorted much less than in the medium of two-level atoms and retain their ability to transfer the atomic population much longer during propagation.
In the present paper, we examine the propagation of frequency-chirped laser pulses through a medium of Lambda atoms, pulses that are capable of producing adiabatic population transfer between the stable states. We compare the propagation of these pulses to their propagation in a medium of two-level atoms, and show that the distortion of the pulses is greatly reduced in the three-level case. In effect, there exists a transparency phenomenon for frequency-chirped laser pulses propagating in the three-level medium that is somewhat similar to electromagnetically induced transparency. It is concluded that the population transfer induced by chirped pulses does populate the excited state during the process to a slight extent, even if the transfer is perfectly adiabatic. Thus the transparency of the medium is also imperfect, the pulse is eventually distorted and its capability to produce the population transfer is lost, even in the perfectly adiabatic limit.
Two approaches of achieving population transfer and coherence are investigated for the three-level Λ system in the adiabatic limit. The effects of the laser pulse sequence on the population transfer efficiency and coherence are studied. Coherent control of quantum state and population is studied by numerical simulation based on self-consistent set of density matrix equations. It can be seen that the counterintuitive pulse sequence is more efficient in population transfer and coherence than the intuitive one.