Propagation of frequency-chirped laser pulses in a medium of atoms with a Λ-level scheme

We study the propagation of frequency-chirped laser pulses in optically thick media. We consider a medium of atoms with a 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 Lambda-atoms and between the ground and excited states of the two-level atoms. We show that associated with this adiabatic population transfer in 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.