Polariton picture of light propagation and storing in a tripod system

Light propagation and storing in a medium of atoms in the tripod configuration, driven by two control pulses are investigated. General situations are studied in which the releasing control pulses need not be replicas of those used at the storage stage and/or additional manipulations have been performed on a pair of atomic coherences, in which the pulse had been written. To completely describe the process we generalize the polariton description by modifying the concept of the dark-state polariton and by introducing a second polariton. The latter is a combination of two atomic coherences and gives account of a part of the signal field which remains trapped inside the medium after the release stage. We show how external manipulations at the storage stage can be taken into account in the polariton analysis and present some formulae which predict the shapes of the released and trapped parts of the pulse.