Generation of two colors echos in a coherently prepared Λ-configuration medium

In an ensemble of incoherently prepared system of Λ-atoms, no photon echo is generated in either excitation channels if the system is excited by two pulses having their center frequency resonant with the first and second excitation channels, respectively. We show, on the other hand, that, for an initially prepared coherent mixture of the lower states, echos in the mixed excitations configuration can, in general, be generated in both channels. We compute the intensities of these echos as function of the sample's length, the pulses area, and the coherence mixing angle and compare them to the echos resulting from the same channel excitations configuration for both coherently and incoherently prepared systems.