Electromagnetically induced transparency under phase modulation of interacting radiations

We report the results of an analytical and numerical analysis of the evolution of nanosecond probe pulses in the case of electromagnetically induced transparency in the Λ-scheme of degenerate quantum transitions. It is assumed that the probe and control fields at the active medium input are elliptically polarised, and one of them is phase modulated. It is shown analytically that probe radiation in a medium is the sum of two normal modes propagating independently of each other. Because the group velocities of normal mode pulses are different, a single probe pulse entering the medium decays inside the medium into separate pulses, each of which transfers the energy of one of the normal modes. Numerical simulations have shown that with a sufficiently large phase modulation, normal modes, whose intensity on the input surface is described by a bell-shaped curve, turn into trains of subpulses in the medium. Phase modulation of the input field has practically no effect on the group propagation velocities of normal modes, but it reduces the transparency of the medium for probe radiation.