Ultraslow stationary pulses in a periodically modulated medium with an active nonlinearity

The paper examines the stationary distribution of electromagnetic waves in a periodically modulated medium with a cubic active nonlinearity, i.e., a medium in which the amplification (or absorption) depends on the light intensity. The active medium is produced by the introduction of two-level absorbing and four-level amplifying particles, pumped by an external source. It is shown that in this type of medium there can exist a stationary solution in the form of a solitary pulse, propagating in the modulation direction with a velocity much less than the velocity of light.