Optical Solitons in Periodic Media with Resonant and Off-Resonant   Nonlinearities
  The properties of optical solitons in periodic nonlinear media are reviewed. The emphasis is on solitons in periodically refractive media (Bragg gratings) incorporating a periodic set of thin layers of two-level systems resonantly interacting with the field. Such media support a variety of bright and dark `gap solitons' propagating in the band gaps of the Bragg gratings, as well as their multi - dimensional analogs (light bullets). These novel gap solitons differ substantially from their counterparts in periodic media with either cubic or quadratic off-resonant nonlinearities. 
G. Kurizki, A.E. Kozhekin, T. Opatrny and B. Malomed