Optical nonlinearity and feedback through Bragg periodicity are the basic ingredients for light localization into gap solitons. We review the basic concepts and model equations for gap solitons in Kerr and quadratic nonlinear media encompassing a one-dimensional Bragg resonance. With specific regard to frequency doubling media, we generalize the concept of a photonic crystal to band-gaps of a nonlinear origin, and finally address the slow character of quadratic gap-solitons with reference to their excitation.
Conti, C., Assanto, G., Trillo, S. (2002). Gap solitons and slow light. JOURNAL OF NONLINEAR OPTICAL PHYSICS AND MATERIALS, 11(3), 239-259 [10.1142/S0218863502001000].