Coupled-mode theory for spatial gap solitons in optically-induced   lattices
  We develop a coupled-mode theory for spatial gap solitons in the one-dimensional photonic lattices induced by interfering optical beams in a nonlinear photorefractive crystal. We derive a novel system of coupled-mode equations for two counter-propagating probe waves, and find its analytical solutions for stationary gap solitons. We also predict the existence of moving (or tilted) gap solitons and study numerically soliton collisions. 
Boris A. Malomed, Elena A. Ostrovskaya, and Yuri S. Kivshar