Optical Gap Solitons in Nonresonant Quadratic Media
  We demonstrate an important role of the process of optical rectification in the theory of nonlinear wave propagation in quadratically nonlinear [or $\chi^{(2)}$ ] periodic optical media. We derive a novel physical model for gap solitons in $\chi^{(2)}$ nonlinear Bragg gratings. 
Takeshi Iizuka and Yuri Kivshar