Large enhancement of third-harmonic generation induced by coupled gap solitons in chi(3) nonlinear photonic crystals.

It is shown that, in a chi((3)) nonlinear photonic crystal, the third-harmonic conversion efficiency can be enhanced by about three orders of magnitude via the formation of self-organized localized states inside the gaps as compared with that in a bulk medium of the same length with a perfect phase-matching condition. These localized states contain both the fundamental and third-harmonic frequencies located in different gaps. Their existence is a result of both the Kerr effect and the large energy transfer between fundamental and third-harmonic waves.