By using only one pump beam, we show theoretically that both forward and backward phase conjugations can be generated simultaneously and efficiently in one-dimensional chi~(3)-nonlinear photonic crystals of only a few micrometers in size. The photonic crystal considered here is a CdS/SiO~2 superlattice. By using realistic optical parameters in our calculations, we find that the phase-conjugate generation efficiency can be enhanced by four to five orders of magnitude as compared with that in a homogeneous CdS medium of the same nonlinearity and length. The enhancement is caused by multiple reflections as well as strong field localization introduced by the shifted band-edge state, the gap localized state, or the shifted defect state.
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