Noncollinear generation of second and third harmonics in three-dimensional photonic crystals of artificial opals is experimentally realized and interpreted in terms of nonlinear diffraction. A factor-of-two enhancement of second-and third-harmonic generation intensity in diffraction maxima directions is observed due to effective phase matching provided by photonic crystal periodicity. Bidirectional diffraction of third harmonics at crystallographic planes ð111Þ and ð 1 11Þ is observed. The broadening of nonspecular diffraction peaks is found to correspond to the polycrystalline structure of the opal samples.

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