Nonlinear diffraction and second-harmonic generation enhancement in silicon-opal photonic crystals

Nonlinear diffraction in three-dimensional silicon-filled photonic crystals of opals is studied. Efficient backward second-harmonic generation (SHG) is observed in the specular direction upon the fundamental radiation reflection from the (111) face of the face-centered-cubic opal lattice. Tuning the fundamental wavelength across the photonic band gap (PBG) results in the 20-times increase of the second-harmonic intensity. The SHG peak has the width of approximately 20nm and is located at the long-wavelength edge of the PBG.