Multiple Bragg wave coupling in photonic band-gap crystals

We report angle and polarization resolved reflectivity from strongly photon ic fee crystals consisting of air spheres in titania. For a 30 degrees rang e of angles of incidence, multiple peaks with distinct polarization depende nce are observed that clearly reveal avoided crossing behavior. Calculated photonic dispersion curves show that the multiple peaks result from band re pulsion of Bloch states due to simultaneous Bragg diffraction by (Ill)and ( 200) planes. Our results demonstrate that many-wave coupling leads to subst antial deviations from simple Bragg diffraction, with significantly flatten ed bands, a requirement for the appearance of complete photonic band gaps.