Influence of optical band structures on the diffraction of photonic colloidal crystals

We have performed optical diffraction studies on colloidal crystals with large refractive index mismatches up to 1.45 and polarizibilities per volume as large as 0.6. These conditions push colloidal crystals into the regime where strong coupling between photonic crystals and the light field occurs. It is found that the photonic band structures result in apparent Bragg spacings that strongly depend on the wavelength of light. The dynamical diffraction theory that correctly describes weak photonic effects encountered in X-ray diffraction, also breaks down. Two simple models are presented that give a much better description of the diffraction of photonic crystals.