A comparative study of colloidal silica spheres: Photonic crystals versus Bragg's law

A comparative study of colloidal crystals made of silica spheres using Bragg's law and a model including a photonic band gap (PBG) was demonstrated. Optical properties of the crystals annealed at various temperatures were characterized by a procedure similar to X-ray diffraction technique. Experimentally, the PBG obtained from the transmission spectra agrees with the model of the photonic bands using two parameters, sphere size and the effective index. This procedure gives a better description than the traditional way using just the Bragg's expression without a band gap, commonly referred to as Zone-folding.