Experiment and Calculations on the Defect-Induced Broadening in One-Dimensional Photonic Bandgaps

Impurity bands are generated by the coupling of defect modes in one-dimensional photonic crystals, which move the bandgap edges towards the shorter and the longer wavelengths. Combining one structure with impurity band and another without it, a greatly broadened photonic bandgap is created when the two structures are well designed. The samples are fabricated by the electron-beam evaporation, which are composed of SiO$_2$ and TiO$_2$. A good agreement between the experimental and the theoretical results is obtained. The further calculations, based on the 4$\times$4 transfer-matrix method, strongly reveal the validity of this approach, so-called the defect-induced broadening. More importantly, the widened omni-directional photonic bandgaps are also realized by this means.