Special kind of photonic crystals with omnidirectional bandgaps

General conditions for obtaining omnidirectional bandgaps (OBG) were derived based on consideration of the permeability of the material. It was found that the OBG is a universal property of a special kind of photonic crystal (PC) comprising two alternate layers with different wave impedances and with the same product of permittivity and permeability (not only the refractive index). Moreover, for this kind of PC, the OBG can be broadened by decreasing the refractive index of the ambient medium and/or increasing the ratio between the wave impedances of the components of the PC. (c) 2006 Optical Society of America.