Omnidirectional zero-n̄ gap in symmetrical Fibonacci sequences composed of positive and negative refractive index materials

The band structures of symmetrical Fibonacci sequences (SFS) composed of positive and negative refractive index materials are studied with a transfer matrix method. A new type of omnidirectional zero-n̄ gaps is found in the SFS. In contrast to the Bragg gaps, such an omnidirectional zero-n̄ gap is insensitive to the incident angles and polarization, and is invariant upon the change of the ratio of the thicknesses of two media. It is found that omnidirectional zero-n̄ gap exists in all the SFS, and it is rather stable and independence of the structure sequence.