Analysis of photonic bandgap structure for a polaritonic photonic crystal in negative-index region

A photonic bandgap (PBG) structure for a polaritonic photonic crystal (PPC) containing lithium tantalate (LiTaO₃) in the negative refractive index (NRI) region has been theoretically investigated. This region consists of a narrow frequency range of anomalous dispersion and a wide range of normal dispersion. The result shows that such PPC has a multiple-PBG structure with one gap located in the anomalous dispersion region and others in the normal dispersion region. PBG in the anomalous region is invariant with filling factor and scaling. This gap is further proven to be independent of the angle of incidence for both TE and TM nodes, leading to the existence of an omnidirectional gap. BPGs within the normal dispersion region are, however, strongly affected by the above three factors, especially for gaps near the upper limit of the NRI region.