Broad Perfect Transparent Band in Asymmetric Photonic Crystals with Graded-Index Films.

Photonic crystals possessing mirror symmetry have been widely investigated to obtain perfect transmission properties. However, complex asymmetric nanostructures can achieve perfect transmission via phase-matching, while one-dimensional photonic crystals exhibit pronounced angle- and polarization-dependent transmission characteristics under oblique incidence. In this study, we systematically investigated the optical properties of one-dimensional asymmetric photonic crystals containing graded-index films, which yielded a broad perfect transparent photonic band that differed from those occurring in one-dimensional asymmetric photonic crystals comprising solid-index films. The transparent band redshifted, and its bandwidth decreased with increasing amplitude of the graded index. Moreover, the broad perfect transparent band obtained over a wide range of incident angles exhibited greater insensitivity to the incident angle for transverse magnetic polarization than for transverse electric polarization. These results provide a theoretical foundation for designing perfect broadband optical devices using photonic crystals with graded-index films.