One-way diffraction effects in photonic crystal gratings made of isotropic materials

The anomalous diffraction effects arising in the intrinsically isotropic photonic crystal gratings with different periods of the front-side and back-side interfaces are studied with the emphasis put on wideband one-way transmission. It is demonstrated that more than 80 percent of the incident-wave energy can be transmitted unidirectionally, i.e., with zero transmission in the opposite direction, by changing side of illumination only, while polarization always remains linear. Most but not all of the related diffraction effects can be predicted using isofrequency contours (IFCs) and wave-vector diagrams, which take into account the IFC-shape-dependent additional transmission channels that can appear due to corrugations. Three main regimes, i.e., isolation and bidirectional and unidirectional translation, can be distinguished, depending on whether the front-side corrugations affect the far field in the half space bounded by the back-side interface, and whether transmission is vanishing only at illumination from the side of one of the interfaces. The conditions providing the existence of these regimes are given and discussed in terms of the features of wave-vector diagrams. In some cases, similar regimes can appear being in contradiction with these conditions. Variation in the angle of incidence and the angle between the characteristic directions of photonic crystal and the interfaces of the corresponding noncorrugated structure allows one extending variety of situations, in which unidirectional transmission can be realized. The general condition of unidirectionality is that the zero diffraction order is not coupled to a Floquet-Bloch wave.