Hybrid two-dimensional subwavelength surface-relief grating-mesh structures.

The homogeneous behavior of periodic two-dimensional subwavelength surface-relief structures that contain both gratings and meshes (inverse gratings) are investigated. It is shown that when effective indices are synthesized near the higher index (substrate region), mesh structures yield larger feature sizes compared with their grating counterparts, whereas grating structures yield larger feature sizes when effective indices are synthesized near the lower index (incident region). For each type of structure investigated, a relation between the parameters of the structure and an effective refractive index is determined. It is shown that an equal area occupied by the high- or low-index media within the grating cell does not, in general, result in equal effective indices. The effective index of the grating is shown to be characterized by both the shape (local distribution) and the area of the high- or low-index medium within the unit grating cell. Finally, the advantages of subwavelength gratings and meshes are combined to produce hybrid grating-mesh structures that are less demanding on the fabrication process.