Theoretical analysis and design guideline for focusing subwavelength gratings

We propose a planar dielectric reflector with focusing ability using concentric circular subwavelength gratings (CC-SWGs). The two-dimensional focusing ability of CC-SWGs is investigated by the rigorous coupled-wave analysis (RCWA) and finite element method (FEM). By designing the concentric circular pattern of the grating surface, a focusing reflector with high numerical aperture (NA) and high reflectivity is constructed. A CC-SWG reflector with a diameter of 32.6μm and a focal length of 6μm is investigated, which exhibits high focusing ability at normal incidence with a radially polarized plane wave. At the reflection focal plane, the full-width-half-maximum (FWHM) of the electric field intensity is 0.89μm. Numerical aperture value as high as 0.93 is achieved for the reflector with very high reflectivity of 92%.