Comparative modeling of amplitude and phase zone plates

In this paper using FDTD-method, we conduct a comparative modeling of a sharp focus of the laser light by using binary-phase and binary-amplitude Fresnel zone plates, with the latter being fabricated as thin metal films of 20-30-nm thickness. Subwavelength local maximums are found near the rings of the silver zone plates, with their intensity being comparable with the intensity in the focus. They can be explained by the excitation of surface plasmons.