The influence of propagating and evanescent waves on the focusing properties of zone plate structures.

The field properties of Fresnel zone plates with wavelength-scale focal distances were numerically investigated using the finite-difference time-domain method. The fields in the focal planes are analyzed using the angular spectrum representation, and the components of the propagating and evanescent waves are reconstructed. It was found that, in the focal plane of silver zone plates, there were more evanescent waves and the propagating waves occurred at higher spatial frequencies relative to glass zone plates. The propagating and evanescent wave components vary with the material and the number of zones in the zone plate structures. Our findings suggest that more evanescent waves and higher spatial frequency components of propagating waves can shape the field and obtain a smaller focus.