Effect of the shadowing in high-numerical-aperture binary phase Fresnel zone plates

Shadowing occurs for the groove zone plate when radiation origin from one zone of the zone plate must pass through another before reaching the focus point. Using the vector diffraction theory, we study the influence of the shadowing on the focusing properties of binary phase Fresnel zone plates (FZPs). The results show that a low-numerical-aperture (NA) FZP can be treated as an ideal in-plane FZP and the shadowing effect can be neglected. For a high-NA FZP, however, the shadowing effect from the etch depth has to be considered. Owing to the shadowing, the intensity of focusing spot decreases markedly and the size of focusing spot increases slightly in a high-NA FZP. The optimal etching depth of an actual FZP is smaller than that of an ideal FZP.