Fresnel Zone Plate (FZP)

Fresnel zone plates are diffractive optics composed of alternating transparent and opaque rings blocking the Fresnel zones (hence the name) of a light source. If the number of these rings is large enough, the FZP act as a simple lens and focuses a parallel beam (source at infinity) to an Airy pattern. This pattern can be used as probe in the same manner as any scanning probe microscopy. The STXM uses this exact configuration where a sample is scanned through the Airy pattern created by the FZP. The width of the outermost zone Δr determines the width of the Airy pattern and therefore, the resolution. We have used focused ion beam lithography (IBL) for robust fabrication of FZPs (see the Fig. 1) with Δr as small as 30 nm in direct milling mode. This is a very efficient way of prototyping FZPs optimized for different applications in essentially a single lithography step. We are also using FIB to machine multilayer type FZPs out of thick multilayer deposits on fibers.

Fig1: An example of a FZP. a) Overview, b) close-up image of the outermost zones and c) a cross-section thereof.