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The principle of achromatic holography for making a FZP. A mask containing two zone plates with focal lengths and is illuminated with spatially coherent EUV light from the left-hand side. At the image plane the diverging and converging diffraction orders from the two zone plates intersect at equal angles to form a zone plate pattern with half the zone width of the parent zone plates. The inner and outer radii of the parent zone plates can be chosen so that in a certain annular range at the image plane, the undesired diffraction orders are absent. A schematic frontal view of the mask is shown in the upper right-hand side.
Intensity distribution calculated with the method of stationary phases near the image plane for (a) monochromatic, and (b) wide spectrum illumination. The fringes are part of the elliptical curves corresponding to the constant optical path difference surfaces between the two sources located at the respective foci of the two parent zone plates. In this calculation we assumed the duty cycles of the two parent zone plates, and hence their diffraction efficiencies to be the same. The focal lengths and the dimensions of the parent zone plates were the same as the ones used in the experiment.
SEM images of the holographically made FZP after the recorded pattern was etched into the film and the support membrane. (a) Low-resolution image showing Moiré fringes in the annular range where the zone structures were formed. The Moiré fringes are formed between the SEM scan lines and the circular zone structures. The schematic inset indicates the area of the ring-shaped FZP where the SEM image was acquired. (b) Higher-resolution image showing etched zones with about zone width. The image was taken approximately in the middle of the annular range shown in (a).
Theoretically calculated and measured knife edge scans across the focused beam. A lithographically made open slit measuring in a thin, but opaque, membrane was used for this purpose. A photodiode behind the slit was used to measure the transmitted light intensity. The measured data was normalized but no background was removed. The calculated curve includes the broadening effect of the illuminating spectrum. For this test the monochromator in the beamline was set to provide a spectral bandwidth of .
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