Abstract
A novel type of zone plate (ZP), termed an inverse-phase composite ZP, is proposed to gain a deeper focus than the standard diffraction-limited depth of focus, with little reduction in spatial resolution. The structure is a combination of an inner ZP functioning as a conventional phase ZP and an outer ZP functioning with third-order diffraction with opposite phase to the inner ZP. Two-dimensional complex amplitude distributions neighboring the focal point were calculated using a wave-optical approach of diffraction integration with a monochromatic plane-wave illumination, where one dimension is the radial direction and the other dimension is the optical-axis direction. The depth of focus and the spatial resolution were examined as the main focusing properties. Two characteristic promising cases regarding the depth of focus were found: a pit-intensity focus with the deepest depth of focus, and a flat-intensity focus with deeper depth of focus than usual ZPs. It was found that twice the depth of focus could be expected with little reduction in the spatial resolution for 10 keV X-ray energy, tantalum zone material, 84 nm minimum fabrication zone width, and zone thickness of 2.645 µm. It was also found that the depth of focus and the spatial resolution were almost unchanged in the photon energy range from 8 to 12 keV. The inverse-phase composite ZP has high potential for use in analysis of practical thick samples in X-ray microbeam applications.
Highlights
Fresnel zone plates (ZPs) are major optical elements in X-ray microscopes
In order to defeat the diffraction limit restricting the relation between spatial resolution and depth of focus, an inversephase composite zone plate is proposed
The structure is a combination of an inner ZP (iZP) functioning as a conventional phase zone plate and an oZP functioning with third-order diffraction with opposite phase to the iZP
Summary
Fresnel zone plates (ZPs) are major optical elements in X-ray microscopes. There are several derivations such as sputteredsliced ZPs (Rudolph et al, 1982; Koyama et al, 2012), multilayer Laue lenses (Maser et al, 2004; Koyama et al, 2008) and total reflection ZPs (Takano et al, 2010). There are other types of optical elements such as grazing-incidence total reflection mirrors of Kirkpatrick–Baez mirrors (Mimura et al, 2007) and Wolter type I mirrors (Aoki et al, 1992), compound refractive lenses (Schroer et al, 2005) and Bragg–Fresnel lenses (Erko et al, 1994). The development of these optical elements has made sub-100 nm-spatial resolution available in X-ray microscopes. 26, 52–58 research papers determined by NA such that DoF / 1/NA2, improved spatial resolution and deeper DoF are incompatible This can be interpreted as an alternative expression of the diffraction limit. It should be noted here that another way to produce a blurred focal spot with an extended DoF has been recently reported using multiple zone plate stacking with misalignment along the optical axis (Li & Jacobsen, 2018)
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