Reconstruction of the near-field distribution in an X-ray waveguide array.

Qi Zhong,Jichang Peng,Zhanshan Wang,Lars Melchior,Tim Salditt,Qiushi Huang

doi:10.1107/s1600576717004630

Qi Zhong, Jichang Peng + Show 4 more

Open Access

https://doi.org/10.1107/s1600576717004630

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Abstract

Iterative phase retrieval has been used to reconstruct the near-field distribution behind tailored X-ray waveguide arrays, by inversion of the measured far-field pattern recorded under fully coherent conditions. It is thereby shown that multi-waveguide interference can be exploited to control the near-field distribution behind the waveguide exit. This can, for example, serve to create a secondary quasi-focal spot outside the waveguide structure. For this proof of concept, an array of seven planar Ni/C waveguides are used, with precisely varied guiding layer thickness and cladding layer thickness, as fabricated by high-precision magnetron sputtering systems. The controlled thickness variations in the range of 0.2 nm results in a desired phase shift of the different waveguide beams. Two kinds of samples, a one-dimensional waveguide array and periodic waveguide multilayers, were fabricated, each consisting of seven C layers as guiding layers and eight Ni layers as cladding layers. These are shown to yield distinctly different near-field patterns.

Highlights

X-ray waveguides (WGs) enable manipulation of X-ray fields at the nanoscale, based on the optics of guide modes
To illustrate the specific field modulating effects which can be achieved by a systematic variation of the waveguide width di for each waveguide i, we have investigated the near field of two different kinds of waveguide structures, namely the aforementioned waveguide array (WGA) and – for comparison – a simpler periodic waveguide multilayer (WGM)
Compared to the Mo/C WGA considered in our earlier report (Zhong et al, 2017), where we presented simulations with a quasi-focal spot of FWHM 37.2 nm, located 180.0 mm behind the exit, the Ni/C WGA used in the present work exhibits a higher numerical aperture and a more desirable near-field distribution owing to the variations in cladding layer thickness cj, yielding a focal spot at 224.6 mm behind the device, with an FWHM of 22.0 nm

Summary

Introduction

X-ray waveguides (WGs) enable manipulation of X-ray fields at the nanoscale, based on the optics of guide modes. The fabrication of 2DWGs was improved by Fuhse & Salditt (2005) and more recently extended from overgrown polymer channels to air channels capped by wafer bonding techniques (Neubauer et al, 2014; Chen et al, 2015) In this form 2DWGs serve as fully operational secondary sources for holographic imaging (Bartels et al, 2015). For purposes of highest beam confinement or to exploit novel geometries, wave guiding in only one dimension as in thin planar films is suitable, owing to the better control of layer sequences that this allows In this way the theoretical limits for beam collimation (Bergemann et al, 2003), notably 8 nm for the given material, could be reached in a planar thin-film waveguide with an optimized cladding material (Mo/C/Mo embedded in Ge; Kruger et al, 2010, 2012)

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