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

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.Multi-waveguide interference can be verified experimentally by reconstructing the near-field from the measured far-field diffraction pattern. This enables a direct visualization of the near-field interference pattern and the diversity of fields that can be created by multi-waveguide design, in particular a secondary quasi-focal spot. Numerical propagation using the design parameters is compared with the phase retrieval results.