Abstract
In X-ray holographic near-field imaging the resolution and image quality depend sensitively on the beam. Artifacts are often encountered due to the strong focusing required to reach high resolution. Here, two schemes for reconstructing the complex-valued and extended wavefront of X-ray nano-probes, primarily in the planes relevant for imaging (i.e. focus, sample and detection plane), are presented and compared. Firstly, near-field ptychography is used, based on scanning a test pattern laterally as well as longitudinally along the optical axis. Secondly, any test pattern is dispensed of and the wavefront reconstructed only from data recorded for different longitudinal translations of the detector. For this purpose, an optimized multi-plane projection algorithm is presented, which can cope with the numerically very challenging setting of a divergent wavefront emanating from a hard X-ray nanoprobe. The results of both schemes are in very good agreement. The probe retrieval can be used as a tool for optics alignment, in particular at X-ray nanoprobe beamlines. Combining probe retrieval and object reconstruction is also shown to improve the image quality of holographic near-field imaging.
Highlights
Preparation of the X-ray probe for coherent imaging applications is indispensable in order to reach high resolution and quantitative contrast
For the last few years, X-ray nano-focus optics have been characterized by far-field ptychographic means, scanning an object in or near the focal plane. While this is correct in principle, we show in this study that the small distortions in the probe which significantly hamper the image quality of full-field imaging can only be properly ‘probed’ in the defocus plane
Note that most alternative phase-retrieval algorithms in the near-field setting, which exploit longitudinal scanning such as the contrast transfer function reconstruction (Cloetens et al, 1999) or the transport of intensity equation (Gureyev & Nugent, 1997; Krenkel et al, 2013), rely on assumptions of the wavefield under reconstruction, which limit the range of their applicability. None of these restrictions apply to the multiple magnitude projections (MMP) or near-field ptychography (NFP) schemes
Summary
Preparation of the X-ray probe for coherent imaging applications is indispensable in order to reach high resolution and quantitative contrast. Note that most alternative phase-retrieval algorithms in the near-field setting, which exploit longitudinal scanning (diversity) such as the contrast transfer function reconstruction (Cloetens et al, 1999) or the transport of intensity equation (Gureyev & Nugent, 1997; Krenkel et al, 2013), rely on assumptions (pure phase object, slowly varying phase, linearity of the propagation) of the wavefield under reconstruction, which limit the range of their applicability. None of these restrictions apply to the MMP or NFP schemes. We close the paper in x5 with some practical considerations on how the presented methods can be used for nano-focus optimization and alignment
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