X-ray holography with a customizable reference

Fourier-transform holography uses the interference between a known reference structure and the unknown object to directly and robustly recover a real-space image from a coherent diffraction pattern. It is a lensless technique, which makes it highly suitable for single-shot imaging with a free-electron laser (FEL) in order to study nanoscale samples, magnetic materials and transient phenomena. The simplest reference is a pinhole because it leads to the simplest analysis to recover the image. However, when using a pinhole resolution can only be improved at the expense of flux, decreasing signal with respect to noise. This has motivated research into extended references, which decouple flux and resolution, such as corner references or uniformly redundant arrays. To date, the choice of extended references has been limited to a few special cases where the analysis methods to recover the image are known.
This paper features a novel method gaining greater flexibility in the design of X-ray holographic experiments.

Figure 1.  (a) Diffraction pattern measured at 32 nm wavelength using FERMI-FEL radiation. (b) Reference modeled from a SEM image;the white rectangle represents the area assumed to contain the object (this area can be overestimated: the size of the object does not need to be known in advance). (c) Reconstructed holographic image of the object using the arbitrary reference waves generated by the structure in (b). (d) Refined object using conventional phasing algorithm seeded with the estimated reference structure. 

This advantage can be used to optimize the signal between the scattering waves from the reference object and the sample, improving the image resolution and fast convergence of phasing algorithms, which is desirable in several fields, running from magnetism, biology to nano-material characterization, and is particularly important for experiments with large datasets such as three-dimensional imaging of objects injected into FEL interaction region via aerosol or liquid. 
Using the scattering end-station DiProI at FERMI-FEL, a new Fourier-transform holography technique was demonstrated with an almost unrestricted choice for the reference as shown in Fig. 1. The flexibility is gained by using a conjugate-gradient algorithm to recover the image. Starting from the Fourier space experimental data (Fig. 1 (a)), such an algorithm considers the autocorrelation function of the exit wave decoupled into its primary components, i.e. the autocorrelation of the scattering object, the autocorrelation of the holographic reference mask (known a priori, Fig. 1(b)) and their cross-correlation terms. These latter contain the holographic information for the reconstruction of the object in real space.

Our demonstration was performed on platinum nanoscale structures deposited on Si3N4 windows and included references that surrounded the object. Fig. 1(c) shows an example of the reconstructed object obtained illuminating the sample with a FEL radiation at 32 nm. The image shows that the morphology of the surrounded object can be reconstructed with a resolution of about 170 nm. The image quality compares favorably with that of conventional phasing algorithms seeded with the assumed reference structure, Fig. 1(d). Notably, our generalized approach does not require references isolated from the object or sparsely located, so that we can more efficiently use the field-of-view. We tested such a novel approach using references with different shapes (open circles, “cartoon clouds”, sparse uniformly redundant arrays) and under different FEL conditions  (single and multi-shots) proving the robustness of the method. This opens up new possibilities for optimizing the signal and resolution, which can aid the development of holographic techniques to meet the demands of resolution and fidelity required for single-shot imaging applications.

This research was conducted by the following research team:

A. V. Martin1, A. J. D’Alfonso2, F. Wang3, R. Bean3, F. Capotondi4, R. A. Kirian3, E. Pedersoli4, L. Raimondi4, F. Stellato3, C. H. Yoon3,5 and H. N. Chapman3,6

1 ARC Centre of Excellence for Coherent X-ray Science, School of Physics, The University of Melbourne, Melbourne, Australia.
2 School of Physics, University of Melbourne, Melbourne, Australia.
3 Center for Free-Electron Laser Science, DESY, Hamburg, Germany.
4 Elettra-Sincrotrone Trieste, Trieste, Italy.
5 European XFEL GmbH, Hamburg, Germany.
6 University of Hamburg, Hamburg, Germany 

Contact person:

Andrew Martin: andrew.martin@unimelb.edu.au


Andrew V. Martin, Adrian J. D’Alfonso, Fenglin Wang, Richard Bean, Flavio Capotondi, Richard A. Kirian, Emanuele Pedersoli, Lorenzo Raimondi, Francesco Stellato, Chun Hong Yoon and Henry N. Chapman. “X-ray holography with a customizable reference”. Nat. Commun. 5, 4661 (2014); DOI: 10.1038/ncomms5661  


Last Updated on Friday, 24 October 2014 13:40