Monolithic focused reference beam X-ray holography

Abstract

Fourier transform holography is a highly efficient and robust imaging method, suitable for single-shot imaging at coherent X-ray sources. In its common implementation, the image contrast is limited by the reference signal generated by a small pinhole aperture. Increased pinhole diameters improve the signal, whereas the resolution is diminished. Here we report a new concept to decouple the spatial resolution from the image contrast by employing a Fresnel zone plate to provide the reference beam. Superimposed on-axis images of distinct foci are separated with a novel algorithm. Our method is insensitive to mechanical drift or vibrations and allows for long integration times common at low-flux facilities like high harmonic generation sources. The application of monolithic focused reference beams improves the efficiency of high-resolution X-ray Fourier transform holography beyond all present approaches and paves the path towards sub-10 nm single-shot X-ray imaging.

X-ray holography with a customizable reference

“…In fact, if an accurate characterization of the reference wave is available, the ILRUFT method can be tailored to best use this information, potentially exceeding the performance of other methods. For example, the ILRUFT method could account for the aberrations of a Fresnel-zone plate reference 13 . It should be noted that other extended references, such as corner or URA references, impose requirements on the uniformity of reference composition and thickness.…”

“…However, adjacent pinholes must be sufficiently separated to produce non-overlapping holograms and consequently the field-of-view is used inefficiently. Alternatively, flux and resolution can be decoupled by using extended references 1,[11][12][13] . The image-reconstruction procedure for extended references involves the application of linear operators, such as convolution 1 , linear filters 11,12 or propagation 13 .…”

“…However, there are only a handful of special cases for which these reconstruction procedures are known. Notwithstanding the specificity of each approach, the aforementioned methods still leave something to be desired as Fresnel-zone plate references have missing information at low frequencies 13 and sharp corner references improve resolution only along a few specific directions 14 . A further limitation is that missing data have a significant impact on the results of existing FTH methods.…”

Extracting depth information of 3-dimensional structures from a single-view X-ray Fourier-transform hologram

“…The field of view is usually restricted by an aperture, fabricated into an X-ray-opaque metal-film mask that is produced in close proximity to the sample substrate. The reference beam allowing for phaseencoding typically originates from a small pinhole adjacent to the object fabricated into the same mask [17], while the use of more complex reference beams is also possible [13,25]. This integrated mask design monolithically couples the source of the reference wave to the object to be imaged and generates the remarkable stability of FTH against drift and vibration during exposure.…”

“…However, the holographic reconstruction of the wave field in the plane of the reference source allows to propagate the angular spectrum of the reconstruction [18,20] along the beam axis and thereby refocus specimen features outside the depth-of-field. It has been shown that the plane of least confusion can be separated from the plane of the reference by numerically propagating the reconstructed specimen wave front according to the physical separation between both planes [21,25]. Similarly, a thoroughly focused reconstruction of an extended 3D object can be obtained by numerically propagating the reconstructed wave fronts of individual object features outside the depth-of-field into focus.…”