Microscopic imaging with high energy x-rays by Fourier transform holography

Experimental results obtained with an x-ray microscope based on Fourier transform holography are discussed. The experiments were done with monochromatic x rays at an energy of 14 keV. The x-ray source was a high brilliance undulator with a small vertical source size providing a spatial coherent beam. The necessity of the increase of the spatial coherence by spatial filtering is discussed together with the generation of a smallest possible reference point source. A Fresnel zone plate was used as a beam splitter. It formed the reference point source by the first order diffracted beam. The sample to be imaged is placed close to the first order focal spot and in parallel the sample is illuminated by the zero order transmitted beam. The hologram is generated by the interference of the reference wave and the object wave at a large distance from the object. A characteristic of these holograms is that a small distance in the object plane gives rise to large fringe spacing in the hologram plane and vice versa. In principle the minimum lateral resolution in a reconstruction of such holograms is not limited by the lateral resolution of the detector but other factors which will be discussed. The holograms were recorded with a charge coupled device based x-ray microscope. The properties of holograms of test structures are investigated by the evaluation of the fringe spacing in the hologram. Fourier transform holograms were successfully numerically reconstructed and structures of 3.5 μm size are visible in the reconstruction.