[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

The difference in optimal thickness is increased for

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The difference in optimal thickness is increased for high energies. A com- parison of the theoretical value of the optimal thickness for the transmission zone plate and two types of Bragg–Fresnel lens is shown in Fig. 3.23. As the Bragg–Fresnel lens substrate we chose a Si (111) crystal with the lattice period of 0.31 nm and a multilayer mirror with a period of 3 nm. Even with the tech- nology with an aspect ratio of about 1:1 (layer thickness equal to a groove width) an outer zone width of the Bragg–Fresnel lens at 30 keV can reach 0.2 µ m. It can be easily fabricated for photon energies as high as 100–200 keV, which is a great problem for conventional zone plates due to the very high material thickness required. The combination of the linear Bragg–Fresnel lens and a bimorph mirror in meridional direction allows a small-range energy scan without loss in spatial resolution. Several lenses fabricated on the same substrate will cover the entire operational energy range of the beamline. Special attention must be paid to beam position stabilization for the microfocus experiments. 3.5.3 Bragg–Fresnel Holographic Optics In 1948, the Nobel Prize Laureate Denis Gabor proposed his famous “method of wave-front reconstruction” which began the era of holography [184]. In the
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186 A. Erko 1960s with the development of optical lasers, E.N. Leith and J. Upatnieks, demonstrated the first wave-front reconstruction by a hologram with a ref- erence beam [185]. Since this first successful hologram demonstration the method of holographic reconstruction has found applications in many different fields of art, science and technology. The possibility of creating holograms in the X-ray range has also been discussed in a number of works. However, such holograms have neither been fabricated nor tested until now except in the case of simple holograms (focusing zone plates). Due to the absence of sensitive and high-resolution materials for X-ray holography, it seems to be more effective to use X-rays only in the reconstruction stage to produce images with micron resolution. A so-called synthetic hologram [186] can be generated by computer and transferred into material using modern methods of microelectronics tech- nology [187]. The holographic optics is becoming even more important with the construction of X-ray lasers in Germany and USA. The first successful reconstruction of a synthetic X-ray hologram was reported in 2001 [32]. A hologram structure can be calculated using specially developed com- puter software and fabricated on the surface of Si (111) monocrystal using e-beam lithography and a metal coating technique. A “white” broad-band synchrotron radiation beam from the BESSY bending magnet source was used for the hologram reconstruction. The computer hologram generation comprises iterative calculations of the hologram phase distribution under the condition of constant value of inten- sity of the complex amplitude A ( x , y ) in the hologram plane using direct Fourier transformations for definition of the image amplitude to be “as close as possible” to the demanded image.
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