The investigations of braggfresnel gratings show the

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The investigations of Bragg–Fresnel gratings show the difference in dif- fraction properties of sagittal and meridional grating structures. Dispersion angles of a sagittal grating are similar to those of a transmission grating and can be calculated using the simple diffraction equation. The main difference of a Bragg–Fresnel grating in comparison with a transmission grating is the value of optimal mask thickness. For a Bragg–Fresnel grating at a small graz- ing incidence angle the thickness can be an order of magnitude less at the same high diffraction efficiency. For example the grating with a gold thickness of 20 nm at 0 . 37 angle has an efficiency of 16%. The same efficient grating at a normal incidence should have the thickness of 1350 nm. The grating mater- ial thickness (aspect ratio) is the most important limitation in production of gratings with nanometer period by modern technology methods. Reduction of an optimal thickness to the nanometer scale opens the possibility of pro- duction of nanometer period gratings with an efficiency of up to 30% for hard X-rays. The investigation of a meridional Bragg–Fresnel grating shows completely different diffraction properties in a comparison with conventional reflection grating. First, for a monochromatic beam, an angular spectrum of the input beam is limited by the value of crystal angular acceptance (rocking curve). All the other angular spectral components are rejected. The output angular spectrum is not limited and all higher diffraction orders are present. The in- clined geometry at small grazing incidence angles makes it possible to enlarge the angle of dispersion of a meridional grating. To check the value of the an- gular dispersion in the meridional direction the performance of a meridional Bragg–Fresnel grating has been measured. A 2- µ m period, 20 nm thick nickel grating was evaporated on the surface of an asymmetrically cut Si [111] crystal with an asymmetry angle of 13 . A diffraction efficiency of 1% was obtained at the energy of 8.2 keV and grazing angle of 0 . 9 . This value corresponds to an effective period for a transmission grating at normal incidence of 25.5 nm. This property of a meridional Bragg–Fresnel grating opens up the possibil- ity of producing a Bragg–Fresnel diffraction grating with nanometer spatial resolution. The definition of a sagittal and meridional grating is given in Fig. 3.21 and Fig. 3.24. In the sagittal geometry, the grating grooves are aligned parallel to the optical plane, as defined by the ingoing and outgoing X-ray beams. In this case diffraction takes place perpendicular to the optical plane. In meridional geometry, the grating grooves are aligned perpendicular to the optical plane and diffraction takes place in the optical plane.
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  • Spring '14
  • MichaelDudley

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