Tham khảo tài liệu 'modern developments in x-ray and neutron optics episode 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 480 A. Erko et al. grating can in general no longer be described as the superposition of two independent diffraction structures but instead by a complex volume interference phenomena. Several methods have been developed for the meridional grating calculations. A description in the kinematic approximation which neglects interactions between incident and diffracted beams was used by several authors 18 . In the calculations 19 rigorous theory based on Maxwell s equations which take into account this interaction dynamical theory was used. Computations can be done using a differential method and modal theory 20 . In the modal theory the solution is sought for the whole structure. Therefore structures with any number of layers can be calculated at the same time. In addition using this method one can quickly compute the dispersive curves in reciprocal space and the Bragg angle scanning of the diffracted orders. However this method is valid only for a lamellar grating. It is not applicable for example for a sawtooth profile grating. In the present chapter the differential technique developed in 21 is used for multilayer meridional Bragg-Fresnel grating calculations. These calculations are carried out layer-by-layer making the method suitable for any kind of profile. Lamellar gratings have been studied by several authors both theoretically and experimentally 22-25 . The nature of diffraction on a three-dimensional grating multilayer structure strongly depends on the optical properties of the materials and the characteristic size of a grating period with the lattice parameter a1 and a multilayer period with the lattice parameter a2. As a consequence two different approximations can be used for different limiting conditions. Multilayer Etched Meridional Grating In the first approximation the double dispersion phenomena of multilayer gratings can be described as a combination of Bragg diffraction on reflecting layers and surface diffraction on a planar grating. In this .
