Bragg-Fresnel Optics: Principles and Prospects of Applications

Abstract

Possibilities of control and focusing of X-rays have been discussed repeatedly since the discovery made by Rontgen. It turned out, however, that applications of the principles of optical element fabrication adopted in visible light, infrared radiation and other wavelength ranges are rather limited due to negligible difference of the refractive index from unity, a relatively large absorption coefficient and the necessity to fabricate optical elements with the accuracy compared to a radiation wavelength. In the last years the methods of X-ray optics acquire further extensive development because of the advances in microstructuring technology, namely, structure fabrication with element sizes up to one hundred angstroms, sputtering technology and that of the growth of thin films of different materials, and due to the advances in the investigation of X-ray diffraction. In our opinion, at present there exists a possibility to set and solve the task of fabricating effective focusing X-ray elements with the structure of three-dimensional Fresnel zones, that is, Bragg Fresnel optics. These elements can be made on the basis of multilayer interference mirrors for the nanometer wavelength range (0.5nm≲λ≲10nm) and semiconductor perfect crystals with heterostructures for the wavelength range of 0.1A≲λ≲5A. The principal peculiarity of Bragg-Fresnel X-ray elements lies in the fact that coherent Bragg scattering by separate layers is used in them. As it will be shown, this phenomenon permits increasing their diffraction efficiency, widening the spectrum range and angular aperture and gives a possibility to realize amplitude and phase modulations of radiation, to switch X-ray elements by an electrical signal, ultrasound and light signals.