The status of polarizing optical elements for the soft x-ray range is reviewed. The criteria for optimum polarization of soft x-ray reflective analyzers, transmission polarizers and phase retarders based on multilayer interference structures are presented. Following these principles, soft x-ray polarizing optical elements have been designed. Designs of broad angular and wavelength range reflective analyzers based on a combination of analytical and numerical methods are also discussed.
The broadband x-ray supermirrors have been designed, fabricated and studied experimentally. They are intended for operating in the photon energy range from 12.4 to 20keV at the fixed grazing incident angle of 0.5°. The choice of material pairs composing a non-periodic multilayer structure is considered from the maximum achievable reflectivity and the layer structure stability. Tungsten and silicon are selected, which have the great index contrast in this energy range and are suitable for fabrication with the use of the vested coating system. The interface width determined by transmission electron microscopy (TEM) lies in the range of 0.4~0.6nm. Following a theoretical approach including analytical and numerical techniques, the designed multilayer mirrors have a practically constant reflectivity of about 24% in the desired photon energy range. The multilayers were deposited onto polished silicon substrates by a high vacuum DC magnetron sputtering system and were characterized by x-ray diffractormeter (XRD) at 8.0keV. The experiments show that the sample has about 10% reflectivity in a wide angular range; The roughness of the interface for both tungsten on silicon layer and silicon on tungsten layer was estimated to be about 0.5nm. Oscillations in the reflectivity curve proved to be larger than the expected ones. They may be caused by the random variation of the layer thickness, as well as the interdiffusion between two materials. Possibilities for further improvements of wideband multilayer mirrors are analysed as well.
The periodic mulitlayer with high reflectivity and small full width at half maximum (FWHM) in hard x-ray range, while the reflectivity of non-periodic multilayer decreases and FWHM increase. Therefore, the optimum coating design must be found out as a compromise between the requirements for the reflectivity and the FWHM. We have used purely numerical techniques to design broad angular multilayer mirror in angle intervals (2.9°-3.1°), which is starting from an appropriate periodic multilayer structure. In our method, the risk of local minimization of the merit function disappears, because we refined the desired depth-distribution of the period using a direct numerical algorithm and the analytical solution as a starting point for computer calculation. The plateau reflectivity can be obtained in a few minutes. The main feature of our approach is the use of an analytical solution as a starting point for direct computer search, and the desired results can be given in a reasonable time. This technique is able to design almost any given reflectivity spectrum both energy- and angle-dependent and in a reasonable amount of time. The periodic and non-periodic W/Si multilayer for grazing incidence multilayer mirrors at the K-edge of Ti (0.275 nm) were both designed and fabricated by high vacuum DC magnetron sputtering coater model JGP560C6, and the multilayer films were characterized by X-ray reflectivity measurements on a laboratory x-ray diffractometer(XRD) and the atomic force microscope (AFM). We find good agreement of the changing trend of surface roughness between the simulation of XRD and measurement of AFM.
The present status of studies on EUV, soft x-ray and x-ray multilayer in the Institute of Precision Optical Engineering (IPOE) is briefly reviewed. With the aim of realizing a Mach-Zender interferometer working at 13.9nm, we have developed a semitransparent beam splitter with multilayer deposited on the back side of a silicon nitride membrane. On the basis of the experimental optical properties of the beam splitter, design has been performed to define the multilayer
structure that provides the highest product of reflectivity and transmission. Optimized Mo/Si multilayer has been successfully deposited on the back side of a silicon nitride membrane by use of the magnetron sputtering. Measurements by means of a reflectometer in Beijing Synchrotron Radiation Facility at 13.9nm and at an angle of 7.2° provide a reflectivity of 20% and a transmission of 22%. Such a beam splitter has been used for X-ray Mach-Zender interferometer at 13.9nm. The broadband multilayer analyzer in the range between 12.4nm and 20nm is designed, and made which can deviate the Quasi-Brewster's angle several degree and show very high polarization. The main feature of our design approach is the use of an analytical solution as a starting point for direct computer search, and the desired results can be given in a reasonable time. The method can be applied in different spectral range for suitable material combination. Supermirrors with broad angular band working at different wavelength such as Cu Kα line are designed, manufactured and measured. The results show that the performance of the supermirrors is in agreement with designed data.
We designed two kinds of supermirrors composed of Carbon and Tungsten in current paper. Using Simplex Optimization method, we designed C/W supermirror with broad incident angular range (0.5°~0.9°) at Cu Kα line (λ=0.154nm). Connecting the analytical method, based on oversimplified analytical and semi-empirical formulas, with the Simplex Optimization method, we designed C/W supermirror with broad photon energy range (16~25keV), at θ=0.5°. The negative effect, due to the interfacial roughness and diffusion between the adjacent sublayers in multilayer, also emerges in simulating the practice performance of the both supermirrors. The supermirrors with broad incident angular range were fabricated by DC magnetron sputtering, because such supermirror is easy to be measured at our lab, using X-ray diffractormeter. The measured reflectivity near the first order Bragg peak is about 30% at Cu Kα line. The roughness factor is about 0.84nm by the fitting data.