Design of a gradient refractive index phase zone plate for soft x-rays

A new type of zone plate has been designed for soft x-rays, based on radial modulation of the refractive indices of the material. It is made with two materials; the concentration of one material increases gradually and that of the other decreases with increasing radius in each pair of zones. When such a zone plate is made with titanium and chromium and their concentrations are optimized for use at the wavelength of 2.74 nm, its x-ray focusing efficiency would be 34%. This value is 3.4 and 1.4 times the efficiencies of the Fresnel zone plate and the pi-radian phase shifting zone plate, respectively.