Refractive Phase Plates for Aberration Correction and Wavefront Engineering

Abstract

The short wavelength of X-rays allows in principle the creation of focal spot sizes down to a few nanometers and below. At the same time, this short wavelength and the resulting interaction with matter puts stringent requirements on X-ray optics manufacturing and metrol-ogy. With the transition from third-generation synchrotron sources to diffraction-limited storage rings of the fourth generation, more beamlines will operate at higher spatial coherence. Thus, more instruments will work with smaller focal spot sizes that are increasingly dominated by diffraction effects instead of a demagnification of the X-ray source. Consequently, the requirements of X-ray optics will increase to ensure best beam characteristics via diffraction-limited optics. Simultaneously, X-ray optics manufacturing strives to achieve higher numerical aper-tures to provide ever decreasing beam sizes. On the forefront of this development are highly specialized nanofocusing beamlines with X-ray optics that push focusing toward 10 nm [1–4] and have the ambi-tious goal to reach 1 nm spot sizes [5]. The fabrication of X-ray optics requires the most advanced technologies, such as lithographic nano-fabrication for diffractive [6] and refractive optics [7], surface figuring with atomic precision for total reflection and multilayer mirrors [8], and thin-film technologies for multilayer optics [9]. All of these technologies have been developed over decades and further advances are expected in the future. Minuscule fluctuations or process anisotropies can cause shape deviations of the X-ray optic with a significant impact on focusing performance. Refractive phase plates in combination with a focusing optic are one solution to overcome these technological limitations. While the weak interaction of hard X-rays with matter and the resulting refractive index decrement δ on the order of 10 6 − pose a challenge for the