Diffractive–refractive optics: low-aberration Bragg-case focusing by precise parabolic surfaces

Abstract

Based on analytical formulae calculations and ray-tracing simulations a low-aberration focal spot with a high demagnification ratio was predicted for a diffractive-refractive crystal optics device with parabolic surfaces. Two Si(111) crystals with two precise parabolic-shaped grooves have been prepared and arranged in a dispersive position (+,-,-,+) with high asymmetry. Experimental testing of the device at beamline BM05 at the ESRF provided a focal spot size of 38.25 microm at a focal distance of 1.4 m for 7.31 keV. This is the first experiment with a parabolic-shaped groove; all previous experiments were performed with circular grooves which introduced extreme aberration broadening of the focal spot. The calculated and simulated focal size was 10.8 microm at a distance of 1.1 m at 7.31 keV. It is assumed that the difference between the measured and calculated/simulated focal spot size and focal distance is due to insufficient surface quality and to alignment imperfection.

X-ray collimation by crystals with precise parabolic holes based on diffractive–refractive optics

“…Here r e is the classical electron radius, F 0 is the structure factor, V is the unit-cell volume, is the wavelength, d hkl is the atomic plane spacing, is the asymmetry angle ( > 0 for the grazing-incidence case), is the Bragg angle, N is the number of diffraction events and b is the asymmetry factor. A plot of formula (2) for d hkl = (111) shows the dependence of the focusing distance over energy or Bragg angle (Oberta et al, 2010). By using only one crystal with a parabolic shaped hole, Fig.…”

“…Based on theoretical calculations (Hrdý & Oberta, 2008) and raytracing simulations (Artemiev et al, 2004), various applications for diffractive-refractive optics were proposed and tested at synchrotron facilities. Recently, the smallest focal spot size by diffractive-refractive optics (Oberta et al, 2010) was achieved and a novel method of higher harmonics separation in space was proposed at the Swiss Light Source (SLS; Hrdý et al, 2011). In addition to beam focusing, another application of diffractive-refractive optics is beam collimation.…”