Christian Fuhse

Front-coupling of a prefocused x-ray beam into a monomodal planar waveguide

A hard x-ray beam of photon energy E=12.5keV has been focused by a Kirkpatrick–Baez mirror system and coupled into the front side of a single-mode x-ray waveguide. The beam dimensions of 3.8×2.5μm2 in the focus of the mirror system have thus been reduced in one direction to 32nm, corresponding to the guiding layer thickness of the waveguide. At the same time the waveguide acts as a coherence filter and leads to a well-defined intensity distribution with steep tails in the near- and far-field regions. The total flux transmitted by the waveguide exceeded 108 photons/s while no significant contributions of radiation transmitted through the absorbing waveguide cladding have been observed.

X-ray waveguide nanostructures : Design, fabrication, and characterization

Two dimensionally confining x-ray channel waveguide structures are fabricated and used for the delivery of nanoscopic x-ray beams. The waveguides can be combined with a high gain Kirkpatrick-Baez-prefocusing mirror system yielding hard x-ray beams with a cross section down to 25nm (full width at half maximum). The incoming synchrotron x-ray beam is coupled in from the front side of the waveguide. Here we address the general design of the x-ray optical devices and their fabrication by e-beam lithography methods.

Finite-difference field calculations for two-dimensionally confined x-ray waveguides

A numerical method for calculation of the electromagnetic field in two-dimensionally confined x-ray waveguides is presented. It is based on the parabolic wave equation, which is solved by means of a finite-difference scheme. The results are verified by a comparison to analytical theory, namely, Fresnel reflectivity and the weakly guiding optical fiber.

Object localization with 10nm accuracy by x-ray phase contrast projection imaging

The present work focuses on the question of localizing single object by hard x-ray phase contrast projection imaging. The authors present a setup where an x-ray channel waveguide defines a “quasi-point source” used to illuminate and image an object in a highly coherent cone beam. Knife edge fluorescence scans revealed a beam diameter of 75nm at a distance of 30μm behind the guide. The recorded image corresponds to an in-line hologram of the object which can be reconstructed numerically. Object translations and associated shifts in the hologram allow for the 10nm localization accuracy.

Coherent propagation of white X-rays in a planar waveguide.

The far-field diffraction pattern of a front-coupled planar waveguide supporting two guided modes has been measured using a white X-ray beam. Interference of the guided modes leads to a characteristic variation of the far-field diffraction pattern for different photon energies. The experiment verifies the predicted properties of the guided modes, shows that these modes superpose coherently, and demonstrates that the electromagnetic field downstream of the waveguide is significantly different from that expected for a hypothetical small slit of the same size.

Projection phase contrast microscopy with a hard x-ray nanofocused beam: Defocus and contrast transfer

We report a projection phase contrast microscopy experiment using hard x-ray pink beam undulator radiation focused by an adaptive mirror system to 100-200 nm spot size. This source is used to illuminate a lithographic test pattern with a well-controlled range of spatial frequencies. The oscillatory nature of the contrast transfer function with source-to-sample distance in this holographic imaging scheme is quantified and the validity of the weak phase object approximation is confirmed for the experimental conditions.