Lahsen Assoufid

Elliptical x-ray microprobe mirrors by differential deposition

A differential coating method is described for fabricating high-performance x-ray microfocusing mirrors. With this method, the figure of ultrasmooth spherical mirrors can be modified to produce elliptical surfaces with low roughness and low figure errors. Submicron focusing is demonstrated with prototype mirrors. The differential deposition method creates stiff monolithic mirrors which are compact, robust, and easy to cool and align. Prototype mirrors have demonstrated gains of more than 104 in beam intensity while maintaining submilliradian divergence on the sample. This method of producing elliptical mirrors is well matched to the requirements of an x-ray microdiffraction Kirkpatrick–Baez focusing system.

Short focal length Kirkpatrick-Baez mirrors for a hard x-ray nanoprobe

We describe progress in the fabrication of short-focal-length total-external-reflection Kirkpatrick-Baez x-ray mirrors with ultralow figure errors. The short focal length optics produce nanoscale beams (<100nm) on conventional (∼64m long) beamlines at third generation synchrotron sources. The total-external reflection optics are inherently achromatic and efficiently focus a white (polychromatic) or a tunable monochromatic spectrum of x rays. The ability to focus independent of wavelength allows novel new experimental capabilities. Mirrors have been fabricated both by computer assisted profiling (differential polishing) and by profile coating (coating through a mask onto ultra-smooth surfaces). A doubly focused 85×95nm2 hard x-ray nanobeam has been obtained on the UNICAT beamline 34-ID at the Advanced Photon Source. The performance of the mirrors, techniques for characterizing the spot size, and factors limiting focusing performance are discussed.

Reconstruction of an astigmatic hard X-ray beam and alignment of K-B mirrors from ptychographic coherent diffraction data.

We have used coherent X-ray diffraction experiments to characterize both the 1-D and 2-D foci produced by nanofocusing Kirkpatrick-Baez (K-B) mirrors, and we find agreement. Algorithms related to ptychography were used to obtain a 3-D reconstruction of a focused hard X-ray beam waist, using data measured when the mirrors were not optimally aligned. Considerable astigmatism was evident in the reconstructed complex wavefield. Comparing the reconstructed wavefield for a single mirror with a geometrical projection of the wavefront errors expected from optical metrology data allowed us to diagnose a 40 μrad misalignment in the incident angle of the first mirror, which had occurred during the experiment. Good agreement between the reconstructed wavefront obtained from the X-ray data and off-line metrology data obtained with visible light demonstrates the usefulness of the technique as a metrology and alignment tool for nanofocusing X-ray optics.

Future metrology needs for synchrotron radiation grazing-incidence optics

Abstract An International Workshop on Metrology for X-ray and Neutron Optics, the first of its kind, was held on March 16–17, 2000, at the Advanced Photon Source at Argonne National Laboratory. Metrology specialists, beamline engineers and scientists, and vendors from around the world met to evaluate current metrology instrumentation and methods used to characterize the surface figure and finish off long grazing-incidence optics used in synchrotron radiation beamlines, and to consider future needs for synchrotron, free-electron laser, and neutron sources. This paper summarizes the discussions on mirror and metrology requirements for the current and next-generation X-ray sources. Some recommended strategies for the needs of the future are also given.

Achromatic nested Kirkpatrick–Baez mirror optics for hard X-ray nanofocusing

A nested Kirkpatrick–Baez mirror pair has been designed, fabricated and tested for achromatic nanofocusing synchrotron hard X-rays. The prototype system achieved a FWHM focal spot of about 150 nm in both horizontal and vertical directions.

Wave-optical simulation of hard-x-ray nanofocusing by precisely figured elliptical mirrors

Computer simulations of nanofocusing by elliptical mirrors are presented wherein the diffraction and propagation of coherent hard x rays are predicted using wave-optical calculations. Surface height data acquired via microstitching interferometry were used to calculate the complex pupil function of a mirror, taking into account the Fresnel reflectivity and treating the surface topography as an aberration to a perfect elliptical mirror. The reflected wave-field amplitude and phase downstream of the mirror were obtained by numerically evaluating the Fresnel-Kirchhoff diffraction integral. Simulated intensity profiles and contours (isophotes) around the focal plane are presented for coherent illumination by a 15 keV point source, which indicate nearly diffraction-limited focusing at the 40 nm level. The effect of high spatial frequency microroughness on nanofocusing was investigated by low-pass filtering the Fourier spectrum of the residual height profile. Simulations using the filtered metrology data confirmed that roughness length scales shorter than 0.1 mm have a minor effect on the focal spot size and intensity.

Profile coating and its application for Kirkpatrick-Baez mirrors

For microfocusing x-ray mirrors, an elliptical shape is essen- tial for aberration-free optics. However, it is difficult to polish elliptical mirrors to x-ray quality smoothness. A differential coating method to con- vert a cylindrical mirror to an elliptical one has been previously reported. The coating was obtained by varying the sputter source power over a moving substrate. Here we report a new method of profile coating: the sputter source power is kept constant, while the substrate is passed over a contoured mask at a constant speed to obtain a desired profile along the direction perpendicular to the direction of substrate motion. The shape of the contour depends on the desired profile and the thickness distribution directly above the gun at the substrate level. The thickness distribution was measured on films coated on Si wafers using a spectro- scopic ellipsometer with computer-controlled X-Y translation stages. A model was developed to fit the measured thickness distribution, which determines the relative thickness weightings. When the substrate moves during a deposition, the film thickness is proportional to the length of the opening on the shield can along the direction of motion. By equating the sum of relative weightings to the required relative thickness at the same position, the length of the opening at that position can be determined. By repeating the same process for the whole length of the required profile, a contour can be obtained for a desired thickness profile. The number of passes and the speed of the substrate are determined according to the required thickness and the growth-rate calibration of a test run. The mir- ror coating profile is determined from the difference between the ideal surface figure of a focus ellipse and the surface figure obtained from a long-trace profiler measurement on the substrate. A Kirkpatrick-Baez (KB) mirror pair was made using Au as a coating material and cylindri- cally polished mirrors as substrates. Synchrotron x-ray results using this KB mirror pair showed a focused spot size of 0.430.4 mm 2 .

Thermal contact resistance across a copper-silicon interface.

An experimental setup to measure the thermal contact conductance across a silicon-copper (Si-Cu) interface is described, and the results obtained are presented. The resulting thermal contact resistance data are used in estimating the thermo-mechanical and optical performance of optical substrates cooled by interfaced copper cooling blocks. Several factors influence the heat transfer across solid interfaces. These include the material properties, interface pressure, flatness and roughness of the contacting surfaces, temperature, and interstitial material, if any. Results presented show the variation of thermal contact conductance as a function of applied interface pressure for a Cu-Si interface. Various interstitial materials investigated include indium foil, silver foil and a liquid eutectic (Ga-In-Sn). As expected, thermal contact resistance decreases as interface pressure increases, except in the case of the eutectic, in which it was nearly constant. The softer the interstitial material, the lower the thermal contact resistance. Liquid metal provides the lowest thermal contact resistance across the Cu-Si interface, followed by the indium foil, and then the silver foil.

All-diamond optical assemblies for a beam-multiplexing X-ray monochromator at the Linac Coherent Light Source

All-diamond optical assemblies holding state-of-the-art type IIa diamond crystals enable the construction of a beam-multiplexing X-ray double-crystal monochromator for hard X-ray free-electron lasers. Details on the design, fabrication and X-ray diffraction characterization of the assemblies are reported.

A hybrid method for X-ray optics simulation: combining geometric ray-tracing and wavefront propagation

A new ’Hybrid Method’ combining ray-tracing and wavefront propagation for X-ray optics simulation is reported. The code is fast and can handle partially coherent sources, making it a useful tool for beamline design and optimization.