Observation and explanation of one-dimensional x-ray speckle patterns from synthetic multilayers

Using the new "Troika" x-ray undulator beamline at the European Synchrotron Radiation Facility, we have succeeded in measuring coherent diffraction ("speckle") patterns from artificial multilayers. The patterns are unusual in that they are unremarkable in the scan direction perpendicular to the Bragg angle, showing a single peak of the width of the Fraunhofer maximum, but have dramatic structure in the direction of the Bragg angle. This is shown to be consistent with the extreme asymmetry of the geometry that causes more than one well-ordered domain of the sample to fall within the coherently illuminated region. The patterns are sufficiently simple that we are able to model them reasonably well by fitting explicit phase parameters to a fixed number of illuminated "blocks." Two fitting procedures are described that may have some general utility.