Effect of focusing optics on x-ray speckle contrast

We investigated the behavior of speckle contrast and size under various experimental conditions using 1.82 keV x-rays. In this paper, we report the comparison of two different setups for x-ray speckle experiments: one employing a focusing zone plate and one in which a pinhole selects the size of the coherent x-ray beam. We found a strong dependence of the speckle contrast and size on the type of setup. In general, the pinhole setup results in higher contrast but smaller speckle size. On the other hand, the zone plate setup allows one to target much smaller areas of interest in the sample, down to submicron dimensions, and also to adjust the speckle size. We anticipate that these results will be useful in future time-correlation spectroscopy experiments.