Probing medium-range structural correlations by fluctuation microscopy

We outline recent advances in the fluctuation microscopy technique for probing medium-range structural correlations in disordered materials. The technique was originally developed for electron microscopy, but has now been extended to optical and x-ray microscopies. We show that fluctuation microscopy can detect trace quantities of C₆₀ in a disordered graphite matrix, even though the diffraction signature from the C₆₀ is essentially undetectable. This result indicates that the technique can be used to discern dilute distributions of macromolecules in an otherwise disordered matrix. We also report preliminary studies of interferometric fluctuation microscopy using cross-correlations in diffraction between coherent double probes. This is a form of holography where the diffraction patterns from two neighboring regions are allowed to overlap and interfere. Young's fringes appear wherever both regions scatter strongly. The cross-correlation can be examined as a function of probe separation to estimate a structure correlation length. This method holds much promise for studying medium-range order, since it isolates the essential four-body terms underpinning the fluctuation microscopy technique.