The interference pattern observed in Young's double-slit experiment is intimately related to the statistical correlations of the waves emitted by the slits. As the waves in the slits become more correlated, the visibility of the interference pattern increases. Here, we experimentally modulate the statistical correlations between the optical fields emitted by a pair of slits in a metal film. The interaction between the slits is mediated by surface plasmon polaritons and can be tuned by the slit separation, which allows us to either increase or decrease the spatial coherence of the emerging fields relative to that of the incoming fields.

When spatially coherent radiation is diffracted by a crystalline object, the field is scattered in specific directions, giving rise to so-called von Laue patterns. We examine the role of spatial coherence in this process. Using the first-order Born approximation, a general analytic expression for the far-zone spectral density of the scattered field is obtained. This equation relates the coherence properties of the source to the angular distribution of the scattered intensity. We apply this result to two types of sources. Quasihomogeneous Gaussian Schell-model sources are found to produce von Laue spots whose size is governed by the effective source width. Delta-correlated ring sources produce von Laue rings and ellipses instead of point-like spots. In forward scattering, polychromatic ellipses are created, whereas in backscattering striking, overlapping ring patterns are formed. We show that both the directionality and the wavelength-selectivity of the scattering process can be controlled by the state of coherence of the illuminating source.