The degree of optical spatial coherence -a fundamental property of light thatdescribes the mutual correlations between fluctuating electromagnetic fields-has proven challenging to control at the micrometer scale. Here we employsurface plasmon polaritons -evanescent waves excited on both surfaces of a thinmetal film- as a means to entangle the random fluctuations of the incidentelectromagnetic fields at the slit locations of a Young's double-slitinterferometer. Strong tunability of the complex degree of spatial coherence oflight is achieved by finely varying the separation distance between the twoslits. Continuous modulation of the degree of spatial coherence with amplitudesranging from 0% up to 80% allows us to transform totally incoherent incidentlight into highly coherent light, and vice versa. These findings pave the wayfor alternative methods to engineer flat optical elements with multi-functionalcapabilities beyond conventional refractive- and diffractive-based photonicmetasurfaces.
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