Spatial coherence of sunlight: First direct measurement

We report the first direct measurement of the spatial coherence of solar beam radiation. Although often perceived as incoherent, direct sunlight exhibits spatial coherence at a sufficiently small scale. These dimensions were recently derived theoretically to be around two orders of magnitude greater than the wavelength. The partial coherence of sunlight raises tantalizing prospects for a new paradigm for solar power conversion via the antenna effect exploited so successfully in radio-frequency and microwave technologies (albeit at frequencies of order 1 PHz for solar). After reviewing the equal-time mutual coherence function of sunlight, we explain the particular suitability of a lateral cyclicshearing interferometer wherein the solar beam is split into two parts that are subsequently recombined with a relative lateral displacement. The method is relatively uncomplicated, inexpensive and obviates the problem of component dispersion (potentially problematic for a light source as broadband as sunlight). The experimental results are in good agreement with the recent theoretical predictions.