Near-field effects in spatial coherence of thermal sources

We present an exact calculation of the cross-spectral density tensor of the near held thermally emitted into free space by an opaque planar surface. The approach, based on fluctuational electrodynamics and the fluctuation-dissipation theorem, yields novel near-held correlation properties. We show that the spatial coherence length of the field close to the surface at a given wavelength lambda may be much smaller than the well-known lambda/2 of blackbody radiation. We also show that a long-range correlation may exist, when resonant surface waves, such as surface-plasmon or surface-phonon polaritons, are excited. These results should have important consequences in the study of coherence in thermal emission and in the modeling of nanometer scale radiative transfer.