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. T he approach, based on fluctuational electrodynamics and the fluctuation-dis sipation theorem, yields novel near-held correlation properties. We show th at the spatial coherence length of the field close to the surface at a give n wavelength lambda may be much smaller than the well-known lambda/2 of bla ckbody radiation. We also show that a long-range correlation may exist, whe n resonant surface waves, such as surface-plasmon or surface-phonon polarit ons, are excited. These results should have important consequences in the s tudy of coherence in thermal emission and in the modeling of nanometer scal e radiative transfer.