Spatial coherence of thermal near fields

We analyze the spatial coherence of the electromagnetic field emitted by a half-space at temperature T close to the interface. An asymptotic analysis allows to identify three different contributions to the cross-spectral density tensor in the near-field regime: thermally excited surface waves, skin-layer currents and small-scale polarization fluctuations. It is shown that the coherence length can be either much larger or much shorter than the wavelength depending on the dominant contribution. (C) 2000 Elsevier Science B.V. All rights reserved.