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.