At a finite temperature, electrons and ions in any matter are under constant thermal agitation, acting as the random current source for thermal emission. The thermally-excited electromagnetic waves have two forms: the propagating modes that can leave the surface of the emitter and radiate freely into the space, and the non-propagating modes (evanescent modes) that do not radiate. The contribution from the propagating modes, or the far-field radiation modes, to the radiative heat flux is well-known and its maximum is governed by Planck’s law of blackbody radiation. The non-propagating modes do not propagate and thus do not carry energy in the direction normal to the surface, unless a second surface is brought close to the first to enable photon tunneling. The contribution from the non-propagating modes to radiative heat flux is the near-field radiative flux.

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