Abstract
Heat transfer between two plates of polar materials at nanoscale distance is known to be enhanced by several orders of magnitude as compared with its far-field value. In this article, we derive accu- rate analytical expressions to quantitatively predict heat fluxes in the near-field. These analytical expressions reveal the physical mechanisms responsible for the enhancement. For two dielectric polar materials and for gaps smaller than 75nm at room temperature the heat transfer is dominated by the surface phonon polariton contribution. Between 75nm and 500 nm, the enhancement is mostly due to frustrated total internal reflection. The paper reports accurate analytical expressions for both contributions. Our analytical results highlight two differences between radiation flux at the nanoscale and in the far field: i)the heat flux spectrum depends on the gap distance, ii) the temperature dependence of the heat transfer coefficient deviates strongly from the T3 law valid for gray bodies in the far-field