  Classical Planck's theory of thermal radiation predicts an upper limit of the
heat transfer between two bodies separated by a distance longer than the
dominant radiation wavelength (far-field regime). This limit can be overcome
when the dimensions of the absorbent bodies are smaller than the dominant
wavelength due to hybrid electromagnetic waves, known as surface
phonon-polaritons (SPhPs). Here, we experimentally demonstrate that the
far-field radiative heat transfer between two non-absorbent bodies can also
overcome Planck's limit, by coating them with an absorbent material to form a
polaritonic waveguide. This super-Planckian far-field thermal radiation is
confirmed by measuring the radiative thermal conductance between two silicon
plates coated with silicon dioxide nanolayers. The observed conductance is
twice higher than Planck's limit and agrees with the predictions of our model
for the SPhP waveguide modes. Our findings could be applied to thermal
management in microelectronics and silicon photonics.
