Coupling Between Conduction and Near-Field Radiative Heat Transfer in Tip-Plane Geometry

Two bodies at different temperature separated by a vacuum gap exchange an energy flux mediated by photons. This radiative heat flux is limited at large distance by the well-known Stefan-Boltzmann’s law. Nevertheless, the development of fluctuational electrodynamics (thanks to the pioneering works of Rytov, Polder and van Hove) allowed to show that in the near field, i.e. at distances smaller than the thermal wavelength (some microns at ambient temperature), the flux can dramatically increase because of photon tunneling. Although this prediction has been confirmed in many experiments, some of them have observed deviations from theoretical results, both in near field and in extreme near field (nanometer and sub-nanometer range of distances). In this sense, one possibly relevant aspect could be the coupling between exchanged radiative flux and conduction. We have shown that in the configuration of two parallel slabs this coupling can induce a temperature profile in each body (neglected in most previous works) and induce, in turn, a saturation of the exchanged flux. This result was later generalized to the configuration of two nanorods, for which the different spectral behavior and the participation of volume modes of the field increases the exchanged flux and induces a non-linear temperature profile even in the diffusive regime. Finally, we present a recent work in which we addressed the coupling between two concentric cylinders of different radii, allowing us to simulate the configuration of a sharp tip in front of a planar substrate. We obtained analytical expressions for both the temperature profiles and the exchanged flux, showing that also in this scenario the conduction-radiation coupling can induce a significant temperature change in the smaller cylinder (the tip), up to tens of degrees in the case of polar materials, along with a strong reduction of the flux (of more than two orders of magnitude) with respect to the scenario of absence of coupling. Our results show that, with an appropriate choice of the materials and geometrical configuration, the effect of conduction-radiation coupling could be within experimental observability.