Near-field radiative transfer by bulk hyperbolic polaritons across vacuum gap

Abstract Thermal radiation has always been treated as a surface phenomenon. A recent study has demonstrated that new radiation channels, the hyperbolic phonon polaritons, can contribute to heat transfer inside bulk hyperbolic materials with a heat flux comparable to conduction. For near-field radiative transfer across a vacuum gap between two hyperbolic materials, hyperbolic phonon polaritons have been considered as surface modes that are responsible for heat transfer enhancement. Here, we analyze near-field radiative transfer due to hyperbolic phonon polaritons, driven by temperature gradient inside the bulk materials. We develop a mesoscale many-body scattering approach to account for the role of hyperbolic phonon polaritons in radiative transfer in the bulk and across a vacuum gap. Our study points out the equivalency between the bulk-generated mode and the surface mode in the absence of a temperature gradient in the material, and hence provide a unified framework for near-field radiative transfer by hyperbolic phonon polaritons. The results also elucidate contributions of the bulk-generated mode and the bulk temperature profile in the enhanced near-field radiative transfer.