Enhancement of Near-Field Thermal Radiation of Graphite

Graphite, one of the few naturally existing hyperbolic materials, is significant in obtaining efficient radiative heat transfer. This paper investigates the influence of optical axes'orientations and thickness on the near-field radiative heat flux between graphite films, and takes an analysis of the underlying mechanism. As optical axes parallel to the energy flow direction, conventional non-resonant hyperbolic modes have a dominant contribution to the radiative heat transfer. For optical axes perpendicular to the energy flow direction, hyperbolic surface plasmon polaritons (HSPPs) will be excited besides the existence of conventional non-resonant hyperbolic modes. The contribution of hyperbolic modes drops with the film thickness decreasing, however, HSPPs can couple with high-k electromagnetic waves, leading to a higher radiative heat flux than that of counterpart bulks. For bulk graphites, the configuration with optical axes parallel to the energy flow direction has a higher radiative heat flux when the gap distance is shorter than 170 nm. However, when the thickness decreases to tens of nanometers, the other configuration may support a more efficient radiative heat exchange.