Fundamental limits to near-field radiative heat transfer and the limited role of nanostructuring.

Near-field radiative heat transfer between bodies at the nanoscale can surpass blackbody limits on thermal radiation by orders of magnitude due to contributions from evanescent electromagnetic fields, which carry no energy to the far-field. Thus far, principles guiding explorations of larger heat transfer beyond planar structures have assumed utility in surface nanostructuring and related enhancements in the density of states. We re-examine this assumption by deriving fundamental limits to near-field radiative heat transfer in polaritonic materials, which can be made to exhibit strong resonant response in the infrared, limited only by intrinsic material losses. We show that at any given wavelength, multiple scattering severely limits the marginal utility of nanoscale texturing for the purpose of enhancing near-field heat transfer, beyond shifting the resonant response of these materials to selective wavelengths. While compact bodies are shown to benefit from stronger material response (larger indices of refraction and smaller losses) up to a size-dependent threshold, the upper bounds on near-field heat transfer between extended structures are found to be practically reached by planar structures at the surface polariton condition, which has ramifications for the ultimate performance of thermophotovoltaics, nanoscale cooling, and related thermal devices operating in the near field.