Quantifying the Spatial Distribution of Radiative Heat Transfer in Subwavelength Planar Nanostructures

Control of nanoscale thermal transport is essential for novel energy conversion, thermal management, and thermal logic devices. Recent experimental and theoretical studies have shown that far-field radiative heat transfer in planar membranes with nanoscale thickness can exceed the blackbody limit. Past computations suggest that the observed enhancements are due to highly directional and spatially confined in-plane heat transfer between these membranes. However, experimental evidence for this confinement is lacking. Here, we perform experiments on submicron-thick, planar silicon nitride membranes to directly quantify the spatial extent over which heat transfer occurs and show that, at room temperature, heat transfer is confined to similar to 10 mu m above and below the plane containing the membranes, a distance comparable to Wien's wavelength. Furthermore, we provide calculations of Poynting fluxes that enable detailed explanations of our experimental observations. The resulting insights could lead to novel approaches and devices for active control of heat flow at the nanoscale.