Mode transition of near-field radiative heat transfer between two subwavelength cylinders

Near-field radiative heat transfer (NFRHT) between nanostructures in the ‘dual nanoscale regime’ has attracted growing interest, especially the dramatic enhancement caused by corner and edge modes in the subwavelength coplanar membranes. However, when device geometry is further confined to subwavelength cylindrical structures — which sustain a rich set of localized electromagnetic multipolar surface modes along their sidewalls — the contributions of such modes to NFRHT remain unexplored. Here, we reveal that reducing nanostructure size drives a mode transition from surface phonon polaritons to multipolar surface modes and its effect on NFRHT between two axially aligned subwavelength silicon carbide (SiC) cylinders. When both radius and vacuum gap are significantly smaller than the thermal photon wavelength, multipolar surface modes can dominate the NFRHT. For 47-nm-radius, 400-nm-height SiC cylinders separated by a 100-nm vacuum gap, the radiative heat transfer coefficient reaches 1.53 times that of semi-infinite SiC plates at room temperature, exhibiting a 610-fold enhancement over the blackbody limit after accounting for geometric view factors. Further, the vacuum gap variation also induces the mode transition and a 2-fold enhancement relative to planar configurations can be achieved with a gap size of 50 nm. These findings provide critical insights into advancing nanoscale thermal management and energy harvesting applications.