Thermal Transport Mediated by Non-Equilibrium Surface Phonon Polaritons

Surface phonon polaritons (SPhPs), hybrid quasi-particles resulting from coupling between infrared photons and optical phonons, could play significant roles in near-field radiation. More recently, it has been predicted that SPhPs can also contribute to heat conduction along polar thin films and nanowires; however, direct and conclusive experimental evidence for SPhP-mediated heat conduction has been scarce. Here we report on significant enhancement of the thermal conductivity of 3C-SiC nanowires when the end(s) of the nanowires are coated with a thin layer of gold that can serve as efficient SPhP launchers. Through measuring the same 3C-SiC nanowires with and without gold coating at the end(s), we show that the SPhPs launched by the gold coating can remarkably enhance the thermal conductivity of the uncoated wire segment from the value of the same wire without gold coating. Interestingly, it is found that the SPhP-mediated thermal conductivity is orders of magnitude higher than the Landauer limit predicted based on the equilibrium Bose-Einstein distribution function over the entire measurement temperature range. We attribute this unexpected discovery to the partial ballistic propagation of SPhPs along the SiC nanowire with a number density well beyond the value determined by the Bose-Einstein distribution function. The understanding of non-equilibrium SPhP transport is supported by the observation that the extracted SPhP-mediated thermal conductivity is proportional to the length of the gold coating at the end(s) of the nanowire, which provides important insights into the SPhP excitation process. The highest SPhP-mediated thermal conductivity at room temperature reaches a value of 5.8 W/m-K, a few times of the phonon-mediated thermal conductivity for some technologically important thin films such as silicon oxide and nitride. This discovery of non-equilibrium SPhP-mediated heat conduction implies novel cooling strategies utilizing SPhPs as effective thermal energy carriers.