(Invited) Thermal Energy Conduction in a Surface Phonon Polariton Crystal

Surface phonon polaritons are coupled states of polar atomic vibrations (optical phonons) and electromagnetic waves that are known to enhance near-field thermal radiation from polar materials. Theory suggests that surface phonon polaritons could also increase the thermal conductivity of nanoscale materials under certain conditions. This concept, however, is difficult to demonstrate experimentally, especially at room temperature. Here we design, predict, and demonstrate a different approach to observe thermal energy conduction by surface phonon polaritons – that is, a surface phonon polariton crystal. Packed beds of silicon dioxide nanoparticles have ultralow thermal conductivity at room temperature and large internal surface area, such that, when several water molecules are adsorbed on the nanoparticles to increase the effective relative permittivity of their surrounding medium (a mix of air and water), thermal energy conduction from surface phonon polaritons can dominate that generated by phonons. Using this material, we resolve experimentally a surface phonon polariton thermal conductivity that is as high as 1.7 times the phonon value near room temperature. Long-range coupling of surface phonon polaritons in the ordered nanoparticle bed, analogous to the extension of phonons in an atomic crystal, enable this enhancement. In contrast to expectations, the surface phonon polaritons dictate the total thermal conductivity of the material due to an apparent quenching of phonon conduction. The effects of nanoparticle diameter and surrounding medium on thermal conductivity data are in excellent agreement with theory, providing strong evidence for significant thermal energy conduction by surface phonon polaritons. We anticipate that the theoretical, and practical, framework established here will enable heat conduction by surface phonon polaritons to be explored in detail experimentally, to build on recent interesting theoretical predictions, and to potentially provide an alternative route to engineer thermally conductive dielectric materials for thermal management.