Low-Dimensional Heat Conduction in Surface Phonon Polariton Waveguide

Heat conduction in solids is governed by Fourier's law, describing a diffusion process primarily driven by phonons and electrons with short wavelengths and mean free paths. Non-Fourier heat conduction, observed in exotic systems, remains elusive in typical solids. Surface phonon polaritons (SPhP) are evanescent surface waves resulting from the collective oscillation of atoms on the surface of polar dielectric materials. They couple thermal photons and optical phonons, potentially contributing significantly to heat conduction, particularly in nanostructures with diminished volumetric heat conduction by phonons and electrons. Unlike thermal radiation, SPhP waves carry a high energy flux along the solid surface, acting as thermal waveguides. Surface phonon polaritons (SPhP) offer a promising avenue for extraordinary heat transfer due to their longer wavelength and propagation length. However, their low energy flux has hindered direct observation of SPhP-mediated heat conduction.<br/><br/>Here, we demonstrate direct observation of enhanced thermal conductivity mediated by SPhP in SiO2 nanoribbon waveguides. To distinguish SPhP from phonons, the waveguide cross-section must be reduced. However, this introduces challenges related to wave leakage and inefficient coupling with thermal reservoirs. Our study addresses these challenges by designing nanoribbon waveguides of SiO2 with controlled SPhP mode size and efficient coupling to thermal reservoirs using an absorber. We directly observe an increased thermal conductivity in these waveguides, surpassing the limit of phonon thermal conductivity by up to 34%. Moreover, we observe non-Fourier behavior in SPhP thermal conductance over 50-100 μm distances at room and high temperature, indicating the unique characteristics of polaritonic heat conduction. Our work represents the first direct observation of enhanced thermal conductivity mediated by SPhP and provides insights into the potential of manipulating heat conduction beyond traditional limits. These findings open up new possibilities for engineering thermal transport in various applications.