Observation of Heat Conduction in Nanoscale Polaritonic Waveguides

Heat conduction in solids is typically mediated by phonons and electrons with short wavelengths and mean free paths. Surface polaritons—hybrid quasiparticles formed by the coupling of photons with matter oscillations—present a novel mechanism for thermal transport, particularly in nanostructures where the polariton wavelength and mean free path could be greater than sample characteristic size and thus unusual transport phenomena could arise. Surface phonon polaritons (SPhPs) originate from the coupling of thermal photons with optical phonons at the surface of polar dielectric materials, while surface plasmon polaritons (SPPs) involve the coupling of photons with free electron oscillations at the interface of a metal and a dielectric. Both phenomena offer promising avenues for extraordinary heat transfer, with polaritons acting as thermal waveguides that carry significant energy flux along surfaces, in contrast to conventional thermal radiation.

Here, we present our studies on the observation of thermal conductivity mediated by SPhPs and SPPs. In the case of SPhPs, we used nanoribbon waveguides made of SiO₂ (a polar dielectric). 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 [1]. For the observation of SPP-mediated heat conduction, we used dielectric nanoribbons coated with thin metal films where both the lattice and electronic thermal conductivities are suppressed. SPPs, which propagate along the metal-dielectric interface, exhibit shorter propagation lengths than SPhPs but are active over a broader spectral range relevant to thermal transport, making them a promising candidate for enhancing thermal conductivity. Indeed, our experiments show enhanced thermal conductivity with unique signatures of SPP-mediated heat conduction. Our findings on heat conduction mediated by polaritons (both SPhPs and SPPs) could open new possibilities for engineering nanoscale thermal transport beyond the realms of electrons and phonons.
Reference
[1] Pei, Y., Chen, L., Jeon, W., Liu, Z. and Chen, R., 2023. Low-dimensional heat conduction in surface phonon polariton waveguide. Nature Communications, 14(1), p.8242.