Thermal transport by surface plasmon polaritons in thin metallic film

Abstract

This study aims to investigate both theoretical and experimental aspects of thermal transport within thin metal films through the mediation of surface plasmon polaritons (SPPs), a type of surface electromagnetic wave arising from the interaction between photons and free electrons. We first experimentally demonstrated boosted in-plane thermal conduction by SPPs propagating along a thin Ti film on a SiO2 substrate. Due to the lossy nature of metal, SPPs can propagate over centimeter-scale distances even along a supported metal film, and the resulting ballistic heat conduction can be quantitatively validated. Further, for a 100-nm-thick Ti film on a SiO2 substrate, a significant enhancement of in-plane thermal conductivity compared to bulk value (~25%) is experimentally shown. Then, we investigated the in-plane thermal conductivity of SPPs propagating along thin noble metal (Au and Ag) films on a SiO2 substrate with a Ti adhesive layer. Our theoretical predictions revealed a decrease of approximately 30% in plasmon thermal conductivity when considering the size effect of the permittivity of thin metal films. By fabricating a sample with the optimal thickness of Au and Ag films, we experimentally demonstrated that the plasmon thermal conductivity of Au and Ag films can be as high as about 20% of their electron contribution. Also, the study employed theoretical validation through fluctuational electrodynamics, which take account of the full-wave nature of surface electromagnetic waves, to substantiate the concept of ballistic heat transport via SPPs. It encompassed the calculation of the heat flux carried by electromagnetic waves along metal films, examining its dependence on film thickness, and comparing these outcomes with results derived from kinetic theory. This study opens new avenues for leveraging SPPs in the context of device-level thermal management of electrical devices by validating the thermal transport of SPPs. These insights can be readily applied to address and mitigate thermal hot-spot issues in microelectronics.