Enhancing In-Plane Thermal Conductivity of Ag and Au Films

In nanophotonic applications based on surface plasmon polariton (SPP) resonance, the introduction of a metallic adhesion layer (e.g., Ti) to stabilize noble metal thin films is widely known to induce severe optical damping. Consequently, it is intuitively presumed that the adhesion layer similarly suppresses SPP-mediated heat transfer due to this intrinsic optical loss. Here, we theoretically and experimentally demonstrate that the reduction in plasmon thermal conductivity does not originate from intrinsic optical damping, but exclusively from a macroscopic geometric dilution effect resulting from the increased total film thickness. By minimizing the Ti adhesion layer thickness from 10 to 1 nm, we ensure the physical continuity of 20 nm-thick Ag and Au films on a substrate while fully preserving their intrinsic SPP propagation characteristics. Utilizing steady-state thermoreflectance (SSTR), we reveal that the thermal conductivity enhancement increases by approximately 53% for Ag and 54% for Au when employing a 1 nm Ti layer compared to a 10 nm Ti layer. Furthermore, this minimization of the Ti layer allows for a reduction in the lateral size required to achieve the same thermal conductivity enhancement by approximately 40–50%. Consequently, minimizing the Ti layer thickness is an effective and practical strategy to avoid the suppression of the thermal conductivity enhancement of metallic thin films and resolve spatial constraints for device thermal management.