Noble Metal Alloys for Plasmonics

The fixed optical properties of noble metals currently limit their use in photonic devices that operate at optical frequencies. To achieve metals with tunable optical response, we present noble metal alloyed thin-films formed by the binary mixture of Ag, Au, and Cu. As the dielectric functions (ε) of the alloys cannot be modeled as the linear combination of the pure metals, we combined transmission and reflection measurements of the alloyed thin films with a B-spline model that takes into account the Kromers–Kronig relation and experimentally determined the dielectric functions. We found that in some cases a mixture can provide a material with higher surface plasmon polariton quality factor than the corresponding pure metals. We independently measured the surface plasmon polariton coupling angle for all alloys using the Kretschmann configuration and found excellent agreement between the two methods when determining ε. Our approach paves the way to implement metallic thin films and nanostructures with on-demand optical responses, overcoming the current limitation of the dielectric function of noble metals.