Nonlinear Charge Density Fluctuation-Driven Plasmonic Enhancement in Au–Ag Thin Films

Abstract Bimetallic alloys of gold and silver have gained considerable attention due to their broad utility in catalysis, biomedicine, and optical technologies, owing to their enhanced surface plasmon polariton (SPP) quality factor compared to the corresponding pure metals. Earlier attempts to understand the central reason for this improved performance, despite their noncommensurate stoichiometries, have not yielded a clear picture. To address this, we performed spectroscopic ellipsometry together with Hall measurements on composition-controlled, equal-thickness Au–Ag alloy films. By examining the interdependence of plasma frequency, carrier concentration, and effective mass, we found that a nonlinear rise in carrier density is the dominant factor behind the improved optical response. To further understand this anomalous carrier density behavior, the Mayadas–Shatzkes (MS) model was applied, revealing how polycrystallinity and grain-boundary scattering influence charge-transport dynamics in the alloys. Additionally, the composition-dependent nonlinear trends in the extracted physical parameters were modeled using a bowing-type polynomial mixing approach. This combined experimental–theoretical framework not only aligns the measured results with a consistent physical model but also accounts for discrepancies based on the MS formalism. By establishing a direct link between microstructural effects, electronic properties, and optical response, this study resolves the origin of the nonlinear optical behavior observed in stoichiometric ratios of bimetallic thin films.