Damping effect in Drude-Lorentz model for ultrathin silver films in the mid-infrared

Ultrathin metal films are critical for mid-infrared optics, but the determination of their optical constants remains challenging due to discrepancies between theory and experiment. We propose a robust Drude-Lorenz model-based method incorporating multi-level scattering and surface/boundary effects to predict the optical constants of ultrathin silver films. Experimental validation using multilayer mid-infrared absorbers shows excellent agreement with measurements. The approach, which can be extended to other metals, provides a versatile tool for exploring ultrathin film properties and applications by elucidating intrinsic physical mechanisms at extreme thicknesses. This work advances the precise design of ultrathin film-based optical devices through improved theoretical-experimental agreement.