Tailoring the Optical Properties of Nanoscale-Thick Metal–Dielectric Ag–SiO₂ Nanocomposite Films for Precision Optical Coating Integration

Thin-metal films can be challenging to process at ultrathin thicknesses (≲10 nm) due to poor wetting (or surface tension compatibility) at dielectric interfaces. Typical thickness dimensions required to induce the onset of coalescence is usually ≳20 nm. Films <20 nm in thickness can result in non-ideal process-dependent film uniformity and morphology that prevent controlled and repeatable ultrathin-film optical properties and behavior. Such thickness limitations undesirably constrain the design and integration of thin-metal films into high-precision multilayer optical coatings (e.g., narrow bandpass filters and induced transmission filters). The co-sputtering of nanocomposite metal–dielectric films offers an appealing route toward ultrathin film coalescence and tailorable optical properties to achieve high-precision optical performance at significantly reduced film thicknesses (e.g., as compared to conventional all-dielectric multilayer optical media). In this work, silver (Ag) nanoparticles and contiguous Ag networks embedded in a silicon dioxide (SiO2) matrix were prepared at ambient substrate temperature via magnetron co-sputtering in a controlled pure argon atmosphere. We show that the structural features and optical properties of nanocomposite Ag–SiO2 films can be manipulated by varying the co-sputtering duration at ∼3–10 nm film thicknesses. Here, the Ag material phase ranges in structure from dispersed nanoparticles to contiguous partially coalesced networks. A distinct optical response transition occurs upon Ag phase transition from nanoparticles to the partially coalesced network. Large differences in the measured optical intensity are observed at these reduced film thicknesses: maximum ΔT = 67%, ΔR = 28%, and ΔA = 46% in the visible and near-infrared regions. Overall, our work shows the tailoring of ultrathin-metal-film optical properties (i.e., the refractive index, n, and extinction coefficient, k) and is expected to provide implementable methodologies toward the design, deposition, and integration of next-generation complex index multilayer optical filters and mirrors exhibiting enhanced precision spectral performance.