Abstract: This study proposes and validates a novel strategy for fabricating ultra-low refractive index films based on nanostructure regulation via secondary chemical etching. Initially, Ag nanowire array-silica composite metamaterial films were prepared using multi-target magnetron co-sputtering technology. After the first-step chemical etching process to remove the metallic phase, an air-nanocolumn array-silica composite metamaterial film was successfully constructed. However, due to the limited porosity achieved by the first etching step, the resulting refractive index could not meet the requirement for further reduction. To address this issue, this work innovatively introduces a secondary chemical etching step to enlarge the diameter of the air nanocolumns, thereby increasing the porosity and successfully obtaining a composite metamaterial with an ultra-low refractive index (<1.2). The optical properties and microstructure of the films were characterized using spectroscopic ellipsometry and scanning electron microscopy. Based on the anisotropic effective medium approximation (EMA) model, the ellipsometric parameters Ψ and Δ were fitted to establish the relationship between porosity and anisotropic refractive indices, clarifying the minimum refractive index achievable by the first etching step. The effects of secondary etching time and etchant concentration on the refractive index were systematically investigated, demonstrating that the proposed ultra-low refractive index tuning method exhibits excellent process repeatability. This enables the controllable preparation of films with an ordinary refractive index ranging from 1.367 to 1.159 and an extraordinary refractive index between 1.392 and 1.191. This research provides a new pathway for the controllable preparation of ultra-low refractive index materials, holding significant application potential in photonic devices such as automotive lenses and display panels.

Key words: Metamaterial film, Low refractive index, Nano-air column array, Chemical etching, Magnetron sputtering