Shedding Light on the Mid-Infrared Complex Refractive Index of Anodic Aluminum Oxide

In the current scientific landscape, the understanding of optical properties in the mid-infrared (mid-IR) range (3–30 µm) is crucial in simulations and models to explore the potential of materials for various applications. However, due to the challenges associated with mid-IR characterization, accurate refractive index (n) and extinction coefficient (κ) data are often lacking in the literature. This study addresses this gap by investigating the mid-IR n and κ spectra of anodic aluminum oxide (AAO) nanostructures anodized under different conditions, using two distinct approaches: IR ellipsometry and a theoretical model based on multilayer reflection and effective medium. The results demonstrate a strong agreement: the anodizing conditions have a significant influence on the optical properties of the AAO nanostructures. These differences enable accurate simulations of the emissivity spectra of AAO nanostructures on Al foils, which align closely with experimental measurements. This theoretical approximation is versatile and extensible to a broad range of materials. Different materials are tested, namely, a sapphire, a polycarbonate film, and a polyethylene terephthalate (PET) film achieving a useful qualitative description. This study paves the way for a novel approach in the engineering of new micro and nano-optical materials, facilitating their evaluation for suitability in mid-IR applications.