I am simulating an FSS structure (unit cell) in CST. You can find the overall structure in one of the figures attached. The FSS structure comprises a metamaterial component (depicted in bluish color) and a mesh-like film stack (depicted in yellowish color). The film stack consists of an aluminum (Al) layer and an alumina (Al2O3) layer. The overall stack can be described as metamaterial-Al-Al2O3.

My objective is to calculate the reflectance of the FSS structure using the frequency domain solver. However, I have observed an unusual behavior in the reflectance plot when considering different film stacks (see attached plot). Specifically, after adding a very thin Al layer (0.1 nm) on top of the aluminum, the reflectance increases dramatically. As the thickness of the Al2O3 layer increases, the reflectance gradually approaches the behavior exhibited by the blue curve. The discontinuous behavior of reflection is very unexpected and I suspect it may be a simulation artifact. I would greatly appreciate any insights or recommendations on how to properly handle thin film simulations and address potential artifacts to ensure accurate reflectance calculations from my structure.

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