Reduction of the Casimir force in Al due to oxidation

The modification of the Casimir force due to the oxidation of an aluminum layer is calculated as a function of the oxidation fraction of the surface. Aluminum has a large plasma frequency, and thus a large Casimir force is expected. As the surface oxidizes, it forms a thin oxide layer that is passivated. The effect of this layer is to decrease the force significantly, even though the oxide layer is only 5 nm thick. To understand the large effect on the force, we calculate the reflectograms in real-frequency space and show the degradation of the evanescent modes due to the growth of the oxide layer.