Observation of the skin-depth effect on the Casimir force between metallic surfaces

We have measured the attractive Casimir force between a metallic plate and a transparent sphere covered with a palladium thin film. When the thickness of the coating is less than the skin-depth of the electromagnetic modes that mostly contribute to the interaction, the force is significantly smaller than that measured with a thick bulk-like film. Our results are in agreement with theoretical predictions and are the first direct evidence of the skin-depth effect on the Casimir force between metallic surfaces.