We present here measurements of the Casimir force gradient in the 60-300 nm range using a commercial Atomic Force Microscope operating in Ultra High Vacuum (UHV). The measurements were carried out in the sphere-plate geometry between a Au sphere and plates consisting of two different classes of material, that is a metal (Au) and a semimetal (HOPG). The variation in the optical properties of the materials produces clearly observed differences in the Casimir force as predicted by calculations based on the quantum theory of optical networks and the Lifshitz theory.