Optical Properties of Ultrathin Films: Evidence for a Dielectric Anomaly at the Insulator-to-Metal Transition

Optical properties of ultrathin layers (<50) of Au and Pb quench condensed on amorphous germanium (α-Ge) have been measured in situ at 10 K. The development of these films from an insulating state to a metallic state is traced as a function of the film thickness as well as the sheet resistance, R_. Of particular interest is the regime of R_ near 3000 Ω where there is an anomaly in the optical transmission. This anomaly is due to a singularity in the dielectric function when the system undergoes an electronic percolation or insulator-to-metal ($I/M$) transition.