Optical properties of ultrathin films: evidence for a dielectric anomaly at the insulator-to-metal transition.

Optical properties of ultrathin layers (<50 A) of Au and Pb quench condensed on amorphous germanium (alpha-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(square). Of particular interest is the regime of R(square) near 3000 Omega 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.