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

Optical properties of ultrathin layers (<50 Angstrom) 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.