Optical Trajectory of the Insulator-to-metal Transition of Ultra-thin Gold Films

We apply interferometric picometrology to measure the complex refractive index and dielectric properties of ultra-thin gold films as a continuous function of thickness from 0.2 nm to 10 nm deposited on thermal oxide on silicon. Three distinct regimes are found for the dielectric constant trajectory in the complex plane. The first regime shows a circular trajectory predicted by the Drude equation tracing the gold bulk-to-thin film transition. The second and third regimes are manifested as metal clusters range from large to small size and sparse coverage. The entire atom-to-bulk transition of gold is revealed from the optical perspective.