  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.
