Recently a new class of optical interference coatings was introduced which comprises ultra-thin, highly absorbing dielectric layers on metal substrates. We show that these lossy coatings can be augmented by an additional transparent subwavelength layer. We fabricated a sample comprising a gold substrate, an ultra-thin film of germanium with a thickness gradient, and several alumina films. The experimental reflectivity spectra showed that the additional alumina layer increases the color range that can be obtained, in agreement with calculations. More generally, this transparent layer can be used to enhance optical absorption, protect against erosion, or as a transparent electrode for optoelectronic devices.
We acknowledge the financial support from the Air Force Office of Scientific Research under grant numbers FA9550-12-1-0289 and FA9550-09-1-0669-DOD35CAP. M. Kats was supported by the National Science Foundation through a Graduate Research Fellowship. M. Kolle was sup- ported by the Alexander-von-Humboldt Foundation. The thin film depositions were performed at the Harvard Center for Nanoscale Systems, which is a member of the National Nanotechnology Infrastructure Network.