Fundamental acoustic vibrations in nanoscale aluminum coatings.

The proper design of optical nanoscale devises, such as Fabry-Perot interferometers used for polariton construction, relies on scientists and engineers understanding the fundamental physical properties of metallic coatings. While properties such as the extinction coefficient, refractive index, and skin depth are well understood and have been modeled for many metals, acoustic phonon vibrations within metallic films at nanoscale thicknesses have only recently become a topic of discussion within the optical community. This is largely because the observance of acoustic phonon vibrations in thin films is relegated to having an impact on ultrafast transient spectroscopy, which has only become a major research tool over the past few decades. Through this manuscript the optical properties of aluminum thin films is discussed. It is determined that the period of the phonon vibration has a linear relationship to film thickness. A formalism relating phonon period to film thickness via the speed of sound is introduced, and it is shown to closely match with the well-stablished Debye equation used to model phonon vibrations.