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As a result of recent developments in nanofabrication techniques, the dimensions of metallic building blocks of plasmonic devices continue to shrink down to nanometer range thicknesses. The strong spatial confinement in atomically thin films is expected to lead to quantum and nonlocal effects, making ultra-thin films an ideal material platform to study light-matter interactions at the nanoscale. Most importantly, the optical and electronic properties of ultra-thin plasmonic films are expected to have a strong dependence on the film thickness, composition, strain, and local dielectric environment, as well as an increased sensitivity to external optical and electrical perturbations. Consequently, unlike their bulk counterparts which have properties that are challenging to tailor, the optical responses of atomically thin plasmonic materials can be engineered by precise control of their thickness, composition, and the electronic and structural properties of the substrate and superstrate. This unique tailorability establishes ultra-thin plasmonic films as an attractive material for the design of tailorable and dynamically switchable metasurfaces.
While continuous ultra-thin films are very challenging to grow with noble metals, the epitaxial growth of TiN on lattice matched substrates such as MgO allows for the growth of smooth, continuous films down to 2 nm. In this study, we present both a theoretical and an experimental study of the dielectric function of ultrathin TiN films of varying thicknesses. The investigated ultrathin films remain highly metallic, with a carrier concentration on the order of 1022 /cm3 even in the thinnest film. Additionally, we demonstrate that the optical response can be engineered by controlling the thickness, strain, and oxidation. The observed plasmonic properties in combination with confinement effects introduce the potential of ultra-thin TiN films as a material platform for tailorable plasmonic metasurfaces.
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