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.
The research of new plasmonic materials , alternative to standard noble metals, for the realization of photonic devices in the THz-to-visible range is continuously increasing. In this regard, new classes of materials such as transparent conductors and phase change materials (PCMs) have been proposed as promising plasmonic and/or hyperbolic metamaterials in the visible and infrared (IR) range. From one hand, transparent conductors (TCs) are electrical conductive materials with a low absorption of light in the visible range. The unique combination of metallicity and transparency makes them appealing for a variety of applications, including photovoltaic cells, flat displays, invisible electronics and waveguides. TCs are obtained by doping wide band-gap semiconductors with metal ions. Yet, the remarkable combination of conductivity in an albeit wide-gap (transparent) material is not fully understood, along with the effect of dopants and defects on charge transport and reflectivity. On the other hand, PCMs can undergo electronic and structural transitions, upon thermal, electrical, chemical or mechanical excitations. Materials that undergo metal-insulator transitions are particularly appealing as they radically modify their electrical and optical properties. This unique property is largely used to realized multi-switchable photonic devices such as plasmonic nanoantennas, ultrafast light emission modulators and near-field thermal transfer device.
Here, by using first principles approaches based on DFT for the characterization of single materials and effective medium theory (EMT) for the characterization of composites, we present the optoelectronic and plasmonic properties of two different classes of metal-oxide materials: transparent conducting oxides (Al-ZnO and Ta-TiO2) and metal-oxides PCM (VO2). In the first case, we investigate the microscopic effects of metal doping (e.g. Al, Cu, Ta)  and defects (e.g. vacancies) [3-4] on the optical and electronic properties of TCOs and how this reflects on the plasmonic response of surface-plasmon polaritons or layered hyperbolic metamaterials, in connection with other dielectric media (e.g. ZnO, ZnS, etc). In the second case, we focus on disordered mixtures and planar homostructures resulting from the coexistence of metallic and semiconducting phases of VO2. This joint-phase combination, which has been experimentally realized, gives rise to an optical metamaterial without the introduction of other different media. This homojunction exhibits tunable optoelectronic properties, with highly anisotropic permittivity, and type-II hyperbolic behaviour in the mid-IR . The possibility of generate volume-plasmon polariton waves in VO2 metamaterial is eventually discussed.
 G.V. Naik, et al., Adv. Mater., 25, 3264–3294, (2013).
 A. Calzolari, et al., ACS Photonics, 1, 703-709, (2014).
 A. Catellani, et al., J. Mater Chem. C, 3, 8419-8424, (2015).
 S. Benedetti et al., PCCP. Phys (2017), in press.
 M. Eaton et al., (2017) submitted.
The development of plasmonic and metamaterial devices requires the research of high-performance materials alternative to standard noble metals. Recently refractory Titanium Nitride has been proposed as a valid alternative to gold,  even for application in harsh and high-temperature environments. Indeed, being refractory this compound exhibits extraordinary mechanical stability over a large range of temperatures (∼2000 ◦ C) and pressures (∼3.5 Mbar), well above the melting point of standard noble metals (∼800 ◦ C). This material is furthermore resistant to corrosion and compatible with silicon technology. TiN has optical and plasmonic properties (color, electron density, plasmon frequency) very similar to gold and has been exploited for the realization of waveguides, broadband absorbers, local heaters, and hyperbolic metamaterials in connection with selected dielectric media (e.g., MgO, AlN, sapphire, etc.).
Even though the fundamental mechanical and optoelectronic properties of TiN have been largely studied so far from experimental and theoretical points of view, very little is know about its plasmonic behavior. Here, we present a fully-first-principles investigation, based on time-dependent density functional theory (TDDFT), of the plasmon properties of stoichiometric titanium nitride. The microscopic origin of plasmonic excitations are analyzed in terms of the fundamental collective and/or radiative exctations of TiN electronic structure. From the simulation of energy-loss spectra at different momentum transfer, we derive the TiN plasmon dispersion relations that are directly accessible by experimental measurements.
We furthermore analyze different interfaces between TiN and conventional semiconductors in order to describe TiN surface-plasmon polaritons for the realization of hyperbolic metamaterials and waveguides. We also investigated the optoelectronic charcateristics of the compound in relation to the crystal phase transition, experimentally observed at very high pressure. The microscopic origin of the plasmon resonances and their dispersions have been discussed on the basis of the analysis of the electronic structure and of the interplay between collective and single-particle excitations, which determine the screening and dissipation effects of the electronic system.
The similarities and the differences with other noble metals, in particular with gold, are thoroughly discussed all along the paper.
Our ab initio results confirm that at standard conditions TiN exhibits plasmonic properties in the visible and near-IR regime, very close to gold, in agreement with experimental data. In contrast with malleable noble metals, the hardness of refractory ceramics allows for the exploitation of plasmonic properties also at high temperature and under pressure, conditions where standard plasmonic materials cannot be used.
 G. V. Naik, V. M. Shalaev, and A. Boltasseva, Science 344, 263 (2014).
 A. Catellani and A. Calzolari, Phys. Rev. B 95, 115145 (2017)
Using simulations from first principles we investigate the microscopic role of doping on the optoelectronic properties of X-doped ZnO (XZO, X=Al, F), as transparent conductive oxide for energy applications. We show how the interplay between (co)dopants and defects affects TCO characteristics of the samples. Finally, we study the plasmonic activity of XZO in the near-IR/visible range and in particular at wavelength relevant for telecommunications (1.5 μm), confirming recent experimental results.