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The research of new plasmonic materials [1], 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) [2] 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 [5]. The possibility of generate volume-plasmon polariton waves in VO2 metamaterial is eventually discussed.
[1] G.V. Naik, et al., Adv. Mater., 25, 3264–3294, (2013).
[2] A. Calzolari, et al., ACS Photonics, 1, 703-709, (2014).
[3] A. Catellani, et al., J. Mater Chem. C, 3, 8419-8424, (2015).
[4] S. Benedetti et al., PCCP. Phys (2017), in press.
[5] M. Eaton et al., (2017) submitted.
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