Tunable Infrared Metasurface of Quasi-Metal Transparent Conductive Oxide for Thermal Management Applications

This study reports a novel class of transparent conductive oxide (TCO) of TiO₂:Nb that exhibits the carrier density comparable to metals for infrared plasmonic applications. Recently, TCOs have emerged as alternative plasmonic materials to replace noble metals, owning to their CMOS compatibility, tunability of optical and structural properties, etc. The plasmonic properties of TCOs are determined by their carrier densities, however, most of the studies, so far, reported the carrier densities (~10²⁰/cm²) much lower than those in nobel metals (~10²²/cm²). It consequently limits the realization and improvement of plasmonic properties in TCOs. In this study, we demonstrate a new method to significantly increase the carrier density in transparent conductive oxides by doping of a small amount of silver (Ag) into conductive oxide thin film (TiO₂:Nb), while maintaining the crystalline structure and the visible transparency of TiO₂ film. The tunability of optical properties of Ag-Nb co-doped TiO₂ in mid-infrared regime was charaterized by ultrafast transient infrared spectroscopy. Thin films were fabricated by co-sputtering Ag and TiO₂:Nb (9% Nb) targets at appropriate ratios. After deposition, the amorphous films underwent the annealing process at 350-400^oC in vacuum. This thermal treatment induced the diffusion and the ion exchange of Ag during the crystallization of Nb-doped TiO₂, resulting in the co-doping of Ag of Nb into the anatase structures of TiO₂. The doping of Ag improves the carrier density while has subtle effect on the visible transparency TiO₂ films. The Hall measurement results of 100 nm-thick Ag-Nb co-doped TiO₂ films show carrier density at 2.1×10²²/cm². This value is two-order larger than reported TCOs and at the same level with nobel metals. The film exhibits the transparency of 85% in the visible regime, which is comparable to TiO₂:Nb without Ag doping. The plasmonic properties of Ag-Nb co-doped TiO₂ were investigated by fabricating metasurface of nanostructures on thin films and characterizing their optical properties. For example, nanorod structures fabricated on a 100 nm-thick Ag-Nb co-doped TiO₂ thin film, with different rod sizes. The corresponding transmittance spectra reveal distinct plasmon dips in the infrared regime, and the decreasing resonant wavenumbers with larger rods, that clearly indicates the plasmonic property in Ag-Nb co-doped TiO₂. Moreover, the plasmon resonance can also be tuned by the doping level of Ag. The achieved plasmonic property can be attributed to the extremely high carrier density and its controllability by Ag doping levels. We also demonstrated the dynamic tuning and switching of optical properties of Ag-Nb co-doped TiO₂ metasurface by thermal and electrical triggers. We observe redshifts of localized surface plasmon resonances arising from a change of the plasma frequency of Ag-Nb co-doped TiO₂, which is governed by the conduction band non-parabolicity. Herein we have demonstrated a simple method of Ag doping for TCOs thin film and demonstrated a novel class of quasi-metal TCOs that exhibit distinquished plasmon properties and tunability in mid-infrared regime. The tunability of optical properties in mid-infrared promises their wide applications for effective usage of solar energy, such as smart windows that allow us adjust light and thermal at ultra-low energy consumption. This work also demonstrates a new scheme to control infrared plasmons for optical switching, telecommunications and sensing.