Natural metamaterial behavior across the phase transition for WxV1−xO2 films revealed by terahertz spectroscopy

Abstract

Metamaterials, which are made of repeated patterns of appropriately arranged small discrete structures, display unusual electromagnetic properties that overwhelm those of conventional materials. The modification of their properties is generally achieved by arranging the structures mechanically or electrically and requires rather complex designs. We report on the study of the complex conductivity of epitaxially-grown tungsten-doped vanadium dioxide (WxV1−xO2) thin films through the semiconductor-to-metal phase transition (SMT) using terahertz time-domain spectroscopy. The modelling of the terahertz conductivity across WxV1−xO2 SMT provides clear insights about the gradual nucleation of VO2 metallic domains among the semiconducting host and evidences the presence of strong carrier confinement and enhanced absorption close to the transition temperature, leading to a strong capacitive response of the electrons. The evolution of the SMT is also strongly affected by W doping, which reduces the scattering time in the metallic state, lowers the transition onset temperature and extends the temperature range over which the transition occurs. The WxV1−xO2 films thus forms an effective medium in the vicinity of the SMT and display the signature of a disordered metamaterial, which has significantly enhanced functionality thanks to its readily thermally-tunable properties over a wide range of temperatures close to room temperature.