Oxygen Vacancy Controlled Hyperbolic Metamaterial Based on Indium Tin Oxide (ITO) Nanotubes with Switchable Optical Properties

Abstract Nanostructured metamaterials can offer optical properties beyond what is achievable in conventional media, such as negative refraction or sub‐wavelength imaging. Due to their structural anisotropy, the class of high aspect ratio metamaterials is of interest for the possibility of achieving hyperbolic optical properties, i.e., both metallic and dielectric behavior based on the excitation direction. Although investigated numerically, the fabrication of tailor‐made metamaterials is complex or often beyond the reach of current technology. For wire metamaterials composed of aligned metallic nanowires in a dielectric matrix, since the free carrier concentration in metals is fixed, light‐matter interaction cannot be adjusted after fabrication. Here, metamaterials based on plasmonic ITO nanotubes with controllable hyperbolic response are introduced. The synthesis is achieved by a template‐based liquid‐phase technique. The tuning mechanism is based on controlling the carrier density in ITO via oxygen vacancy concentration. The process is reversible, the photonic features are activated by creating oxygen vacancies and can be switched off by filling them up again. Further, it is shown that the carrier concentration can also be controlled via a static electric field. Optical simulations support the experimental findings and highlight the parameters that determine the optical response of the metamaterial.