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

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 behavior, i.e., materials with both metallic and dielectric optical response based on the excitation direction. Although widely investigated numerically, the fabrication of tailor-made metamaterials is very complex or often beyond range using 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/ changed anymore after fabrication. Here, we introduce high aspect ratio metamaterial based on plasmonic ITO nanotubes with controllable hyperbolic response. The synthesis is achieved by a scalable template-based liquid-phase technique. Our 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.