  Hyperbolic metamaterials (HMMs) are engineered materials with a hyperbolic
isofrequency surface, enabling a range of novel phenomena and applications
including negative refraction, enhanced sensing, and subdiffraction imaging,
focusing, and waveguiding. Existing HMMs primarily work in the visible and
infrared spectral range due to the inherent properties of their constituent
materials. Here we demonstrate a THz-range Dirac HMM using topological
insulators (TIs) as the building blocks. We find that the structure houses up
to three high-wavevector volume plasmon polariton (VPP) modes, consistent with
transfer matrix modeling. The VPPs have mode indices ranging from 126 to 531,
10-100x larger than observed for VPP modes in traditional media while
maintaining comparable quality factors. We attribute these properties to the
two-dimensional Dirac nature of the electrons occupying the topological
insulator surface states. Because these are van der Waals materials, these
structures can be grown at a wafer-scale on a variety of substrates, allowing
them to be integrated with existing THz structures and enabling next-generation
THz optical devices.
