  In nanophotonics, small mode volumes, high-quality factor (Q) resonances, and
large field enhancements without metals, fundamentally scale with the
refractive index and are key for many implementations involving light-matter
interactions. Topological insulators (TI) are a class of insulating materials
that host topologically protected surface states, some of which exhibit
extraordinary high permittivity values. Here, we study the optical properties
of TI bismuth telluride (Bi2Te3) single crystals. We find that both the bulk
and surface states contribute to the extremely large optical constants, with
the real part of the refractive index peaking at n~11. Utilizing these
ultra-high index values, we demonstrate that Bi2Te3 metasurfaces are capable of
squeezing light in deep subwavelength structures, with the fundamental magnetic
dipole (MD) resonance confined in unit cell size smaller than {\lambda}/10. We
further show that dense ultrathin metasurface arrays can simultaneously provide
large magnetic and electric field enhancements arising from the surface
metallic states and the high index of the bulk. These findings demonstrate the
potential of chalcogenide TI materials as a platform leveraging the unique
combination of ultra-high-index dielectric response with surface metallic
states for metamaterial design and nanophotonic applications in sensing,
non-linear generation, and quantum information.
