Deep‐Subwavelength Resonant Meta‐Optics Enabled by Ultra‐High Index Topological Insulators

In nanophotonics, small mode volumes, high‐quality factor 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 (TIs) are a class of insulating materials that host topologically protected surface states, some of which exhibit extraordinarily high permittivity values. Here, the optical properties of TI bismuth telluride (Bi 2 Te 3 ) single crystals are studied. It is found 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, it is demonstrated that Bi 2 Te 3 metasurfaces are capable of squeezing light in deep‐subwavelength structures, with the fundamental magnetic dipole (MD) resonance confined in unit cell sizes smaller than λ/10. It is further shown that dense ultrathin metasurface arrays can simultaneously provide large magnetic and electric field enhancements arising from the high index of the bulk and the surface metallic states. These findings demonstrate the potential of chalcogenide TIs 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.