Deep-subwavelength resonant metaphotonics enabled by high-index topological insulator bismuth telluride

TL;DR

This paper demonstrates that topological insulator bismuth telluride can be used to create deep subwavelength nanophotonic resonators with high field enhancements, leveraging its ultra-high refractive index and surface states.

Contribution

The study reveals the optical properties of Bi2Te3, showing its potential for deep subwavelength resonances and enhanced fields in metasurfaces, combining high index and metallic surface states.

Findings

Refractive index of Bi2Te3 peaks at ~11.

Magnetic dipole resonance confined in <λ/10 unit cell.

Ultrathin metasurfaces exhibit large field enhancements.

Abstract

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.