Advanced Optical Materials 2302797

Topological insulators generally have dielectric bulk and conductive surface states. Consequently, some of these materials are shown to support polaritonic modes at visible and THz frequencies. At the same time, the optical properties of topological insulators in the mid-infrared (mid-IR) remain poorly investigated. Here, near-field imaging is employed to probe the mid-IR response from the exfoliated flakes of bismuth (Bi)/selenide (Se)/telluride (Te)/antimony (Sb) crystals with varying stoichiometry – Bi₂Se₃, Bi₂Te₂Se, and Bi_1.5Sb_0.5Te_1.7Se_1.3 – in pristine form as well as covered by thin flakes of hexagonal boron nitride (hBN) is employed. Contrary to theoretical expectations, all three materials exhibit a dielectric response with a high refractive index and with a loss below the experimental detection limit. Particularly, the near-field mapping of propagating phonon-polaritons in hBN demonstrates that all three van der Waals crystals with different stoichiometry act as a practically lossless dielectric substrate with an ultra-high refractive index of up to 7.5 in Bi₂Te₂Se. Such a unique dielectric crystal will be of great advantage for numerous nanophotonic applications in the mid-IR.