High-Index and Low-Loss van der Waals Topological Insulators for Mid-Infrared Nanophotonics

Strong spin-orbit coupling in topological insulators (TIs) leads to an electronic band inversion and the topological Dirac surface states in the bandgap. These surface states are chiral and thus topologically protected from back-scattering by the time-reversal symmetry. Therefore, charge carriers in surface states are free to move parallel to the surface and conduct current, while the insulating bulk remains dielectric.<br/>Such a conductive surface provides a condition for the manifestation of surface plasmons at optical frequencies in bismuth and antimony chalcogenides. For example, a particularly strong plasmonic response in visible and ultraviolet spectra has been reported in Bi_1.5Sb_0.5Te_1.8Se_1.2 (BSTS). Propagating surface plasmons in the visible spectrum have been directly observed with the scattering-type scanning near-field optical microscope (s-SNOM) in Bi₂Te₂Se (BTS). On the other hand, bulk Bi₂Se₃ (BS) is a polar dielectric and supports the THz plasmon-phonon-polaritons. THz near-field imaging by s-SNOM has been recently used to demonstrate the existence of the plasmon-phonon-polaritons in both BS and BTS.<br/>According to the theoretical models developed for these three TI materials (BS, BTS, and BSTS), all of them are expected to have a high-index low-loss dielectric bulk and a conductive surface at mid-infrared (mid-IR) frequencies. Other topological insulators such as Bi₂Te₃ and Bi:Sb:Te family are known to have a very high refractive index, but their bulk is lossy in the mid-IR.<br/>Motivated by this, we attempt to observe the interaction between the highly-confined hyperbolic phonon-polaritons (HPP) in hexagonal boron nitride (hBN) and the topological surface states in BS, BTS, and BSTS of optimized chemical composition Bi_1.5Sb_0.5Te_1.7Se_1.3. To this end, we place thin (30–115 nm-thick) exfoliated flakes of hBN on top of the exfoliated TI crystals and obtain near-field images of propagating HPP using s-SNOM. Surprisingly, the measured HPP dispersion does not reveal any unambiguous signatures of the conductive surface states in any of the three materials. To confirm this observation, we analyze the complex near-field signal over the samples with hBN and across bare TI flakes on a silicon (Si) substrate. Again, our rigorous near-field analysis does not exhibit any clear features of the conductive surface states in the TI crystals. At the same time, all experiments demonstrate the ultra-high refractive index and practically lossless nature of the three TI crystals in the tested mid-IR range (950 – 1600 cm^–1). Despite the possible existence of conductive surface states, our results suggest that in practice BS, BTS, and BSTS can be used as low-loss and ultra-high-index van der Waals dielectrics for numerous mid-IR applications.<br/>This discovery is particularly important in the context of rapidly advancing field of mid-IR hyperbolic polaritons in van der Waals crystals which recently demonstrated a plethora of new phenomena such as ghost and sheer phonon-polaritons in low-symmetry crystals, nanolight manipulation in twisted van der Waals crystals, and electrical manipulation of hybrid plasmon-phonon-polaritons in van der Waals heterostructures. All these new applications may greatly benefit from low-loss and ultra-high-index van der Waals dielectrics which would provide polaritons with significantly increased momenta without introducing additional loss.