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

Abstract

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 (Bi2Te3) 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 Bi2Te3 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.

Unveiling Local Optical Properties Using Nanoimaging Phase Mapping in High-Index Topological Insulator Bi2Se3 Resonant Nanostructures

“…These results show that the NBs are ultracompact resonators with a dielectric-like response within cross-section sizes as small as ∼λ/8. The achieved confinement is comparable to other high-index chalcogenide Mie resonators such as Bi 2 Te 3 and PbTe, with respective unit cell sizes of ∼λ/11 and ∼λ/7, , and smaller than the reported confinements in polaritonic systems, such as hexagonal boron nitride (hBN) or SiC with unit cell sizes down to ∼λ/61. − A finite-difference time-domain (FDTD) simulation (Details on FDTD simulations can be found in the Supporting Information), incorporating the extracted optical constants (Figure a) and the NB dimensions, is in good agreement with the experimentally obtained results (Figure c). Figure d shows the electric ( E , top) and magnetic ( H , bottom) field profiles of the fundamental resonant mode .…”

“…Topological insulators (TIs) constitute a distinct category of materials characterized by a bulk energy gap (insulator-like), while also hosting time-reversal symmetry-protected gapless surface states. − Fundamentally, TIs exhibit strong spin-orbit coupling, with the edge states allowing a flow of unidirectional supercurrent . These novel quantum states have been predicted theoretically as well as experimentally observed in several types of materials and systems such as CdTe/HgTe/CdTe quantum wells, strained HgTe, Sb 2 Te 3 /Sb 2 Te 3– y Se y , − bismuth selenide (Bi 2 Se 3 ), ,− Bi 2 Te 3 , , Bi 2– x Sb x Te 3– y Se y , MnBi 2 Se 4 , ZrTe 5 , and more, with extensive research toward realizing novel electronic devices. , Recently, the unique properties of TIs have been explored in the context of electromagnetic and photonic systems. − Among these, chalcogenide TIs are especially intriguing owing to their unique properties of low bulk bandgap, , exceptionally high values of permittivity, , and strong optical anisotropy . Recent studies have shown that these TIs are capable of generating high harmonics, , confining light in deep sub-wavelength structures, demonstrating unique plasmonic properties, , and hosting high-mobility surface states across a wide spectral range (visible to THz). ,, These findings highlight the vast potential of TIs in realizing novel devices with unparalleled properties. ,,,,− …”

“…Figure a shows that the complex RI dispersion is characterized by very high index values peaking at n ∼ 6.42 for λ ∼ 2.56 μm, which can be attributed to the strong dielectric polarizability of the bulk. This is followed by a Drude-like descent for longer wavelengths, reflecting the combined contribution of both the bulk free carriers, arising from unintentional doping, ,,− and from the topological metallic surface states. These latter effects are also responsible for the losses ( k ) in the system, which we examine in the following sections.…”

“…22−30 Among these, chalcogenide TIs are especially intriguing owing to their unique properties of low bulk bandgap, 16,31 exceptionally high values of permittivity, 32,33 and strong optical anisotropy. 34 Recent studies have shown that these TIs are capable of generating high harmonics, 29,35 confining light in deep sub-wavelength structures, 32 demonstrating unique plasmonic properties, 36,37 and hosting highmobility surface states across a wide spectral range (visible to THz). 13,38,39 These findings highlight the vast potential of TIs in realizing novel devices with unparalleled properties.…”

“…20,21 Recently, the unique properties of TIs have been explored in the context of electromagnetic and photonic systems. 22−30 Among these, chalcogenide TIs are especially intriguing owing to their unique properties of low bulk bandgap, 16,31 exceptionally high values of permittivity, 32,33 and strong optical anisotropy. 34 Recent studies have shown that these TIs are capable of generating high harmonics, 29,35 confining light in deep sub-wavelength structures, 32 demonstrating unique plasmonic properties, 36,37 and hosting highmobility surface states across a wide spectral range (visible to THz).…”