Visible Range Plasmonic Modes on Topological Insulator Nanostructures

Abstract

CommuniCationplasmon resonances in the entire visible spectrum by varying the partial ratio of the building elements in TI compounds, [9,13] bearing high potential for photonics applications. Recent studies revealed potential benefits of using TI visible range plasmons for light harvesting applications. [7,9] TIs form an intrinsic core-shell structure with metal-like surface state layer and insulating bulk, while its practical applications in optical or electronic devices require low residual conductivity of the bulk. [7] Bi 2-x Sb x Te 3-y Se y topological insulators show well pronounced surface dominated electronic transport and highly insulating bulk interior, [5,14] compared to simpler and more studied compounds such as Bi 2 Se 3 , Bi 2 Te 3 , Sb 2 Te 3 . [5] In this work, we report first near-field optical observations of visible range plasmons in Bi 1.5 Sb 0.5 Te 1.8 Se 1.2 (BSTS [6,[14][15][16] ) nano-and microstructures. By using scattering-type technique we accessed both, the amplitude and the phase of the plasmons field, and revealed dipolar and higher-order plasmon modes on artificially carved nanostructures and naturally exfoliated flakes.BSTS was synthesized from high-purity Bi, Sb, Te, and Se by modified Bridgman method as in our previous works. [6,14] The resulting material forms high quality single crystals, which were mechanically exfoliated on commercial thermally oxidized silicon substrate (285 nm of SiO 2 ). This method allows easy fabrication of the TI micro-and nanoflakes with typical thicknesses ranging from few to several hundred nm. We investigate both naturally occurring flakes, following exfoliation, and artificially fabricated nanostructures, carved by further focused ion beam (FIB) milling of the flakes (see details in Supporting Information). All samples were capped with 5 nm of SiO 2 to prevent the TI from atmosphere exposure. The capping has been realized by physical vapor deposition in high vacuum (3 × 10 −7 mbar), while the sample was attached to water-cooled holder.For experimental imaging of TI plasmons, we use polarization sensitive scattering-type scanning near-field optical microscope (s-SNOM) [17] operating at wavelength of 633 nm. s-SNOM is based on Neaspec commercial instrument which we upgraded with a balanced detection. To excite/detect localized plasmons in BSTS, we elaborate sp-geometry scheme [18] for measurements (see sketch of the experiment in Figure 1a). Here the sample is illuminated in oblique configuration with s-polarized cw laser radiation. The incident laser beam excites electromagnetic oscillations in the TI nanostructure, which are mapped by recording s-SNOM tip-scattered signal at each point of the scan. P-polarized component of the scattered light is selected by a polarizer placed in front of the detector. Pseudoheterodyne interferometric detection at nonlinear harmonics of the tip tapping frequency (fourth and third in this work) [19] allows us to measure amplitude and phase of the scattered Surface plasmons are electromagnetic excitations existing at ...

Interband transitions in semi-metals, semiconductors, and topological insulators: a new driving force for plasmonics and nanophotonics [Invited]

“…BSTs and BSTSs belong to the last generation of topological insulators which have attracted the attention of the scientific community in relation with their outstanding surface transport properties. The experimentally determined dielectric function of the Bi1.5Sb0.5Te1.8Se1.2 was reported by Ou et al [15], and Dubrovkin et al [65]. Yin et al [66] reported quantum simulated dielectric functions for several ternary and quaternary compounds: Bi2Se2Te, Bi2SeTe2, BiSbTeSe2, BiSbTe2Se.…”

“…The optical and resonant properties of nanostructures based on V-VI topological insulators are since recently focusing the attention [15,65,[91][92][93][94][95][96]. Resonances Zhao et al [92] also demonstrated an active switching of these resonances upon crystalline to amorphous phase transition and discussed their potential for enhancing the performance of polymer solar cells.…”

Plasmonics of topological insulators at optical frequencies

“…12,13 In addition, multiple plasmon modes were recently reported in solution-synthesized Bi 2 Te 3 nanoplates 14 and nano-discs and flakes of BSTS. 15 Ellipsometric data indicated the presence of high optical conductivities, possibly linked to the existence of topologically protected surface states. If true, BSTS or other TIs would be a radically new material platform for broadband nanoplasmonic devices that can be modulated optically, 16 through injection of electrons, 17 or by applied magnetic fields.…”

Artifact free time resolved near-field spectroscopy

“…The pulse duration of mid-IR radiation is typically 40-200 fs, which is sufficient to gain access to timescales where electron-phonon, and electron-spin coupling have not yet brought the electronic system into thermal equilibrium [50]. In the visible range several interesting spectral features such as excitonic modes in transition metal dichalcogenides [61,62], plasmonic modes in metals and topological insulators [63], as well as resonances related to interband transitions in insulators, across charge transfer and Mott-Hubbard gaps can be observed. The pulse duration of visible radiation, which can be in the range of 4-40 fs, also enables indirect access to resonant modes in the infrared spectral range such as coherent phonons and Raman active modes [64,65].…”