Plasmon–Phonon Interactions in Topological Insulator Microrings

Abstract

morphologies. [6][7][8][9] Plasmon hybridization, [ 10,11 ] Fano resonances, [ 12,13 ] and electromagnetically induced transparency [ 14 ] are among the feats that have been realized and broadly used to understand and design plasmonic devices. The range of applications of plasmon excitations is vast and includes optical sensing, [15][16][17][18] quantum electrodynamics, [ 19,20 ] nonlinear optics, [ 21,22 ] photovoltaic technologies, [ 23 ] and medical diagnosis and treatment. [ 24,25 ] Extensive experimental efforts are currently underway to fi nd materials with improved plasmonic performance, in particular in the mid-infrared and terahertz parts of the electromagnetic spectrum. Examples of such materials are low-Tc [ 26 ] and high-Tc superconductors, [ 14 ] conductive oxides, [ 27 ] and graphene. [28][29][30][31][32][33][34] The latter exhibits a peculiar electronic structure, which enables unprecedented levels of electrooptical tunability via chemical or electrostatic doping: [35][36][37] electrons in graphene behave as massless Dirac fermions characterized by a linear energy/ momentum dispersion relation and a vanishing density of states at the Fermi energy, so that a few additional charge carriers produce a large shift in the chemical potential. Dirac charge carriers are also found in 3D topological insulator (TI) materials-i.e., quantum systems characterized by an insulating electronic gap in the bulk, whose opening is due to strong spin-orbit interaction, and gapless surface states atThe great potential of Dirac electrons for plasmonics and photonics has been readily recognized after their discovery in graphene, followed by applications to smart optical devices. Dirac carriers are also found in topological insulators (TIs)-quantum systems having an insulating gap in the bulk and intrinsic Dirac metallic states at the surface. Here, the plasmonic response of ring structures patterned in Bi 2 Se 3 TI fi lms is investigated through terahertz (THz) spectroscopy. The rings are observed to exhibit a bonding and an antibonding plasmon modes, which we tune in frequency by varying their diameter. An analytical theory based on the THz conductance of unpatterned fi lms is developed, which accurately describes the strong plasmon-phonon hybridization and Fano interference experimentally observed as the bonding plasmon is swiped across the prominent 2 THz phonon exhibited by this material. This

Plasmonics of topological insulators at optical frequencies

“…At higher frequencies, the dispersion of the optical conductivity is given by the two-dimensional (2D) Drude model: 10 ð1ÀiotÞ . The contribution of surface free carriers to the optical response can then be evaluated from the expression:…”

“…Thus, TIs are ideal candidates to realize exotic plasmonic phenomena. Localized plasmons have recently been observed in TIs at THz frequencies (in Bi 2 Se 3 ) 9,10 as well as visible-to-UV frequencies (in single crystal Bi 1. 5 Sb 0.5 Te 1.8 Se 1.2 , or BSTS).…”

Terahertz and mid-infrared plasmons in three-dimensional nanoporous graphene

“…In micro- and nano- ordered structures based on conventional metals and 2D electron gas systems35242526272829, the plasmon frequency depends on both the charge-carrier density and the geometrical parameters of the structure. In a disordered system one still expects a dependence on these parameters, although disorder may affect both the plasmon frequency, linewidth and intensity30.…”

Plasmons in topological insulator cylindrical nanowires

“…It could be interesting to consider not only the presence of Dirac surface plasmons but also of other plasmon excitations 42 , in particular of surface plasmons 43 related to bulk charge carriers which have been observed very recently in EELS experiments 44,45 . Finally, it could be useful to study plasmon features at finite temperatures analyzing the effects of electronphonon couplings 46,47 , in particular, in the case of wires freely suspended or grown on a substrate 48 . Work in this direction is in progress.…”