Plasmonics of topological insulators at optical frequencies

The development of nanoplasmonic devices, such as plasmonic circuits and metamaterial superlenses in the visible to ultraviolet frequency range, is hampered by the lack of low-loss plasmonic media. Recently, strong plasmonic response was reported in a certain class of topological insulators. Here, we present a first-principles density functional theory analysis of the dielectric functions of topologically insulating quaternary (Bi,Sb)2(Te,Se)3 trichalcogenide compounds. Bulk plasmonic properties, dominated by interband transitions, are observed from 2 to 3 eV and extend to higher frequencies. Moreover, trichalcogenide compounds are better plasmonic media than gold and silver at blue and UV wavelengths. By analyzing thin slabs, we also show that these materials exhibit topologically protected surface states, which are capable of supporting propagating plasmon polariton modes over an extremely broad spectral range, from the visible to the mid-infrared and beyond, owing to a combination of inter- and intra-surface band transitions. Energy losses occurring when plasmonic devices manipulate light waves at the nanoscale can be minimized using dielectrics with metallic surface states. Topological insulators are a new class of quantum material that confine charge carriers to nanoscale surface layers on an otherwise non-conducting crystal. Using high-level theoretical simulations, Cesare Soci from Singapore's Nanyang Technological University and colleagues from Singapore and UK show that bismuth selenide—based topological insulators respond predictably to a range of plasmon wavelengths, and could serve as a platform for optical, electrical or magnetic modulation of defined light pulses. While metals such as silver or gold tend to dissipate blue and ultraviolet plasmonic light wavelengths, the team's computations suggest that transitions between energy bands located on the surface and in the bulk crystal will work together to yield a broadband optical response. The optical and plasmonic properties of (Bi,Sb)2(Te,Se)3 trichalcogenide topological insulator crystals are studied systematically by first-principles density functional theory. These materials exhibit bulk plasmonic properties, dominated by interband transitions, which are better than gold and silver at blue and UV wavelengths. Moreover, topologically protected surface states are also capable of supporting propagating plasmon polariton modes over an extremely broad spectral range, due to a combination of interband and intraband transitions.