  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 2-3 eV and extend to higher frequencies.
Moreover, trichalcogenide compounds are better plasmonic media than gold and
silver at blue and UV wavelengths. By analysing 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.
