  The development of metamaterials, data processing circuits and sensors for
the visible and UV parts of the spectrum is hampered by the lack of low-loss
media supporting plasmonic excitations and drives the intense search for
plasmonic materials beyond noble metals. By studying plasmonic nanostructures
fabricated on the surface of topological insulator
$\mbox{Bi}_{1.5}\mbox{Sb}_{0.5}\mbox{Te}_{1.8}\mbox{Se}_{1.2}$ we found that it
is orders of magnitude better plasmonic material than gold and silver in the
blue-UV range. Metamaterial fabricated from
$\mbox{Bi}_{1.5}\mbox{Sb}_{0.5}\mbox{Te}_{1.8}\mbox{Se}_{1.2}$ show plasmonic
resonances from 350 nm to 550 nm while surface gratings exhibit
cathodoluminescent peaks from 230 nm to 1050 nm. The negative permittivity
underpinning plasmonic response is attributed to the combination of bulk
interband transitions and surface contribution of the topologically protected
states. The importance of our result is in the identification of new mechanisms
of negative permittivity in semiconductors where visible-range plasmonics can
be directly integrated with electronics.
