  One of the most striking manifestations of electronic properties of
topological insulators is the dependence of the photocurrent direction on the
helicity of circularly polarized optical excitation. The helicity dependent
photocurrents, underpinned by spin-momentum locking of surface Dirac electrons,
are weak and easily overshadowed by bulk contributions. Here we show that the
chiral response can be enhanced by nanostructuring. The tight confinement of
electromagnetic fields in the resonant nanostructure enhances the
photoexcitation of spin-polarized surface states of topological insulator
Bi1.5Sb0.5Te1.8Se1.2, leading to an 11-fold increase of the circular
photogalvanic effect and an unprecedented photocurrent dichroism
(\r{ho}circ=0.87) at room temperature. The control of spin-transport in
topological materials by structural design is a previously unrecognised ability
of metamaterials that bridges the gap between nanophotonics and
spin-electronics, providing new opportunities for developing polarization
sensitive photodetectors.
