Topological Insulator Metamaterial with Giant Circular Photogalvanic Effect

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.