Strong nonlinear terahertz response induced by Dirac surface states in Bi2Se3 Topological Insulator

Electrons with a linear energy/momentum dispersion are called massless Dirac electrons and represent the low-energy excitations in exotic materials like Graphene and Topological Insulators (TIs). Dirac electrons are characterized by notable properties like a high mobility, a tunable density and, in TIs, a protection against backscattering through the spin-momentum looking mechanism. All those properties make Graphene and TIs appealling for plasmonics applications. However, Dirac electrons are expected to present also a strong nonlinear optical behavior. This should mirror in phenomena like electromagnetic induced transparency (EIT) and harmonic generation. Here, we demonstrate that in Bi2Se3 Topological Insulator, an EIT is achieved under the application of a strong terahertz (THz) electric field. This effect, concomitant determined by harmonic generation and charge-mobility reduction, is exclusively related to the presence of Dirac electron at the surface of Bi2Se_3, and opens the road towards tunable THz nonlinear optical devices based on Topological Insulator materials.