Terahertz Investigation of Dirac Materials: Graphene and Topological Insulators

Light-matter interaction in two-dimensional Dirac materials exhibits intriguing features in comparison to conventional semiconductors. Recent discoveries of graphene and three-dimensional topological insulators (TIs) have demonstrated novel terahertz (THz) optoelectronics, such that control over the electronic properties of Dirac-type carriers can be conducted by optical techniques. In this paper, we review recent investigations of graphene and TIs using broadband THz radiation and ultrashort optical pulses. After discussing state-of-the-art progress in graphene and TI investigations (Sec. I), we present ultrafast optical techniques that employ optical-pump THz-probe spectroscopy (Sec. II). In Sec. III, broadband THz responses in Dirac materials are examined according to semi-classical theories, and corresponding physical rationales are extended to elucidate Dirac plasmons in graphene and TIs (Sec. IV). Finally, brief summaries with research outlooks for future THz applications of graphene and TIs are provided (Sec. V).