Dirac Fermion and Plasmon Dynamics in Graphene and 3D Topological Insulators

Light-matter interactions illuminate the nature of solids and provide a second look on associated carrier dynamics. In particular, graphene and 3D topological insulators (3D TI) with broadband electromagnetic excitation have revealed to host exotic dynamic interactions. Much of Dirac-point physics arises from discrete lattice symmetries and nontrivial Z(2) classification of Bloch states. In this review, ongoing spectroscopic works on graphene and 3D TI are presented, where special attention is on the far-infrared terahertz (THz) spectroscopy that characterizes the intraband dynamics. The constant absorbance of graphene upon interband particle-hole generation exhibits nonclassical results of gapless dispersion, which can be elucidated by the relativistic Dirac equations with zero rest mass. The massless Dirac fermions on graphene and 3D TI surface exhibit distinct phenomena compared to the conventional systems. First, the transient responses after pulse excitation reveal the semimetallic states of the gapless electronic dispersion, where the Fermi energy in equilibrium determines the crossover from metallic to semiconducting states. Second, the 2D Dirac plasmon dispersion of graphene and 3D TI surface exhibits unconventional density dependence in comparison to the massive electronic systems. The novel optical phenomena in graphene and 3D TI surface can serve for advanced optoelectronic and optospintronic applications.