Terahertz-driven hot Dirac fermion and plasmon dynamics in the bulk-insulating topological insulator Bi2Se3

For degenerately doped three-dimensional topological insulators (3D TIs), the interaction dynamics between Dirac fermions and collective charge oscillations (plasmons) are considerably complicated. In particular, the low-energy contribution near band inversion is largely obscure because of the bulk-surface interaction as well as the two-dimensional electron gas (2DEG). We have circumvented these issues by performing high-field terahertz (THz) pump-probe spectroscopy in bulk insulating ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ TI microribbon array structures. Plasmon blueshift of 11% was found to occur within 4 ps under 0.433 MV/cm THz excitation compared to the near-equilibrium state. Although the conventional optical pump or engineered microribbon TIs exhibits a similar plasmon blueshift, our observation is readily explained by the intrinsic plasmon behavior from the increased electronic temperature (${T}_{\mathrm{e}}\ensuremath{\cong}1430\phantom{\rule{4pt}{0ex}}\mathrm{K}$) and reduced chemical potential ($\ensuremath{\mu}\ensuremath{\cong}16\phantom{\rule{4pt}{0ex}}\mathrm{meV}$) without considering extrinsic contributions. Our findings may apply to other classes of Dirac materials to investigate the coupled low-energy interactions with collective quasiparticles.