Topologically protected surface states present rich physics and promising spintronic,
optoelectronic, and photonic applications that require a proper understanding of their
ultrafast carrier dynamics. Here, we investigate these dynamics in topological insulators (TIs)
of the bismuth and antimony chalcogenide family, where we isolate the response of Dirac
fermions at the surface from the response of bulk carriers by combining photoexcitation with
below-bandgap terahertz (THz) photons and TI samples with varying Fermi level, including …