  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 with TI samples with varying Fermi level,
including one sample with the Fermi level located within the bandgap. We
identify distinctly faster relaxation of charge carriers in the
topologically-protected Dirac surface states (few hundred femtoseconds),
compared to bulk carriers (few picoseconds). In agreement with such fast
cooling dynamics, we observe THz harmonic generation without any saturation
effects for increasing incident fields, unlike graphene which exhibits strong
saturation. This opens up promising avenues for increased THz nonlinear
conversion efficiencies, and high-bandwidth optoelectronic and spintronic
information and communication applications.
