Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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 vary-
ing 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 femtosec-
onds), 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 effciencies, and high-bandwidth optoelectronic
and spintronic information and communication applications.