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Topologically-protected surface states present rich physics and promisingspintronic, optoelectronic and photonic applications that require a properunderstanding of their ultrafast carrier dynamics. Here, we investigate thesedynamics in topological insulators (TIs) of the bismuth and antimonychalcogenide family, where we isolate the response of Dirac fermions at thesurface from the response of bulk carriers by combining photoexcitation withbelow-bandgap terahertz (THz) photons with TI samples with varying Fermi level,including one sample with the Fermi level located within the bandgap. Weidentify distinctly faster relaxation of charge carriers in thetopologically-protected Dirac surface states (few hundred femtoseconds),compared to bulk carriers (few picoseconds). In agreement with such fastcooling dynamics, we observe THz harmonic generation without any saturationeffects for increasing incident fields, unlike graphene which exhibits strongsaturation. This opens up promising avenues for increased THz nonlinearconversion efficiencies, and high-bandwidth optoelectronic and spintronicinformation and communication applications.

Terahertz signatures of ultrafast Dirac fermion relaxation at the surface of topological insulators at room temperature