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

TL;DR

This study reveals ultrafast relaxation dynamics of Dirac fermions on topological insulator surfaces at room temperature using terahertz spectroscopy, highlighting potential for advanced spintronic and optoelectronic applications.

Contribution

It isolates and characterizes the ultrafast surface state dynamics in topological insulators using below-bandgap terahertz excitation, demonstrating significantly faster relaxation than bulk carriers.

Findings

Surface Dirac fermions relax in hundreds of femtoseconds.

Bulk carriers relax in a few picoseconds.

THz harmonic generation shows no saturation at high fields.

Abstract

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.