This research investigates ultrafast carrier dynamics in topological insulators (TIs) using photoexcitation with below-bandgap terahertz (THz) photons. By combining this technique with TI samples of varying Fermi levels, the response of Dirac fermions at the surface is isolated from bulk carrier responses. The study finds significantly faster relaxation of charge carriers in the topologically protected Dirac surface states (few hundred femtoseconds) compared to bulk carriers (few picoseconds). The observed THz harmonic generation, without saturation effects for increasing incident fields, contrasts with graphene's behavior and suggests potential for enhanced THz nonlinear conversion efficiencies.

S. Kovalev, K.-J. Tielrooij, J.-C. Deinert, I. Ilyakov, N. Awari, M. Chen, A. Ponomaryov, M. Bawatna, T. V. A. G. de Oliveira, L. M. Eng, K. A. Kuznetsov, D. A. Safronenkov, G. Kh. Kitaeva, P. I. Kuznetsov, H. A. Hafez, D. Turchinovich, M. Gensch