THz Generation and Detection on Dirac Fermions in Topological Insulators

TL;DR

This paper demonstrates that Dirac fermions in topological insulators like Bi2Se3 can efficiently generate and absorb terahertz radiation, revealing their role in surface phonon interactions and carrier dynamics.

Contribution

It provides experimental evidence of THz generation from topological insulator surfaces and highlights the importance of Dirac fermions in THz absorption and surface phonon behavior.

Findings

THz waves can be generated from Bi2Se3 surfaces using femtosecond pulses.

Dirac fermions significantly enhance THz absorption compared to bulk carriers.

Surface phonon modes are confirmed to be associated with Dirac fermions.

Abstract

This study shows that a terahertz (THz) wave can be generated from the (001) surface of cleaved Bi$_{\textrm{2}}$Se$_{\textrm{3}}$ and Cu-doped Bi$_{\textrm{2}}$Se$_{\textrm{3}}$ single crystals using 800 nm femtosecond pulses. The generated THz power is strongly dependent on the carrier concentration of the crystals. An examination of the dependence reveals the two-channel free carrier absorption to which Dirac fermions are indispensable. Dirac fermions in Bi$_{\textrm{2}}$Se$_{\textrm{3}}$ are significantly better absorbers of THz radiation than bulk carriers at room temperature. Moreover, the characteristics of THz emission confirm the existence of a recently proposed surface phonon branch that is normalized by Dirac fermions.