Time-resolved terahertz dynamics in thin films of the topological insulator Bi$_{2}$Se$_3$

TL;DR

This study investigates the ultrafast terahertz response of thin Bi2Se3 topological insulator films, revealing distinct surface and bulk carrier dynamics with implications for optoelectronic device design.

Contribution

It provides the first detailed time-resolved THz spectroscopy analysis distinguishing surface and bulk carrier responses in Bi2Se3 thin films.

Findings

Thinner films show decreased conductivity and fast relaxation (~10 ps) after photoexcitation.

Thicker films exhibit increased conductivity with a decay time of about 5 ps.

Surface carriers are long-lived and can be independently accessed in thin films.

Abstract

We use optical pump--THz probe spectroscopy at low temperatures to study the hot carrier response in thin Bi$_2$Se$_3$ films of several thicknesses, allowing us to separate the bulk from the surface transient response. We find that for thinner films the photoexcitation changes the transport scattering rate and reduces the THz conductivity, which relaxes within 10 picoseconds (ps). For thicker films, the conductivity increases upon photoexcitation and scales with increasing both the film thickness and the optical fluence, with a decay time of approximately 5 ps as well as a much higher scattering rate. These different dynamics are attributed to the surface and bulk electrons, respectively, and demonstrate that long-lived mobile surface photo-carriers can be accessed independently below certain film thicknesses for possible optoelectronic applications.