Impact of ultrafast transport on the high-energy states of a photoexcited topological insulator

TL;DR

This study demonstrates how ultrafast transport mechanisms in topological insulators can be selectively controlled to influence electron dynamics, thermalization, and potential optoelectronic applications, by transitioning between bulk-conducting and insulating regimes.

Contribution

It introduces a method to selectively switch ultrafast transport channels in topological insulators, revealing their role in electron decay and thermalization processes.

Findings

Ultrafast transport channels can be switched on and off in TIs.

Inhibiting transport causes a thermalization bottleneck.

Transport influences electron-hole recombination rates.

Abstract

Ultrafast dynamics in three-dimensional topological insulators (TIs) opens new routes for increasing the speed of information transport up to frequencies thousand times faster than in modern electronics. However, up to date, disentangling the exact contributions from bulk and surface transport to the subpicosecond dynamics of these materials remains a difficult challenge. Here, using time- and angle-resolved photoemission, we demonstrate that driving a TI from the bulk-conducting into the bulk-insulating transport regime allows to selectively switch on and off the emergent channels of ultrafast transport between the surface and the bulk. We thus establish that ultrafast transport is one of the main driving mechanisms responsible for the decay of excited electrons in prototypical TIs following laser excitation. We further show how ultrafast transport strongly affects the thermalization and scattering dynamics of the excited states up to high energies above the Fermi level. In particular, we observe how inhibiting the transport channels leads to a thermalization bottleneck that substantially slows down electron-hole recombination via electron-electron scatterings. Our results pave the way for exploiting ultrafast transport to control thermalization time scales in TI-based optoelectronic applications, and expand the capabilities of TIs as intrinsic solar cells.