Ultrafast Evolution of Bulk, Surface and Surface Resonance States in Photoexcited Bi$_{2}$Te$_{3}$

TL;DR

This study uses time-resolved circular dichroism photoemission spectroscopy to investigate femtosecond charge dynamics and the interplay between bulk, surface, and resonance states in photoexcited Bi$_{2}$Te$_{3}$, revealing ultrafast transport and scattering processes.

Contribution

It introduces a novel application of circular dichroism in time-resolved photoemission to disentangle ultrafast dynamics of bulk and surface states in a topological insulator.

Findings

Photoexcitation mainly involves bulk states with subsequent transport to the surface.

Surface resonance states significantly influence ultrafast interactions between bulk and surface states.

Femtosecond timescale disentanglement of unpolarized bulk and spin-textured surface dynamics.

Abstract

We use circular dichroism (CD) in time- and angle-resolved photoemission spectroscopy (trARPES) to measure the femtosecond charge dynamics in the topological insulator (TI) Bi$_{2}$Te$_{3}$. We detect clear CD signatures from topological surface states (TSS) and surface resonance (SR) states. In time-resolved measurements, independently from the pump polarization or intensity, the CD shows a dynamics which provides access to the unexplored electronic evolution in unoccupied states of Bi$_{2}$Te$_{3}$. In particular, we are able to disentangle the unpolarized electron dynamics in the bulk states from the spin-textured TSS and SR states on the femtosecond timescale. Our study demonstrates that photoexcitation mainly involves the bulk states and is followed by sub-picosecond transport to the surface. This provides essential details on intra- and interband scatterings in the relaxation process of TSS and SR states. Our results reveal the significant role of SRs in the subtle ultrafast interaction between bulk and surface states in TIs.