Clarifying ultrafast carrier dynamics in ultrathin films of the topological insulator Bi2Se3 using transient absorption spectroscopy

TL;DR

This study uses transient absorption spectroscopy to clarify ultrafast carrier relaxation in ultrathin Bi2Se3 topological insulator films, revealing the role of surface states and phonon cascades in electron dynamics.

Contribution

First demonstration of transient absorption spectroscopy for ultrafast carrier dynamics in Bi2Se3, highlighting the influence of Dirac surface states and phonon cascades.

Findings

TA spectra cover the visible region, indicating two-photon excitation.

Carrier relaxation involves polar optical phonon cascade emission.

Presence of a second Dirac cone (SS2) slows electron relaxation.

Abstract

Ultrafast carrier dynamics in the topological insulator Bi2Se3 have recently been intensively studied using a variety of techniques. However, we are not aware of any successful experiments exploiting transient absorption (TA) spectroscopy for these purposes. Here we demonstrate that if the ~730 nm wavelength pumping (~1.7 eV photon energy) is applied to ultrathin Bi2Se3 films, TA spectra cover the entire visible region, thus unambiguously pointing to two-photon excitation (~3.4 eV). The carrier relaxation dynamics is found to be governed by the polar optical phonon cascade emission occurring in both the bulk states and the Dirac surface states (SS), including SS-bulk-SS vertical electron transport and being also exclusively influenced by whether the Dirac point is presented between the Dirac cones of the higher energy (~1.5 eV) Dirac SS (known as SS2). We have recognized that SS2 act as a valve substantially slowing down the relaxation of electrons when the gap between Dirac cones exceeds the polar optical phonon and resonant defects energies. The resulting progressive accumulation of electrons in the gapped SS2 becomes detectable through the inverse bremsstrahlung type free carrier absorption.