Ultrafast dynamics in topological insulators

C W Luo,G D Gu,K H Wu,T Kobayashi,H J Wang,J Y Juang,B L Young,J.-Y Lin,R Sankar,T M Uen,H Berger,S A Ku,F C Chou,H.-J Chen,Markus Betz,Abdulhakem Y Elezzabi,Jin-Joo Song,Kong-Thon Tsen

doi:10.1117/12.2001954

Abstract

Ultrafast dynamics of carriers and phonons in topological insulators CuxBi2Se3-y (x=0, 0.1, 0.125, y=0, 1) was studied using femtosecond optical pump-probe spectroscopy. One damped oscillation was clearly observed in the transient reflectivity changes (ΔR/R), which is assigned to the coherent optical phonon (A1g 1). According to the red shift of A1g1 phonon frequency, the Cu atoms in CuxBi2Se3 crystals may predominantly intercalated between pair of the quintuple layers. Moreover, the carrier dynamics in the Dirac-cone surface state is significantly different from that in bulk state, which was investigated using optical pump mid-infrared (mid-IR) probe spectroscopy. The rising time and decay time of the negative component in ΔR/R, which is assigned to carrier relaxation in Dirac cone, is 1.62 ps and 20.5 ps, respectively.

Highlights

Topological insulators (TIs) [1,2,3,4,5,6,7,8] and two-dimensional (2D) materials such as graphene [9], MoS2, WS2, and MoSe2 [10] are of great interests because of their unique physical properties and applications
We report the ultrafast dynamics of carriers and phonons in topological insulator Bi2Se3, CuxBi2Se3 (x = 0, 0.1, 0.125) single crystals
By time-resolved pump-probe spectroscopy, one damped fast oscillation was clearly observed in the transient reflectivity changes (∆R/R) for

Summary

Introduction

Topological insulators (TIs) [1,2,3,4,5,6,7,8] and two-dimensional (2D) materials such as graphene [9], MoS2, WS2, and MoSe2 [10] are of great interests because of their unique physical properties and applications. TIs have a spin degenerate and fully gapped bulk state but exhibit a spin polarized and gapless electronic state on the surface [8]. The issues associated with electron–phonon interaction, carrier lifetime, carrier dynamics, energy loss rate, and low-energy electronic responses are very important for optimizing device performance These ultrafast dynamic properties of the materials can be resolved by pump-probe spectroscopy. We provide a brief introduction to the materials, time-resolved pump-probe spectroscopy, and some ultrafast dynamic properties of Bi-based topological insulators

Bismuth-based topological insulators

Degenerate pump-probe spectroscopy

Optical pump and mid-infrared probe spectroscopy

Time-resolved spectroscopy in a topological insulators

Interband relaxations in topological insulators

Intraband relaxations in topological insulators

Findings

Conclusion