Direct extraction of spin-polarized current from terahertz emission of topological insulators

Chien-Ming Tu^1*, Wen-Yen Tzeng¹, Tien-Tien Yeh¹, Pei-Yu Chuang², Min-Hsien Yang³, Jian-Ruei Chen³, Cheng-Maw Cheng², Jung-Chun A. Huang³, Chih-Wei Luo¹

¹Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
²National Synchrotron Radiation Research Center, Hsinchu, Taiwan
³Department of Physics, National Cheng Kung University, Tainan, Taiwan

* Presenter:Chien-Ming Tu, email:cmtu@nctu.edu.tw

Topological insulators (TIs) represent a new quantum phase of matter with a Dirac cone-like conducting topological surface states (TSSs) that shows time-reversal protection from backscattering due to spin-momentum-locking. It has been demonstrated that helicity-dependent photocurrent on TIs can be manipulated by the polarization of incident light. These works highlight the potential of TIs on applications of spintronics in the future. Here, we demonstrate the direct extraction of the spin-polarized photocurrent from terahertz emission of topological insulators heterostructure Sb₂Te₃/Bi₂Te₃ by using a photoelastic modulator (PEM). The spin-polarized photocurrent originates from circular photogalvanic effect (CPGE) associated with topological surface states, and the polarity-reversals of the emitted terahertz waveform were observed as the incident angle θ of optical pulse changes its sign, i.e., θ -> -θ. In azimuthal measurements, φ-scan, the polarity of the emitted terahertz radiation keeps the same. These observations totally agree with the characteristics of TSSs. Compared to the traditional technique-rotating quarter wave-plate, not only the CPGE component can be directly but also data-acquisition-time is largely reduced.

Keywords: terahertz, topological insulators, spin-polarized current, ultrafast dynamics