Observation of THz Radiation from Topological Insulators
Chien-Ming Tu1*, Tien-Tien Yeh1, Wen-Yen Tzeng1, Shin-An Ku1, Chih-Wei Luo1, Jenh-Yih Juang1, Kaung-Hsiung Wu1, Jiunn-Yuan Lin2, Takayoshi Kobayashi1,3, Cheng-Maw Cheng4, Ku-Ding Tsuei4, Helmuth Berger5, R. Sankar6, Fang-Cheng Chou6
1Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
2Institute of Physics, National Chiao Tung University, Hsinchu, Taiwan
3Advanced Ultrafast Laser Research Center and Department of Engineering Science, The University of Electro-Communications, Tokyo, Japan
4National Synchrotron Radiation Research Center, Hsinchu, Taiwan
5Institut de Physique des Nanostructures, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
6Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
* presenting author:Chien-Ming Tu, email:cmtu@nctu.edu.tw
We report on the time-domain measurements of THz radiation generated in topological insulators (TIs) n-type Cu0.02Bi2Se3 and p-type Bi2Te3 single crystals by ultrafast optical pulse excitation. The observed polarity-reversal of the THz pulse originated from transient current is unusual, and can not be reconciled with the photo-Dember effect used to explain THz generation from conventional narrow bandgap semiconductors. The second Dirac surface state (SS) and bulk bands (BBs) are found to be indispensable for the explanation of the unusual phenomenon. Thanks to the existence of 2nd SS and BBs, TIs manifest an effective wide band gap in THz generation. The present study demonstrates that ultrafast THz generation and detection provide valuable information of the optical coupling and the electronic structure of TIs.

Keywords: topological insulator, ultrafast THz generation, second surface state, carrier dynamics