THz Generation and Detection on Dirac Fermions in Topological Insulators

C. W. Luo, C. C. Lee, H. -J. Chen, C. M. Tu, S. A. Ku, W. Y. Tzeng, T. T. Yeh, M. C. Chiang, H. J. Wang, W. C. Chu, J. -Y. Lin, K. H. Wu, J. Y. Juang, T. Kobayashi, C. -M. Cheng, C. -H. Chen, K. -D. Tsuei, H. Berger, R. Sankar, F. C. Chou, H. D. Yang

Published 2013-01-26Version 1

This study shows that a terahertz (THz) wave can be generated from the (001) surface of cleaved Bi$_{\textrm{2}}$Se$_{\textrm{3}}$ and Cu-doped Bi$_{\textrm{2}}$Se$_{\textrm{3}}$ single crystals using 800 nm femtosecond pulses. The generated THz power is strongly dependent on the carrier concentration of the crystals. An examination of the dependence reveals the two-channel free carrier absorption to which Dirac fermions are indispensable. Dirac fermions in Bi$_{\textrm{2}}$Se$_{\textrm{3}}$ are significantly better absorbers of THz radiation than bulk carriers at room temperature. Moreover, the characteristics of THz emission confirm the existence of a recently proposed surface phonon branch that is normalized by Dirac fermions.