Snapshots of Dirac fermions near the Dirac point in topological insulators

Chih-Wei Luo*, H. J. Wang, S. A. Ku, H. J. Chen, T. T. Yeh, Jiunn-Yuan Lin, Kaung-Hsiung Wu, Jenh-Yih Juang, Ben-Li Young, Takayoshi Kobayashi, C. M. Cheng, C. H. Chen, K. D. Tsuei, R. Sankar, F. C. Chou, K. A. Kokh, O. E. Tereshchenko, E. V. Chulkov, Yu M. Andreev, G. D. Gu

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

75 Scopus citations


The recent focus on topological insulators is due to the scientific interest in the new state of quantum matter as well as the technology potential for a new generation of THz optoelectronics, spintronics and quantum computations. It is important to elucidate the dynamics of the Dirac fermions in the topologically protected surface state. Hence we utilized a novel ultrafast optical pump mid-infrared probe to explore the dynamics of Dirac fermions near the Dirac point. The femtosecond snapshots of the relaxation process were revealed by the ultrafast optics. Specifically, the Dirac fermion-phonon coupling strength in the Dirac cone was found to increase from 0.08 to 0.19 while Dirac fermions were away from the Dirac point into higher energy states. Further, the energy-resolved transient reflectivity spectra disclosed the energy loss rate of Dirac fermions at room temperature was about 1 meV/ps. These results are crucial to the design of Dirac fermion devices.

Original languageEnglish
Pages (from-to)5797-5802
Number of pages6
JournalNano letters
Issue number12
StatePublished - 11 Dec 2013


  • Dirac fermion dynamics
  • Dirac fermion-phonon coupling
  • Topological insulator
  • ultrafast optical pump mid-infrared probe spectroscopy


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