Optical bulk-boundary dichotomy in a quantum spin Hall insulator

Junfeng Han, Pengcheng Mao, Hailong Chen, Jia Xin Yin, Maoyuan Wang, Dongyun Chen, Yongkai Li, Jingchuan Zheng, Xu Zhang, Dashuai Ma, Qiong Ma, Zhi Ming Yu, Jinjian Zhou, Cheng Cheng Liu, Yeliang Wang, Shuang Jia, Yuxiang Weng, M. Zahid Hasan, Wende Xiao, Yugui Yao

Research output: Contribution to journalArticlepeer-review

1 Scopus citations


The bulk-boundary correspondence is a critical concept in topological quantum materials. For instance, a quantum spin Hall insulator features a bulk insulating gap with gapless helical boundary states protected by the underlying Z2 topology. However, the bulk-boundary dichotomy and distinction are rarely explored in optical experiments, which can provide unique information about topological charge carriers beyond transport and electronic spectroscopy techniques. Here, we utilize mid-infrared absorption micro-spectroscopy and pump–probe micro-spectroscopy to elucidate the bulk-boundary optical responses of Bi4Br4, a recently discovered room-temperature quantum spin Hall insulator. Benefiting from the low energy of infrared photons and the high spatial resolution, we unambiguously resolve a strong absorption from the boundary states while the bulk absorption is suppressed by its insulating gap. Moreover, the boundary absorption exhibits strong polarization anisotropy, consistent with the one-dimensional nature of the topological boundary states. Our infrared pump–probe microscopy further measures a substantially increased carrier lifetime for the boundary states, which reaches one nanosecond scale. The nanosecond lifetime is about one to two orders longer than that of most topological materials and can be attributed to the linear dispersion nature of the helical boundary states. Our findings demonstrate the optical bulk-boundary dichotomy in a topological material and provide a proof-of-principal methodology for studying topological optoelectronics.

Original languageEnglish (US)
Pages (from-to)417-423
Number of pages7
JournalScience Bulletin
Issue number4
StatePublished - Feb 26 2023

All Science Journal Classification (ASJC) codes

  • General


  • BiBr
  • Edge states
  • Mid-infrared absorption micro-spectroscopy
  • Pump–probe micro-spectroscopy
  • Quantum spin Hall effect
  • Topological insulator


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