Centre for Quantum Physics, Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement (Ministry of Education), School of Physics, Beijing Institute of Technology, Beijing 100081, China; Yangtze Delta Region Academy of Beijing Institute of Technology, Jiaxing 314000, China; Beijing Key Laboratory of Nanophotonics & Ultrafine Optoelectronic Systems, Beijing Institute of Technology, Beijing 100081, China.
Centre for Quantum Physics, Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement (Ministry of Education), School of Physics, Beijing Institute of Technology, Beijing 100081, China; Analysis & Testing Center, Beijing Institute of Technology, Beijing 100081, China.
Sci Bull (Beijing). 2023 Feb 26;68(4):417-423. doi: 10.1016/j.scib.2023.01.038. Epub 2023 Feb 2.
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 Z 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 BiBr, 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.
体边对应是拓扑量子材料中的一个关键概念。例如，量子自旋霍尔绝缘体具有体绝缘能隙和无能隙的螺旋边界态，这些边界态由底层的 Z 拓扑保护。然而，体边二分法和区别在光学实验中很少被探索，这些实验可以提供关于拓扑电荷载流子的独特信息，超越了传输和电子光谱技术。在这里，我们利用中红外吸收微光谱和泵浦探测微光谱来阐明最近发现的室温量子自旋霍尔绝缘体 BiBr 的体边光学响应。得益于中红外光子的低能量和高空间分辨率，我们可以明确分辨出边界态的强吸收，而体吸收则被其绝缘能隙抑制。此外，边界吸收表现出强烈的偏振各向异性，与拓扑边界态的一维性质一致。我们的红外泵浦探测显微镜进一步测量到边界态的载流子寿命大大增加，达到纳秒量级。这个纳秒寿命比大多数拓扑材料的寿命长一到两个数量级，可以归因于螺旋边界态的线性色散性质。我们的发现证明了拓扑材料中的光学体边二分法，并为研究拓扑光电子学提供了一种原理证明的方法。
