【摘要】 量子自旋霍尔绝缘体拥有具有带隙的体态和无能隙的一维拓扑边缘态.然而,由于拓扑边缘态局域在纳米尺度,所以很难用光学手段直接观测并区分体态和边缘态,这就限制了对拓扑边缘态独特的光学特性和光电响应等方面的研究和应用. Bi4Br4体材料的台阶存在一维非平庸的边缘态,多个台阶累加的效应使得通过较为宏观的光学和光电分析手段来研究一维边缘态成为可能.该工作利用中远红外显微吸收光谱和泵浦探测光谱,对比研究了Bi4Br4体和边缘的光学响应行为.研究发现Bi4Br4的边缘在小于带隙(约0.22 e V)的波段存在明显的强于体内的红外吸收,且该吸收呈现较强的各向异性.室温下的红外泵浦探测首次观察到边缘处载流子激发态超长的弛豫时间(1.5 ns),该激发态载流子寿命要比体内载流子寿命长两个量级.该工作证实体态和拓扑边缘态具有明显不同的光学和光电响应行为,为未来设计新型红外探测器提供了材料和物理基础.更多还原

【Abstract】 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 bounda ry 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-ofprincipal methodology for studying topological optoelectronics.更多还原