Herein, experimental work on the ultrafast electron dynamics in the topological surface state (TSS) of three‐dimensional (3D) topological insulators (TIs) observed with time‐ and angle‐resolved two‐photon photoemission (2PPE) is reviewed. The focus is laid on the generation of ultrafast photocurrents and the time‐resolved observation of their decay. 2PPE not only allows to unambiguously relate the photocurrents to the spin‐polarized electronic surface states. Probing of the asymmetric momentum distribution of the electrons carrying the current makes it possible to study the microscopic scattering processes that govern the unusual electron transport in the time domain. Ultrashort mid‐infrared pump pulses permit not only a direct optical excitation of the TSS in Sb₂Te₃ but also lead to a strong asymmetry of the TSS population in momentum space. Two‐dimensional band mapping of the TSS shows that this asymmetry is in fact representative of a macroscopic photocurrent, while the helicity‐dependence of the photocurrent is found to be small. The time‐resolved observation of the photocurrent decay reveals a huge mean free path of the electrons in the TSS.

中文翻译：

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三维拓扑绝缘子表面态中光电流的超快动力学

在这里，审查了在三维（3D）拓扑绝缘体（TIs）的拓扑表面状态（TSS）中超快电子动力学的实验工作，该时间是在时间和角度分辨的两个光子光发射（2PPE）下观察到的。重点放在超快光电流的产生和其衰减的时间分辨观察上。2PPE不仅可以明确地将光电流与自旋极化电子表面态相关。对载流电子的不对称动量分布的探测，使得研究时空中异常电子传输的微观散射过程成为可能。超短的中红外泵浦脉冲不仅允许对Sb ₂ Te ₃中的TSS进行直接光激发但也会导致动量空间中TSS种群的强烈不对称性。TSS的二维带状映射显示，这种不对称实际上代表了宏观光电流，而光电流的螺旋度依赖性很小。时间分辨的对光电流衰减的观察揭示了TSS中电子的巨大平均自由程。