光抽运对石墨烯时间分辨光电发射实验解释的影响
Ramifications of Optical Pumping on the Interpretation of Time-Resolved Photoemission Experiments on Graphene
摘要：
在泵浦探测时间和角度分辨光电子能谱（TR-ARPES）实验中，泵浦脉冲的存在增加了一个新的复杂程度的光电过程相比，传统的ARPES。这是由泵引起的真空空间电荷效应和表面光电压，以及由于在样品基板系统中的内部反射的多个泵激发证明。这些过程可能会严重影响正确的解释数据掩盖了平衡电子动力学固有的样品。在这项研究中，我们表明，这种效应确实影响碳化硅（SiC）衬底上的石墨烯的TR-ARPES数据。特别是，我们发现一个时间和激光能量密度依赖的光谱位移和扩大所获得的光谱，并明确地显示存在的双泵浦激发。这些效应的动力学比石墨烯样品中的电子动力学慢，从而允许我们在时域中对信号进行去卷积。我们的研究结果表明，复杂的泵相关的过程中，应始终考虑在实验装置和数据分析。
英文摘要：
In pump-probe time and angle-resolved photoemission spectroscopy (TR-ARPES) experiments the presence of the pump pulse adds a new level of complexity to the photoemission process in comparison to conventional ARPES. This is evidenced by pump-induced vacuum space-charge effects and surface photovoltages, as well as multiple pump excitations due to internal reflections in the sample-substrate system. These processes can severely affect a correct interpretation of the data by masking the out-of-equilibrium electron dynamics intrinsic to the sample. In this study, we show that such effects indeed influence TR-ARPES data of graphene on a silicon carbide (SiC) substrate. In particular, we find a time- and laser fluence-dependent spectral shift and broadening of the acquired spectra, and unambiguously show the presence of a double pump excitation. The dynamics of these effects is slower than the electron dynamics in the graphene sample, thereby permitting us to deconvolve the signals in the time domain. Our results demonstrate that complex pump-related processes should always be considered in the experimental setup and data analysis.
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