红外泵浦-探测纳米显微镜研究剥离石墨烯中的超快和纳米尺度等离子体现象
Ultrafast and Nanoscale Plasmonic Phenomena in Exfoliated Graphene Revealed by Infrared Pump-Probe Nanoscopy
摘要：
抽运-探测光谱是探索基本激发、集体模式和能量转移过程的超快动力学的核心。通常使用传统的衍射限制光学进行，泵浦-探测实验固有地对局部化学、成分和电子不均匀性进行平均。在这里，我们规避这一缺陷，并介绍泵浦探测红外光谱与~20 nm的空间分辨率，远低于衍射极限，这是使用散射扫描近场光学显微镜（S-SNOM）完成。这种技术使我们能够在技术上显著的中红外（MIR）频率下研究SiO2上的剥离石墨烯单层，其中局部光学电导率通过经由s-SNOM尖端的表面等离子体激元的激发而变得实验上可访问。在近红外（NIR）频率下的光泵浦促使200飞秒（fs）时间尺度上的等离子体行为发生明显变化。泵诱导的，增强的等离子体响应的起源被确定为有效电子温度增加到几千开尔文，直接从与等离子体共振相关的Drude重量推导。
英文摘要：
Pump-probe spectroscopy is central for exploring ultrafast dynamics of fundamental excitations, collective modes and energy transfer processes. Typically carried out using conventional diffraction-limited optics, pump-probe experiments inherently average over local chemical, compositional, and electronic inhomogeneities. Here we circumvent this deficiency and introduce pump-probe infrared spectroscopy with ~20 nm spatial resolution, far below the diffraction limit, which is accomplished using a scattering scanning near-field optical microscope (s-SNOM). This technique allows us to investigate exfoliated graphene single-layers on SiO2 at technologically significant mid-infrared (MIR) frequencies where the local optical conductivity becomes experimentally accessible through the excitation of surface plasmons via the s-SNOM tip. Optical pumping at near-infrared (NIR) frequencies prompts distinct changes in the plasmonic behavior on 200 femtosecond (fs) time scales. The origin of the pump-induced, enhanced plasmonic response is identified as an increase in the effective electron temperature up to several thousand Kelvin, as deduced directly from the Drude weight associated with the plasmonic resonances.
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