Nanoscale-Resolved Spatial Mapping of Tip-Mediated Terahertz Emission from Semiconductors
Nanoscale-Resolved Spatial Mapping of Tip-Mediated Terahertz Emission from Semiconductors
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DOI：
10.1007/s10762-023-00908-3
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发表时间：
2023-02
期刊：
影响因子：
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通讯作者：
A. Pizzuto;E. Castro-Camus;D. Mittleman
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作者：
A. Pizzuto;E. Castro-Camus;D. Mittleman
散射型扫描近场光学显微镜（s-SNOM）已成为亚波长分辨成像和光谱学的有力工具。该技术在太赫兹或其他长波长范围内特别有用，其中衍射极限甚至禁止解析微米大小的物体。这种方法也可以大大提高非线性测量的空间分辨率，如激光太赫兹发射显微镜（LTEM），其中宽带太赫兹脉冲产生后，由近红外（NIR）脉冲光激发。然而，在所有现有的近场LTEM实验中，泵浦光斑已经被定位到散射探针，以耦合到在照明区域的中心处产生的THz辐射。在这里，我们展示了firstnonlocalnear-field LTEM实验，其中一个超快的NIR脉冲光激发样品的位置是横向移动的近场探头的位置耦合太赫兹信号的远场。泵浦点的增加的横向位移产生更大的时间延迟，在所发射的宽带THz脉冲的到达，与泵的位置和探针尖端之间的地下偶极子的漂移一致。蒙特卡罗模拟证实了偶极形成的时移，这反过来又产生了相对延迟的太赫兹发射。仿真结果与实验结果吻合良好。这种非局部的S-SNOM方法LTEM提供了一个新的机会，研究横向传输的纳米尺度，并可能是特别有用的各向异性材料。
Scattering-type scanning near-field optical microscopy (s-SNOM) has become a powerful tool for subwavelength-resolved imaging and optical spectroscopy. The technique is particularly useful in the terahertz or other long-wavelength regimes, where the diffraction limit prohibits resolving even micron-sized objects. This approach can also drastically improve the spatial resolution of nonlinear measurements such as laser terahertz emission microscopy (LTEM), in which a broadband THz pulse is generated after photoexcitation by a near-infrared (NIR) pulse. However, in all prior near-field LTEM experiments, the pump spot has been localized to the scattering probe, to couple to the generated THz radiation at the center of the illuminated region. Here, we demonstrate the firstnonlocalnear-field LTEM experiments, in which an ultrafast NIR pulse photoexcites a sample at a location that is laterally shifted from the location of the near-field probe which couples the THz signal to the far field. Increasing lateral shifts of the pump spot produce larger time delays in the arrival of the emitted broadband THz pulse, consistent with drift of the subsurface dipole between the pump location and probe tip. Monte Carlo simulations corroborate the time shift for the dipole formation, which in turn produces the THz emission with a relative delay. The simulation results show excellent agreement with experiments. This nonlocal s-SNOM approach to LTEM offers a new opportunity for studying lateral transport on the nanoscale and may be particularly useful in anisotropic materials.