在达到5 nm空间分辨率的IR纳米显微镜中理解材料边界的图像对比度
Understanding the image contrast of material boundaries in IR nanoscopy reaching 5 nm spatial resolution
摘要：
散射型扫描近场光学显微镜（s-SNOM）允许纳米级分辨红外（IR）和太赫兹（THz）成像，因此具有从材料到生物科学的多种应用。然而，对材料边界处的图像对比度形成的定量准确理解以及因此的空间分辨率是一个令人惊讶的未探索领域。在这里，我们介绍了商业硬盘驱动器（HDD）的写/读磁头作为基础研究的最合适的测试样本，因为它具有垂直于其超光滑表面的清晰的材料边界。我们获得了前所未有的和意想不到的见解的s-SNOM图像形成过程中，没有地形引起的文物，往往掩盖和人为修改纯近场光学对比度。在金属-电介质边界上，我们观察到红外和太赫兹照明的非点对称线轮廓，这是完全由数值模拟证实的。我们解释了我们的研究结果的样品依赖的限制和筛选的近场在尖端顶点，这将是至关重要的准确理解和正确的解释高分辨率的s-SNOM图像的纳米复合材料。我们还表明，与超尖锐的钨尖的明显宽度（从而分辨率）的尖锐的材料边界可以减少到约5纳米。
英文摘要：
Scattering-type scanning near-field optical microscopy (s-SNOM) allows for nanoscale resolved Infrared (IR) and Terahertz (THz) imaging, and thus has manifold applications ranging from materials to biosciences. However, a quantitatively accurate understanding of image contrast formation at materials boundaries, and thus spatial resolution is a surprisingly unexplored terrain. Here we introduce the write/read head of a commercial hard disk drive (HDD) as a most suitable test sample for fundamental studies, given its well20 defined sharp material boundaries perpendicular to its ultra-smooth surface. We obtainunprecedented and unexpected insights into the s-SNOM image formation process, free of topography-induced artifacts that often mask and artificially modify the pure near-field optical contrast. Across metal-dielectric boundaries, we observe non-point-symmetric line profiles for both IR and THz illumination, which are fully corroborated by numericalsimulations. We explain our findings by a sample-dependent confinement and screening of the near fields at the tip apex, which will be of crucial importance for an accurate understanding and proper interpretation of high-resolution s-SNOM images of nanocomposite materials. We also demonstrate that with ultra-sharp tungsten tips the apparent width (and thus resolution) of sharp material boundaries can be reduced to about 5 nm.
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