Opt Express. 2023 Feb 13;31(4):7012-7022. doi: 10.1364/OE.483804.
Scattering-type scanning near-field optical microscopy (s-SNOM) allows for nanoscale optical mapping of manifold material properties. It is based on interferometric recording of the light scattered at a scanning probe tip. For dielectric samples such as biological materials or polymers, the near-field amplitude and phase signals of the scattered field reveal the local reflectivity and absorption, respectively. Importantly, absorption in s-SNOM imaging corresponds to a positive phase contrast relative to a non-absorbing reference sample. Here, we describe that in certain conditions (weakly or non- absorbing material placed on a highly reflective substrate), a slight negative phase contrast may be observed, which can hinder the recognition of materials exhibiting a weak infrared absorption. We first document this effect and explore its origin using representative test samples. We then demonstrate straightforward simple correction methods that remove the negative phase contrast and that allow for the identification of weak absorption contrasts.
散射型近场光学显微镜（s-SNOM）可实现多种材料性质的纳米级光学测绘。它基于在扫描探针尖端散射光的干涉记录。对于介电样品（如生物材料或聚合物），散射场的近场幅度和相位信号分别揭示了局部反射率和吸收率。重要的是，相对于非吸收参考样品，s-SNOM 成像中的吸收对应于正的相位对比度。在这里，我们描述了在某些条件下（放置在高反射衬底上的弱吸收或非吸收材料），可能会观察到轻微的负相位对比度，这可能会阻碍对表现出弱红外吸收的材料的识别。我们首先使用代表性的测试样品记录了这种效果，并探讨了其起源。然后，我们演示了简单的校正方法，可以消除负相位对比度，并能够识别弱吸收对比度。
