Polar domain walls are currently at the focus of intensive research owing to their unusual and highly localized functional properties, which bear great potential for technological applications. They can present unusual topological features, like swirling polar structures or defect lines. The prediction of possible non-Ising and chiral internal structures of polar domain walls has been a particularly important development in this topic over the past years. This Tutorial highlights the capabilities of non-linear optics to probe these newly discovered aspects in polar non-Ising type domain walls through the second-harmonic generation (SHG) process. Fundamental symmetry properties of domain walls are presented in the context of recent advances on chiral and abnormal polar structures. We introduce the basics of the SHG and its ability to probe the symmetry down to the nanoscale, and we explain how to obtain insight into the non-Ising character of polar domain walls by combining the SHG polarimetry analysis with modeling.

中文翻译：

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使光消散在非绝缘的极性畴壁上：二次谐波产生显微镜和极化分析的洞察力

极性畴壁由于其不寻常且高度局部化的功能特性，目前正成为广泛研究的重点，而这些性能特性在技术应用中具有巨大的潜力。它们可以表现出不寻常的拓扑特征，例如旋转的极性结构或缺陷线。在过去的几年中，对极性畴壁可能的非Ising和手性内部结构的预测一直是该主题中特别重要的发展。本教程重点介绍了非线性光学器件通过二次谐波生成（SHG）过程探测极性非Ising型畴壁中这些新发现的方面的能力。畴壁的基本对称性质是在手性和异常极性结构的最新进展的背景下提出的。