TWIST-NANOSPEC: Development of Advanced Optical Nano-spectroscopy Techniques for Twistronics

涡旋电子学利用了一种新的可调原子晶格周期性，称为莫尔<s:1>超晶格（mSL），在少原子厚的范德华异质结构中，它不仅在凝聚态物质中高度可调的量子相的基础物理学中很重要，而且在未来的光电和量子技术应用中也很有前景。尽管涡旋电子学有着巨大的兴趣和巨大的潜力，但目前有限的表征工具可以直接应用于纳米尺度上的msl的详细研究，从而阻碍了这一充满活力的新兴领域的进展。在这里，我们的目标是开发新的纳米成像技术，直接观察扭曲过渡金属二硫族化合物双层（由两个单层相互旋转而成）中的msl，利用几种光学方法，包括使用散射型扫描近场光学显微镜实现的线性和非线性光谱学，该显微镜配备了可在可见光，近红外和中红外的宽波长范围内进行光谱的激光器。莫尔激子是半导体msl中的一种基本激发，我们将在线性体系中以及对激子态对称性和异质层原子晶格敏感的共振二、三次谐波实验中进行探测。此外，我们的技术将能够在具有封装介质或导电层的样品中“看到表面以下”，这是表面扫描技术无法实现的特征。利用我们开发的技术，mSL有望在室温下具有高对比度，非接触和高分辨率的优势。我们新开发的纳米成像和纳米光谱学方法将为一种独特但潜在的非常广泛的扭曲异质结构的行为提供根本性的新见解，也将适用于其他纳米材料系统（如钙钛矿），其中纳米尺度上的纳米成像和光谱学也是非常可取的。

Twistronics exploits a new tunable atomic lattice periodicity called moiré superlattices (mSL) in few-atom-thick van der Waals heterostructures, which is shown and predicted not only to be important in fundamental physics towards highly tunable quantum phases in the condensed matter but also promising for future optoelectronic and quantum technology applications. Despite the great interest and high potential of twistronics, limited characterisation tools exist that can be directly applied for detailed investigation of mSLs on the nanoscale thus far impeding progress in this vibrant emerging field. Here, we aim to develop new nano-imaging techniques to directly observe mSLs in twisted transition metal dichalcogenides bilayers (built from two monolayers rotated with respect to each other) by utilizing several optical methods, including the linear and nonlinear optical spectroscopy realised using scattering-type scanning near-field optical microscope equipped with lasers enabling spectroscopy in a wide range of wavelengths in the visible, near-infrared and mid-infrared. Moiré excitons, a fundamental excitation in semiconducting mSLs will be probed in the linear regime as well as in the resonant second- and third-harmonic generation experiments sensitive to the symmetry of the excitonic states and the heterobilayer atomic lattice. Furthermore, our techniques will enable to "see below the surface" in samples with encapsulated dielectric or conducting layers, a feature unaccessible for surface scanning techniques. With our developed techniques, the mSL is expected to be visualised with advantages of high contrast, non-contact, and high resolution at room temperature. Our newly developed approaches for nano-imaging and nano-spectroscopy will provide fundamentally new insights in the behaviour of a unique but potentially very extended class of twisted heterostructures and will also be applicable in other nano-materials systems (such as perovskites), where nano-imaging and spectroscopy on the nanoscale are also highly desirable.