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

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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.