Chasing Polaritons: A Pathway to investigate the optoelectronic properties of van der Waals heterostructures

在本项目中，我们将利用我们首次建立的可见光激发波长可调近场扫描光学显微镜，对二维范德华异质结构的激子极化进行成像，观察层间激子和光学莫尔条纹。过渡金属二硫族化合物（TMDs）的Van-der-Waals异质结构显示出一层电子与另一层空穴之间的束缚态对应的特殊激发。由于抑制了辐射衰变，这些层间激子和它们的三角子预计会显示出优越的寿命和传播长度。我们将通过记录纳米级分辨率的近场干涉模式来成像层间激子和三角极子在实际空间中的传播。二维异质结构的第二个显著特征是由于两个不对齐的晶格（取向，晶格常数）堆叠而形成的波纹图案。我们想通过记录吸收系数的近场图像来成像局部原子构型的变化，吸收系数对电子势的微小变化非常敏感。通过将激光调谐到与莫尔晶格跃迁的共振中，我们将观察到由于带弯曲和超晶格内的局域光学量子点而引起的极化子传播畸变。我们的实验将针对不同的材料组合和相对的单层取向进行。将纳米显微镜与远场和近场光谱学相结合，我们将提取激发能、寿命、传播长度和光-物质相互作用的耦合强度。我们将开发利用光谱学表征TMD范德华异质结构的技术。我们的项目将受益于SPP 2D材料在样品制备，表征和理论建模方面的强大合作。

In this project we will image exciton polaritons of 2D van der Waals heterostructures to observe interlayer excitons and optical moiré patterns using a wavelength-tunable near-field scanning optical microscope with visible laser excitation, which was first build by us. Van-der-Waals heterostructures of transition-metal dichalcogenides (TMDs) show peculiar excitations that correspond to bound states between an electron in one and a hole in the other layer. These interlayer excitons and their trions have been predicted to show superior lifetime and propagation length, because of suppressed radiative decay. We will image the propagation of the interlayer exciton and trion polaritons in real space by recording near-field interference patterns with nanoscale resolution. The second remarkable feature of 2D heterostructures are moiré patterns that form due to the stacking of two misaligned lattices (orientation, lattice constant). We want to image the change in local atomic configuration by recording near-field images of the absorption coefficient that is highly sensitive to small changes in the electronic potential. By tuning the laser into resonance with the transitions in the moiré lattice we will observe the distortion of the polariton propagation due to band bending and localized optical quantum dots within the superlattice. Our experiments will be performed for various materials combinations and relative monolayer orientation. Combining nanoscale microscopy with far-field and near-field spectroscopy we will extract excitation energies, lifetimes, propagation length, and the coupling strength of light-matter interaction. We will develop techniques for characterizing TMD van-der-Waals heterostructures using optical spectroscopy. Our project will benefit from strong collaborations within the SPP 2D Materials for sample preparation, characterization, and theoretical modeling.