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

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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.