Investigation of van der Waals heterostructure interlayer coupling on large length scales through widefield pump-SHG probe microscopy*

The strong light absorption, rich exciton diversity and non-epitaxial device construction make transition metal dichalcogenide (TMD) heterobilayers (HBLs) a popular candidate for next-generation optoelectronic devices. Strong interlayer coupling affords TMD HBL-based devices their novel capabilities which depends on a close spatial proximity between the constituent monolayers. One indicator of close interlayer contact is the increase in second harmonic generation (SHG) intensity in the presence of an optical pump [1]. The energetic offset of the band edges between the constituent monolayers drives the photoexcited electrons to reside in one layer and holes in the other, resulting in an SHG-active transient dipole at the interface. Here, we investigate on a large spatial scale (100s µm) the interlayer coupling of a WS₂-MoSe₂ HBL constructed from a popular gold-tape exfoliation technique [2]. A hyperspectral widefield microscope spatially monitors pump-induced changes of sub-band gap SHG from the HBL. We find there are abrupt differences in the pumped SHG intensity and therefore the layer coupling across the lateral extent of the HBL. These findings are not present in basic characterization methods such as photoluminescence and reflection contrast spectroscopy, which both exhibit strong interlayer coupling for the studied HBL. Our work suggests the widefield pump-SHG probe microscopy technique is supplementary to well-established characterization methods that describe interactions at the interface in TMD HBLs.