Crystal quality control in 2D TMD structures using polarization-resolved SHG

2D transition metal dichalcogenides (TMDs) offer a unique platform for fundamental science and technological applications. They can be assembled in vertically stacked heterostructures with new physical properties that depend on the relative orientation (twist-angle) between the constituent monolayers. While large-area crystal growth techniques such as CVD are successfully used to produce 2D TMDs, the presence of grain boundaries and defects affect their crystal quality. Here, we demonstrate polarization-resolved second harmonic generation (PSHG) microscopy as an all-optical, high-resolution and minimally-invasive technique for the quality control of TMD-based structures. By fitting, pixel-by-pixel, experimental PSHG images of sub-micron resolution into suitable theoretical models, we can spatially-resolved map the main crystallographic axis, as well as the twist-angle in the overlapping region of TMD van der Waals superlattices. By measuring the mean of the corresponding distributions, we define the standard deviation as crystal quality factor.