  The interface between two different semiconductors is crucial in determining
the electronic properties at the heterojunction, therefore novel techniques
that can probe these regions are of particular interest. Recently it has been
shown that heterojunctions of two-dimensional transition metal dichalcogenides
have sharp and epitaxial interfaces that can be used to the next generation of
flexible and on chip optoelectronic devices. Here, we show that second harmonic
generation (SHG) can be used as an optical tool to reveal these atomically
sharp interfaces in different lateral heterostructures. We observed an
enhancement of the SH intensity at the heterojunctions, and showed that is due
to a coherent superposition of the SH emission from each material. This
constructive interference pattern reveals a phase difference arising from the
distinct second-order susceptibilities of both materials at the interface. Our
results demonstrate that SHG microscopy is a sensitive characterization
technique to unveil nanometric features in layered materials and their
heterostructures.
