  Atomically thin two-dimensional (2D) materials can be vertically stacked with
van der Waals bonds, which enable interlayer coupling. In the particular case
of transition metal dichalcogenide (TMD) bilayers, the relative direction
between the two monolayers, coined as twist-angle, modifies the crystal
symmetry and creates a superlattice with exciting properties. Here, we
demonstrate an all-optical method for pixel-by-pixel mapping of the twist-angle
with resolution of 0.23 degrees, via polarization-resolved second harmonic
generation (P-SHG) microscopy and we compare it with four-dimensional scanning
transmission electron microscopy (4D-STEM). It is found that the twist-angle
imaging of WS2 bilayers, using the P-SHG technique is in excellent agreement
with that obtained using electron diffraction. The main advantages of the
optical approach are that the characterization is performed on the same
substrate that the device is created on and that it is three orders of
magnitude faster than the 4D-STEM. We envisage that the optical P-SHG imaging
could become the gold standard for the quality examination of TMD
superlattice-based devices.
