  Two-dimensional (2D) graphene and graphene-related materials (GRMs) show
great promise for future electronic devices. Nevertheless, GRMs result distinct
properties under the influence of the substrate that serves as support through
uneven compression/ elongation of GRMs surface atoms. Strain in GRM monolayers
is the most common feature that alters the interatomic distances, band
structure, providing a new degree of freedom that allows regulation of their
electronic properties and introducing the field of straintronics. Having an
all-optical detection, a minimally invasive tool that rapidly probes strain in
large areas of GRM monolayers, would be of great importance in the research and
development of novel 2D devices. Here, we use Polarization-resolved Second
Harmonic Generation (P-SHG) optical imaging to identify strain distribution,
induced in a single layer of WS2 placed on a pre-patterned Si/SiO2 substrate
with cylindrical wells. By fitting the P-SHG data pixel-by-pixel, we produce
spatially resolved images of the crystal armchair direction. In regions where
the WS2 monolayer conforms to the pattern topography, a distinct cross-shaped
pattern is evident in the armchair image owing to strain. The presence of
strain in these regions is independently confirmed using a combination of
atomic force microscopy and Raman mapping.
