  Layered franckeite is a natural superlattice composed of two alternating
layers of different compositions, SnS$_2$- and PbS-like. This creates
incommensurability between the two species along the planes of the layers,
resulting in spontaneous symmetry-break periodic ripples in the \textit{a}-axis
orientation. Nevertheless, natural franckeite heterostructure has shown
potential for optoelectronic applications mostly because it is a semiconductor
with 0.7 eV bandgap, air-stable, and can be easily exfoliated down to ultrathin
thicknesses. Here, we demonstrate that few-layer franckeite shows a highly
anisotropic nonlinear optical response due to its lattice structure, which
allow for the identification of the ripple axis. Moreover, we find that the
highly anisotropic third-harmonic emission strongly varies with material
thickness. These features are further corroborated by a theoretical nonlinear
susceptibility model and the nonlinear transfer matrix method. Overall, our
findings help to understand this material and propose a characterization method
that could be used in other layered materials and heterostructures to assign
their characteristic axes.
