  The in-plane anisotropic properties of two-dimensional (2D) group IV
monochalcogenides provide an additional degree of freedom which can be used in
future optoelectronic devices. Here, it is shown that the third harmonic
generation (THG) signal produced by ultrathin tin (II) sulfide (SnS) is
in-plane anisotropic with respect to the incident linear polarization of the
laser field. We fit the experimental polarization-resolved THG (P-THG)
measurements with a nonlinear optics model, which accounts for the orthorhombic
crystal structure of 2D SnS. We calculate the relative magnitudes of the
\{chi}^(3) tensor components by recording and simultaneously fitting both
orthogonal components of the P-THG intensity. Furthermore, we introduce a THG
anisotropy ratio, whose calculated values compare the total THG intensity when
the excitation linear polarization is along the armchair crystallographic
direction with the case when it is along the zigzag direction. Our results
provide quantitative information on the anisotropic nature of the THG process
in SnS, paving the way to a better understanding of anisotropic nonlinear
light-matter interactions, and the development of polarization-sensitive
nonlinear optical devices.
