  We report observation of strong and anisotropic third harmonic generation
(THG) in monolayer and multilayer ReS$_2$. The third-order nonlinear optical
susceptibility of monolayer ReS$_2$, $\left| \chi^{(3)} \right|$ is on the
order of $10^{-18} $ m$^2$/V$^2$, which is about one order of magnitude higher
than reported results for hexagonal-lattice transition metal dichalcogenides
such as MoS$_2$. A similar magnitude for the third-order nonlinear optical
susceptibility was also obtained for a multilayer sample. The intensity of the
THG field was found to be dependent on the direction of the incident light
polarization for both monolayer and multilayer samples. A point group symmetry
analysis shows that such anisotropy is not expected from a perfect $1T$
lattice, and must arise from the distortions in the ReS$_2$ lattice. Our
results show that THG measurements can be used to characterize lattice
distortions of two-dimensional materials, and that lattice distortions are
important for the nonlinear optical properties of such materials.
