Strong and Anisotropic Third Harmonic Generation in Monolayer and Multilayer ReS${}_2$

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${}_2$, $\mid {\chi }^{(3)}\mid$ 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.