Strong and anisotropic third-harmonic generation in monolayer and multilayer ReS2

We report observation of strong and anisotropic third-harmonic generation (THG) in monolayer and multilayer ReS2. The third-order nonlinear optical susceptibility of monolayer ReS2, vertical bar chi((3))vertical bar, 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 MoS2. 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 ReS2 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.