Third-harmonic generation in ultrathin films of MoS2.

We observe optical third-harmonic generation in atomically thin films of MoS2 and deduce effective third-order nonlinear susceptibilities on the order of 10(-19) m(2)/V(2), which is comparable to that of commonly used semiconductors under resonant conditions. By measuring the susceptibility as a function of light wavelength, we find significant enhancements of the susceptibility by excitonic resonances. The demonstrated third-harmonic generation can be used for nonlinear optical identification of MoS2 atomic layers with high contrast, better distinguishing power of multilayers, and less restrictions to substrate selections. The size of the third-order nonlinear susceptibility suggests feasibility of exploring other types of third-order nonlinear optical effects of MoS2 two-dimensional crystals.