Tenth-Order Multiphoton Excitation and Saturable Second Harmonic Generation in Polyoxometalate-Exfoliated Molybdenum Disulfide

Nanomaterials are promising alternatives to traditional bulk crystals for nonlinear optics and energy conversion materials. In this study, we report the observation of saturable second harmonic generation and high-order (up to 10th) multiphoton photo-luminescence in colloidal MoS2 nanoflakes prepared by redox exfoliation which generates polyoxometalate clusters. Fourier trans-form nonlinear optical spectroscopy enabled the resolution of these high-order signals through two-dimensional nonlinear excitation/ detection correlation spectra. Complementary modeling of the nonlinear interferograms suggested mechanisms for the observed high-order signals, which involve saturation of transitions resonant with the harmonic energy and multiple competing orders of multiphoton absorption. The saturable second harmonic generation and high-order multiphoton photoluminescence, which were not observed in MoS2 prepared by chemical vapor deposition, are related to the formation of polyoxometalate clusters during the redox exfoliation process. Further studies demonstrated that polyoxometalates can induce high-order nonlinear optical effects in other colloidal semiconductors with resonances at the harmonic energy, suggesting a general route to drastically alter the nonlinear optical response of nanomaterials with molecular adsorbates. This response is due in part to the interfacial charge transfer between polyoxometalate species and the nanomaterial. Fourier transform nonlinear optical spectroscopy offers a route to resolving the effects of molecular adsorbates, which are obscured in traditional nonlinear optical measurements.