Abstract:
　　Owing to the specific two-dimensional (2D) confinement of electron motion and the absence of interlayer perturbation, 2D semiconductors possess unique optoelectronic properties and has become a research hot-spot in recent years. Whereas the electronic and luminescent properties of 2D transition metal dichalcogenide (TMDC) nanosheets, say, MoS2, MoSe2, WS2, etc., have been generating much research interest, the ultrafast nonlinear optical (NLO) properties remain largely unexplored. Realized that the sizable and thickness-dependent bandgap offers TMDCs a huge potential in the development of photonic devices with high performance and unique functions, we studied extensively the ultrafast NLO property of a range of TMDC nanosheets. 2D TMDC nanosheets with high-quality layered nanosheets were prepared using liquid-phase exfoliation and CVD techniques. Ultrafast saturable absorption, two-photon absorption, ultrafast nonlinear photoluminescence were observed from the 2D nanostructures. The exciting results open up the door to 2D photonic nano-devices, such as optical switches, pulse shaping devices, mode-lockers, optical limiters, etc., capable of ultrafast response and broadband tunability.