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As a class of semiconductors, transition metal dichalcogenides (TMDs) have the formula MX2, where M stands for a transition metal (i.e., Mo, W, Ti, Nb, etc.) and X stands for a chalcogen (i.e., S, Se or Te). TMDs show graphene-like layered structure. Strong covalent bonds in layers and weak van der Waals interaction between layers allow TMDs to form a robust 2D nanostructure. In a TMD monolayer, the single transition metal layer is sandwiched between the two chalcogen layers. Owing to the specific 2D confinement of electron motion and the absence of interlayer coupling perturbation, 2D layered TMDs show unique photonics-related physical properties, e.g.,
1) Sizable and layer-dependent bandgap, typically in the 1-2 eV range;
2) Indirect-to-direct bandgap transition as the decreasing of the number of monolayer;
3) Fairly good photoluminescence and electroluminescence properties;
4) Remarkable excitonic effects, i.e., high binding energy, large oscillator strength and long lifetime.
In combination of the ultrafast carrier dynamics and molecular-scale thickness, the prominent properties manifest the 2D TMDs a huge potential in the development of photonic devices and components with high performance and unique functions.
We have extensively studied the ultrafast nonlinear absorption and nonlinear refraction of layered MX2 (X=S, Se, Te) over broad wavelength (Vis-NIR) and time (fs-ps-ns) ranges. Large area MoS2 neat films with controllable thicknesses were fabricated from liquid-exfoliated MoS2 dispersions by vacuum filtration. The MoS2 films show superior broadband ultrafast saturable absorption (SA) performance, in comparison with the graphene films and the MoS2 dispersions. Very recently, we observed giant two-photon absorption (TPA) coefficient in a WS2 monolayer. The order of magnitude of TPA coefficient in WS2 monolayer (~100 cm/MW) exceeds that of the conventional semiconductors (e.g., CdTe, GaAs, ZnS, ZnO, etc.) by a factor of 3-4. This is also the first Z-scan performance on an optical medium with a thickness as tiny as 0.75 nm. Moreover, a comprehensive study on the layer-dependent nonlinear photonic effect was carried out in MoS2 mono- and few-layers by CVD growth. SA to TPA transition was confirmed when the thickness changes from few-layer to monolayer. In addition, a spatial self-phase modulation method has been applied to tune the nonlinear refractive index of TMD dispersions.
The above-mentioned works have opened up a door towards 2D semiconductor based nonlinear photonics, spectroscopy and relevant photonic devices.
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