As a class of semiconductors, transition metal dichalcogenides (TMDCs) 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). TMDCs show graphene-like layered structure. Strong covalent bonds in layers and weak van der Waals interaction between layers allow TMDCs to form a robust 2D nanostructure. In a TMDC 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 TMDCs 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 TMDCs a huge potential in the development of photonic devices and components with high performance and unique functions.
In this work, the saturation of two-photon absorption (TPA) in four types of layered TMDCs (MoS2, WS2, MoSe2, WSe2) was systemically studied both experimentally and theoretically. It was demonstrated that the TPA coefficient is decreased when either the incident pulse intensity or the thickness of the TMDCs nanofilm is increased, while TPA saturation intensity has the opposite performance, under the excitation of 1.2 eV photons with pulse width of 350 fs. A three-level excitonic dynamics simulation indicates that the fast relaxation of the excitonic dark states, the exciton-exciton annihilation and the depletion of electrons in the ground state contribute significantly to TPA saturation in TMDC nanofilms. Large third order nonlinear optical response makes these layered 2D semiconductors strong candidate materials for optical modulation and other photonic applications.