Saturation of Two-Photon Absorption in Layered Transition Metal Dichalcogenides: Experiment and Theory

The saturation of two-photon absorption (TPA) in four types of layered transition metal dichalcogenides (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 TMDC nanofilms increases, while TPA saturation intensity has the opposite behavior, under the excitation of 1.2 eV photons with a 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 responses make these layered 2D semiconductors strong candidate materials for optical modulation and other photonic applications.