Direct Observation of Degenerate Two-Photon Absorption and Its Saturation in WS2and MoS2Monolayer and Few-Layer Films

The optical nonlinearity of WS2and MoS2monolayer and few-layer films was investigated using the Z-scan technique with femtosecond pulses from the visible to the near-infrared range. The nonlinear absorption of few- and multilayer WS2and MoS2films and their dependences on excitation wavelength were studied. WS2films with 1–3 layers exhibited a giant two-photon absorption (TPA) coefficient as high as (1.0 ± 0.8) × 104cm/GW. TPA saturation was observed for the WS2film with 1–3 layers and for the MoS2film with 25–27 layers. The giant nonlinearity of WS2and MoS2films is attributed to a two-dimensional confinement, a giant exciton effect, and the band edge resonance of TPA.