Direct Observation of Degenerate Two-Photon Absorption and Its Saturation in WS<sub>2</sub> and MoS<sub>2</sub> Monolayer and Few-Layer Films
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Abstract
The optical nonlinearity of WS<sub>2</sub> and MoS<sub>2</sub> monolayer and few-layer films was investigated using the <i>Z</i>-scan technique with femtosecond pulses from the visible to the near-infrared range. The nonlinear absorption of few- and multilayer WS<sub>2</sub> and MoS<sub>2</sub> films and their dependences on excitation wavelength were studied. WS<sub>2</sub> films with 1–3 layers exhibited a giant two-photon absorption (TPA) coefficient as high as (1.0 ± 0.8) × 10<sup>4</sup> cm/GW. TPA saturation was observed for the WS<sub>2</sub> film with 1–3 layers and for the MoS<sub>2</sub> film with 25–27 layers. The giant nonlinearity of WS<sub>2</sub> and MoS<sub>2</sub> films is attributed to a two-dimensional confinement, a giant exciton effect, and the band edge resonance of TPA
