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Direct Observation of Degenerate Two-Photon Absorption and Its Saturation in WS2 and MoS2 Monolayer and Few-Layer Films

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journal contribution
posted on 28.07.2015, 00:00 by Saifeng Zhang, Ningning Dong, Niall McEvoy, Maria O’Brien, Sinéad Winters, Nina C. Berner, Chanyoung Yim, Yuanxin Li, Xiaoyan Zhang, Zhanghai Chen, Long Zhang, Georg S. Duesberg, Jun Wang
The optical nonlinearity of WS2 and MoS2 monolayer 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 WS2 and MoS2 films and their dependences on excitation wavelength were studied. WS2 films with 1–3 layers exhibited a giant two-photon absorption (TPA) coefficient as high as (1.0 ± 0.8) × 104 cm/GW. TPA saturation was observed for the WS2 film with 1–3 layers and for the MoS2 film with 25–27 layers. The giant nonlinearity of WS2 and MoS2 films is attributed to a two-dimensional confinement, a giant exciton effect, and the band edge resonance of TPA.