Direct Observation of Degenerate Two-Photon Absorption and Its Saturation in WS 2 and MoS 2 Monolayer and Few-Layer Films
- 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
- JWJun Wang
ACS Nano · 2015 · American Chemical Society
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Abstract
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) × 10(4) 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.