Optical nonlinearity in a silver-decorated reduced graphene oxide–molybdenum disulfide (Ag–rGO–MoS₂) nanocomposite was experimentally investigated via the Z-scan technique using a Q-switched Nd:YAG nanopulsed green laser. An interesting switching behaviour from saturable to reverse saturable absorption with varying input on-axis intensity of the laser was demonstrated. Under low-intensity laser excitation, Ag–rGO–MoS₂ displayed ground-state bleaching, which resulted in saturable absorption (SA) behaviour. Here, the prominent SPR phenomenon of Ag enforced plasmon absorption and is assigned to the SA process. Interestingly at higher intensity, the material switched its nonlinearity to reverse saturable absorption (RSA), attributed to two different mechanisms of two-photon absorption (2PA). At moderate laser pulse energies, the sample underwent sequential 2PA, which is underpinned by calculated varying nonlinear optical parameters and excited-state absorption cross-sections. However, at higher intensities, nonlinear absorption coefficients and excited-state absorption cross-sections remained constant, indicating the occurrence of genuine 2PA. In addition, theoretical estimation of the 2PA cross-section validates the observed transition. Thus, experimental evidence for the validity of nonlinear absorption theory (intensity-dependent transition from linear to nonlinear SA to nonlinear RSA to genuine 2PA via sequential 2PA) is provided for the first time in the literature.

中文翻译：

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深入了解掺银还原氧化石墨烯-二硫化钼 (Ag-rGO-MoS2) 混合物中的非线性吸收跃迁


使用Q开关Nd:YAG纳米脉冲绿光激光器通过Z扫描技术对银装饰的还原氧化石墨烯-二硫化钼(Ag-rGO- _MoS2 )纳米复合材料中的光学非线性进行了实验研究。演示了随着激光输入轴上强度的变化，从可饱和吸收到反向可饱和吸收的有趣切换行为。在低强度激光激发下，Ag–rGO–MoS ₂表现出基态漂白，从而导致可饱和吸收（SA）行为。在这里，Ag 的显着 SPR 现象强制等离子体吸收，并被分配给 SA 过程。有趣的是，在较高强度下，该材料将其非线性转变为反向饱和吸收（RSA），这归因于两种不同的双光子吸收（2PA）机制。在中等激光脉冲能量下，样品经历了连续 2PA，其基础是计算出的不同非线性光学参数和激发态吸收截面。然而，在较高强度下，非线性吸收系数和激发态吸收截面保持恒定，表明出现了真正的 2PA。此外，2PA 横截面的理论估计验证了观察到的转变。因此，文献中首次提供了非线性吸收理论（从线性到非线性 SA 到非线性 RSA 到真正 2PA 的强度相关过渡，通过顺序 2PA）有效性的实验证据。