▎ 摘　　要

We experimentally show that Difference Frequency Generation (DFG) can efficiently occur in a graphene ink for a visible pump-probe radiation in consequence of the second order nonlinear optical response of graphene. DFG induces the local field enhancement over the graphene sheets and subsequently motivates Surface Plasmons (SPs). We then show that SPs can appear as internal feedback entities to realize Optical Bistability (OB) if a large nonlinear response is guaranteed. However, the figure of merit is the experimental demonstration of ultra-low rise/fall threshold OB process, not only by using no external feedback procedure but also caused by a single visible guide beam (in contrast to the double pump-probe radiation). The reason is inferred as the presence of strong Raman scattering which in turn triggers a probe radiation interacting with the guide beam as the pump radiation. We repeat the experiment for different wavelengths/nonlinear medium lengths. We deduce that the smaller lengths as well as the larger wavelengths demonstrate better OB features. We previously bring a simple theory, yet mathematically rigorous to interpret the effects. Then, conforming to the Ishikawa's theory of the graphene nonlinear optical response as the interplay of both interband and intraband transitions, we indicate the agreement between the experimental observations and simulation results. Our experimental results can delineate a plain route for Raman scattering and the other elastic scattering mechanisms within the complex medium of graphene ink, breeding thus the connected theory development. On the other hand, since the low threshold OB can be used to sense SPs' formation elaborately, our proposed approach to control SPs via a single visible beam can be extended to the potential applications like the optical switching, plasmonic wave generation, sensing, etc. with the purpose of energy saving. (C) 2019 Elsevier B.V. All rights reserved.