石墨烯超快载流子动力学及其在全光调制中的应用研究

Graphene has great potential applications in high-speed devices, such as, optical modulator and high-frequency transistor. The performances of these electric and photonic devices are greatly related to the carrier dynamics of graphene. Although there are extensive studies on the photo-excited carrier dynamics, the carrier dynamics of graphene still remain unclear and the underlying physical mechanisms of carrier dynamics are controversial. The relationship between carrier dynamics and Fermi energy is not clear. The study of carrier dynamics with total reflected light under the arrangement of enhancing the interaction between light and matter was seldom involved. In addition, the study of using photo-excited carriers to realize all-optical modulating is scarce and being called. In this project, we will perform some work on the photo-excited carrier dynamics, including fundamental and application study. The following contents will be included: firstly, the anisotropy of carrier dynamics of graphene will be probed with tilted probe beam and total internal reflected beam. Secondly, with the help of field effect transistor the dependence of graphene ultrafast carrier dynamics and saturable absorption on Fermi energy will be measured. Thirdly, we will try to realize all-optical modulating in graphene by using photo-excited carrier. Based on this research, a more comprehensive knowledge on graphene ultrafast carrier dynamics will be obtained and the possible answer to present controversies may be found, in the other hand, the application of graphene photo-excited carriers will be extended to all-optical modulating.

石墨烯在光调制器、高频晶体管等高速光电子器件上有着潜在的应用，而这些高速光电子器件的性能同石墨烯材料内部载流子动态过程相关。虽然人们已经对石墨烯载流子动态过程进行了深入的研究，但仍存在很多不清楚和有争议的地方，载流子动力学过程同费米能的关系还不明确，对于石墨烯在全内反射等与光相互作用增强的新型结构下的载流子动态过程研究还很少。此外，利用石墨烯光激发载流子来实现全光调制的研究目前还很缺乏。因此，本项目将围绕石墨烯光激发载流子开展以下几个方面的研究：①使用倾斜光束和全内反射结构探测石墨烯超快载流子动力学过程的各向异性；②借助石墨烯场效应晶体管研究石墨烯载流子动力学过程和饱和吸收强度对费米能的依赖性；③利用石墨烯光激发载流子对光吸收的影响实现全光调制。期望通过该研究，对石墨烯载流子动力学过程有一个更全面的认识并为现存争议找到可能的答案，同时拓展石墨烯光激发载流子在光调制上的应用。

独特的能带结构导致石墨烯具有优异的光学和电学性质，这些性质使石墨烯在光电子器件上有着广泛的应用。理解石墨烯中光激发载流子的动力学过程和石墨烯的瞬态光学响应将有助于把握石墨烯在光电子器件上的应用，能为设计和优化这些器件的性能提供理论基础。在本项目中，我们使用泵浦-探测技术研究了石墨烯光激发载流子动量空间的分布演化和瞬态光学响应。首先，通过非简并泵浦-探测信号的泵浦偏振和入射方向无关性，揭示了石墨烯对平面内和平面外光场吸收的各向同性，再通过简并泵浦-探测信号峰值的泵浦偏振依赖性揭示了石墨烯中光激发载流子动量空间的分布是各向异性的，结合非简并结果将光激发载流子动量空间的分布随载流子弛豫的演化给描绘出来，并证明载流子-声子散射参与了载流子热化。基于此，我们研究了石墨烯光致消光系数改变的偏振依赖性和机制，相关结果有利于分析任意结构中石墨烯瞬态光学响应的偏振依赖性。另外，我们分析了石墨烯泵浦-探测差分信号符号同样品摆放的关系，发现翻转石墨烯样品会导致差分反射信号的符号发生改变，而差分透射信号不会发生改变。最后，研究了入射方向和载流子分布的各向异性单独或者共同作用下，石墨烯瞬态光反射和透射的偏振依赖性。我们发现入射方向可以赋予瞬态光学响应偏振依赖性，而且可以主导这种偏振依赖性。非平衡载流子分布的各向异性可以改变这种偏振依赖性，改变方式（包括增强、减弱甚至反转）决定于各向异性分布的形式。结合这两种机制，可以产生多样化的偏振依赖性。这些结果将能被用于石墨烯光调制器和光开关等光子器件性能的优化。