Controllable Broadband Nonlinear Optical Response of Graphene Dispersions

作为第一种被实验证实的二维晶体材料，石墨烯凭借其独特的结构和物理化学性质掀起了新一轮纳米研究热潮。石墨烯不仅拥有优良的电学，热学和力学性质，同时也具有独特的光子学性质。随着人们对石墨烯光电性质理解的不断加深和多元化的大批量、低成本合成方法的实现，石墨烯光子学实验研究从2008年萌芽，到现在短短几年间，得到了迅速和广泛的发展。研究表明，石墨烯在光子器件方面具有巨大的应用价值。本文采用液相剥离技术制备石墨烯分散液，利用改进的开孔Z扫描装置，通过调整气压实现了石墨烯的可控宽带非线性光学响应；研究了Ag/G复合材料在低沸点IPA溶液中分散后的非线性光学性质。主要结果如下： 1．实验和理论分析都揭示，用于分散石墨烯的溶剂的表面能只有与石墨的表面能很好的匹配时，才能将克服石墨层片之间范德华力的能量消耗降到最低，从而将石墨有效的剥离成单层或少层。因此，实验中我们选用N-甲基吡咯烷酮、二甲基甲酰胺、胆酸钠水溶液作为分散剂，通过液相剥离技术制备能够稳定存在的石墨烯分散液。同时，对石墨烯分散液进行了透射电子显微镜表征，统计数据显示，分散液中单层石墨烯片数目占总石墨片数目的30%。 2．为了改变石墨烯分散液周围的气压状态，我们设计并制作了一个密封装置（包括一个不锈钢密封腔和一个抽真空装置），可以方便快捷地实现石墨烯分散液上方气压的精确控制，气压调节范围从0.1 MPa（常压）到0.004 MPa。该密封装置稳定性高，操作简单，气压精确可调，可以实现各种非线性散射介质光学响应的定量控制。 3．对于纳秒脉冲激光辐照的石墨烯分散液而言，热致微气泡引起的非线性散射是其产生非线性光学响应的主要机制。结合密封装置，通过改进的Z扫描技术，我们研究了气压的变化对石墨烯分散液非线性光学响应的影响。研究发现，对于532 nm和1064 nm纳秒激光脉冲，通过控制气压，均可以实现石墨烯分散液非线性光学响应的可控调节。气压越低，非线性光学响应越强。与石墨烯相比，富勒烯的非线性光学性质不受气压变化的影响，因为其非线性响应的主要机制为非线性吸收。这项工作提供了一种简单的方法，可以很好地区别非线性纳米材料的非线性散射和非线性吸收机制。 4．纯石墨烯具有优异的非线性光学响应，在低入射能量下主要机制为饱和吸收，在高入射能量下主要机制为热致非线性散射。Ag纳米颗粒同样拥有很好的非线性光学性质，其主要机制为双光子吸收和自由载流子吸收。我们初步研究了Ag/G复合材料在不同能量密度下的非线性光学性质，及不同Ag含量的复合材料相似的非线性光学性质。通过对石墨烯复合材料的研究，初步了解基于不同非线性机制的各个组分对整体非线性行为的贡献大小，为以后的工作奠定了基础。As the first experimental confirmed 2D crystal material, graphene gave rise to a new research round due to its unique structures and physical properties. Graphene has been demonstrated to possess unique optical and photonic properties, in addition to the outstanding electronic, mechanical and thermal properties. With the deep understanding of photoelectric properties of graphene and the realization of high-production, low-cost synthetic method, the nonlinear optical research on graphene has obtained rapid and extensive development since 2008. The results indicate that graphene has great application prospects in nanophotonic devices. In this dissertation, we report the synthesis of graphene suspensions by using liquid-phase exfoliation technique. A modified open-aperture Z-scan apparatus in combination with a vacuum system was used to realize the broadband nonlinear optical response of graphene dispersions by tuning vacuum pressure. In addition, the nonlinear optical properties of Ag/G/IPA under different energy densities were studied. 1. Experimental and theoretical analyses reveal that the surface energies of the selected solvents match very well that of graphite, resulting in a minimal energy cost of overcoming the van der Waals forces between two graphene sheets, hence the effective exfoliation to graphene with single or few layers. In our experiments, we selected several solvents, say, N-methyl-2-pyrrolidone (NMP), N, N-dimethyl-formamide (DMF) and sodium cholate (SC), to prepare high quality graphene dispersions by liquid-phase exfoliation technique. The dispersions were stable against sedimentation over several weeks. Those graphene dispersions were characterized by using transmission electron microscopy, and the thickness statistics showed that the number fraction of monolayer graphene (number of monolayer/total number of graphite flakes) in each dispersion was close to 30%. 2. In order to tune the pressure over graphene dispersions, we designed and fabricated the sealing equipment, including a stainless steel seal chamber and a vacuum device, which can control the atmospheric pressure over graphene dispersions conveniently and tune the atmospheric pressure at a range of 0.1 MPa to 0.004 MPa. The sealing device, with advantages of high stability, simple to operation, adjusting pressure precisely, is suitable for the study of quantitative control of nonlinear optical response of nonlinear scattering medium. 3. Nonlinear scattering, originating from laser induced micro-bubbles, is regarded as the principal mechanism for nonlinear optical (NLO) response of graphene dispersions at ns timescale. A modified open-aperture Z-scan apparatus in combination with a vacuum system was used to study the effect of vacuum pressure on the NLO property of graphene dispersions. We showed that the atmospheric pressure can be utilized to control and tune the nonlinear responses of the graphene dispersions for ns laser pulses at both 532 nm and 1064 nm. The lower the vacuum pressure was, the larger the NLO response was. In contrast, the NLO property of fullerene was found to be independent of the pressure change, due to its nature of nonlinear absorption. This work afforded a simple method to distinguish the nonlinear scattering and absorption mechanisms for NLO nanomaterials. 4. As has been reported recently, pure graphene dispersions exhibit remarkable NLO response as a result of saturable absorption (SA) at low incident intensities and thermally induced nonlinear scattering (NLS) at high input energies. Ag nano-particles also have a strong NLO effect, which is attributed to two-photon absorption (TPA) as well as to free-carrier absorption (FCA). The nonlinear optical properties of Ag/Graphene composites under different energy density were researched preliminary and varying Ag loadings displayed a similar NLO behavior with the increase in the incident energy density. Through our preliminary research on graphene dispersions, the role of each component with different nonlinear mechanisms in OL response can be distinguished and laid a foundation for later work