液相剥离石墨烯的空间自相位调制研究
文献类型：学位论文
| 作者 | 王高中 |
| 学位类别 | 硕士 |
| 答辩日期 | 2013 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 王俊 |
| 关键词 | 石墨烯 液相剥离法 自相位调制 塌缩效应 三阶非线性极化率 非线性折射率 |
| 其他题名 | Spatial Self-phase Modulation of Liquid-Phase-Exfoliated Graphene |
| 中文摘要 | 石墨烯由于具有优良的力学、热学、电学和光学性能而引发前所未有的关注，尤其在光学领域，在不同的条件下，石墨烯呈现卓越的双光子吸收、非线性散射、激发态吸收和饱和吸收等非线性光学性能。石墨烯可用于开发透明电极、发光器件、光伏电池、光电探测器 、激光锁模器、光开关、激光防护设备、生物传感器等器件。然而，对石墨烯非线性的相干散射只有少数报道。 在本文中，我们用波长为633nm的连续光研究了石墨烯非线性的相干光学行为。论文的主要研究内容和结果如下： 1. 分别用采胆酸钠和NMP为分散剂以液相剥离法制备了高品质石墨烯分散液。拉曼光谱和透射电镜结果表明，液相剥离法得到了高品质石墨烯分散液；此外还通过测量石墨烯分散液的透射谱计算了比色皿中石墨烯的NMP分散液的有效层数。另外，还制备了表面光滑、石墨烯均匀分布的石墨烯/PVA薄膜。将石墨烯有效地分散在PVA溶液中，用热蒸发法制备出表面粗糙度低于17 nm/(1 mm×1 mm)的石墨烯/PVA薄膜。所得石墨烯薄膜的厚度约为68 µm。 2. 计算了石墨烯的非线性折射率（n_2）和三阶非线性极化率（χ^（3） ）。所使用的激光器为氦氖激光器，采用波长为633nm连续光进行聚焦，通过空间自相位调制(SSPM)技术测量了石墨烯的非线性折射率和三阶非线性极化率。用不同浓度的石墨烯分散液和不同长度的比色皿做了多组对比实验，得出单层石墨烯的χ^（3） 高达~10-7 esu，非线性折射率n_2为~10-9 m2/W。结果证实了SSPM技术是表征材料非线性性能行之有效的可靠技术。此外，还通过基尔霍夫衍射理论计算SSPM衍射后的光强分布，结果和实验值相符合。同时，在文章中定性分析了石墨烯/PVA薄膜的SSPM效应。 3. 分析了SSPM衍射图案的塌缩效应。在SSPM效应中，衍射图案形成之后的几秒之内，从一系列同心圆环变型为不对称的环，这称为SSPM的塌缩效应。研究发现，石墨烯分散液的非线性折射率的改变是引起SSPM衍射图案塌缩的主要原因。而石墨烯分散液折射率改变是由热致对流和气泡的产生引起。石墨烯分散液吸收了激光的部分能量后，在内部形成温度梯度，引起一定的对流，导致光束通过的地方的非线性折射率分布不均匀。另外气泡的产生也会改变石墨烯分散液的非线性折射率。 4. 通过研究SSPM塌缩效效应，计算材料非线性折射率的变化量。选用长度为10 mm的比色皿，有效石墨烯层数为71层，在入射功率为~54 W/cm2，石墨烯分散液温度从20℃升到100℃时，非线性折射率的相对变化率从为~20.0%升到~37.5%。 |
| 英文摘要 | Graphene has drawn a lot of attention because of its remarkable mechanical, thermal and electrical properties especially its optical properties. Graphene presents outstanding nonlinear optical performances such as excellent two-photon absorption, nonlinear scattering, excited state absorption and satutated absorption. Graphene has potential applications in areas of transparent electrodes, Light-Emitting devices, solar cells, photoelectric detectors, mode-locking devices, photoswitches, laser protecting devices and biosensors. However, few investigations have been reported on coherent light scattering of graphene nanosheets. In this paper, we studied the coherent nonlinear optical light scattering of graphene nanosheets using a continuous-wave (CW) laser at 633 nm. The major research contents and results are as follows: 1. High quality graphene dispersions were prepared in N-methyl-2-pyrrolidone and Sodium Cholate solution by liquid-phase exfoliation method, respectively. The graphene dispersion was characterized using transmission electron microscopy and Raman spectrometer. The transmission spectra of graphene dispersion was utilized to calculate the effective number of graphene monolayers in a Quartz cuvette. In addition, graphene and PVA (Polyvinyl Alcohol) composite films were prepared by thermal evaporation. The surface roughness of graphene and PVA composite films was less than 17 nm/ (1 mm ×1 mm). 2. The nonlinear refractive index n_2 and the third-order nonlinear susceptibility χ^（3） of graphene were calculated, respectively. Spatial self-phase modulation (SSPM) technique was utilized to characterize n_2 and "χ" ^(("3" ) ) of graphene nanosheets using a focused He-Ne laser beam at 633 nm. n_2 and "χ" ^(("3" ) )of graphene monolayer were deduced as high as ~10-9 m2/W and ~10-7 esu, respectively, for a range of dispersions with different optical pathlengths and effective numbers of graphene monolayer. The results indicates that the spatial SPM method is a quick and efficient NLO technique for "χ" ^(("3" ) ) characterization of graphene. The intensity distribution of the SSPM patterns calculated by Kirchhoff’s diffraction theory has a good agreement with the experimantal results. Besides, SSPM effect of graphene and PVA composite film was studied qualitatively. 3. The distortion effect of SSPM diffraction pattern was analyzed. The SSPM pattern, whose original shape is a series of perfect concentric circles, distorted in a few seconds after the incident laser beam horizontally passing through the graphene dispersions. This is the distortion effect of SSPM. We found that the distortion effect of SSPM pattern was caused by the change of n_2 of graphene dispersion induced by thermal convection and bubbles. The graphene dispersion was heated by absorption part of the incident laser, forming temperature gradient inside graphene dispersion, which leads to some convection inside graphene dispersion. These will cause different distribution of n_2 in places where the laser beam passes through. Meanwhile, bubbles induced by laser heating also changed the distribution of n_2 inside the graphene dispersion. 4. The distortion effect was applied to calculate the change of nonlinear refractive index ∆n_2 of materials. When the thickness of graphene dispersion is 10 mm, effective numbers of graphene monolayers is 71 and the incident power is ~54 W/cm2, ∆n_2/n_2 is calculated to be ~20% at 20 oC and increases gradually to ~37.5% at 100 oC. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/16749]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 王高中. 液相剥离石墨烯的空间自相位调制研究[D]. 中国科学院上海光学精密机械研究所. 2013. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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