Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu, Sichuan 610054, China.
Department of Mechanical Engineering, University of Houston, Houston, Texas 77204, USA.
Nanoscale. 2017 Mar 9;9(10):3547-3554. doi: 10.1039/c6nr08487g.
The interaction of light with atomically thin nanomaterials has attracted enormous research interest in order to understand two-dimensional (2D) electron systems and develop novel opto-electronic devices. The observations of spatial self-phase modulation and the associated multiple diffraction ring patterns in liquid suspensions of 2D nanomaterials are believed to be excellent examples of strong laser interaction with 2D nanomaterials and this phenomenon has been attributed to their large electronic third-order susceptibilities. By performing a series of control experiments with liquid suspensions of graphene and graphene oxide flakes in different solvents at various temperatures under an increasing modulation frequency of laser illumination, we first show that the diffraction ring pattern has little dependence on the type of nanomaterial but strongly depends on the duration of laser illumination. A laser induced local refractive index change is then monitored by a weaker probe beam, resulting in the divergent diffraction of the probe beam that indicates a lower self-induced refractive index in the center of the pump laser beam than at its periphery: a clear signature of the thermal lens effect. Finally, we use computational fluid dynamics to simulate laser induced temperature and index changes of the suspensions. The evolution of diffraction rings is well correlated to the transient temperature distribution. Our understanding of complex laser interactions with nanomaterial suspensions and the associated thermal lens effect paves the way for further basic studies and fluid opto-electronic applications of 2D nanomaterials.
为了理解二维（2D）电子系统并开发新型光电设备，原子层薄纳米材料与光的相互作用引起了极大的研究兴趣。在二维纳米材料的液体悬浮液中观察到的空间自相位调制和相关的多个衍射环图案，被认为是强激光与二维纳米材料相互作用的极好例子，这种现象归因于它们大的电子三阶极化率。通过在不同温度下的不同溶剂中对石墨烯和氧化石墨烯片的液体悬浮液进行一系列控制实验，并在激光照射的调制频率增加的情况下，我们首先表明，衍射环图案几乎不依赖于纳米材料的类型，而强烈依赖于激光照射的持续时间。然后，通过较弱的探测光束来监测激光诱导的局部折射率变化，导致探测光束的发散衍射，这表明在泵浦激光束的中心的自诱导折射率低于其外围：热透镜效应的明显特征。最后，我们使用计算流体动力学来模拟悬浮液中的激光诱导温度和折射率变化。衍射环的演化与瞬态温度分布很好地相关。我们对纳米材料悬浮液中复杂激光相互作用及其相关热透镜效应的理解，为进一步的二维纳米材料基础研究和流体光电应用铺平了道路。
