基于可调界面二维自由电子气等离激元的介观二阶与三阶光学非线性的研究

Recent years, by coupling photons with free electron gas, interface plasmon polaritons (IPP) can be excited in subwavelength scale near the interface. As the result, the intensity of the electromagnetic field is intensified several orders of magnitude due to light confinement, consequently, optical nonlinearity was strengthened greatly. The associated investigations bring forth nonlinear plasmonics and propel its advancement. It is expected that new optical material systems, exotic photonic devices and photonic circuits can be designed and implemented out of this growing field. Grabbing this opportunity, it may infuse life into national economy in the growing global competitions, envisioning photonics as one of potential main driving forces. In this project, semiconductor coated polar crystal slabs will be studied. Based on electrostatic modification and phase grating mediated excitation, IPPs will be excited to strengthen optical nonlinearity in the subwavelength scale. We will mainly experimentally investigate the second and third nonlinearity, generation of photons in new frequencies, and optical modulation and other related effects. In addition, a theoretical frame applicable to the material systems will be developed, based on the nonlinear theory for the bulk systems. The new material systems and the unique way in exciting IPPs show various advantages over their conventional metallic counterparts, such as ultra-low loss and favorable tunability.

近年来，人们通过耦合光子和自由电子气，在导体和电介质界面的亚波长尺度内激励界面等离激元，由此引发的电磁场局域使其强度增大数个量级。由此带来的好处是在相对弱的入射光的情形下引起显著的非线性光学效应。相关的研究孕育了快速发展的非线性等离子学，且被用于设计研发许多新奇的材料系统、光子学器件乃至光子回路。抓住此契机，无疑有利于国家在以光子学为核心引擎之一的科技角逐中立于不败之地。本课题采用半导体薄膜覆盖的被誉为光子学“硅”的铌酸锂极性晶片，结合静电改性所致的界面二维电子气与位相光栅，巧妙地激发界面等离激元，从而极大地提高电磁场强度而使光学非线性效应显著增强。我们将重点从实验出发研究二阶与三阶光学非线性效应相关的新频率光的产生与光学调制等现象，继而利用体块材料中非线性光学理论框架来建立适于亚波长尺度的理论模型。本课题所采用的材料系统与激励等离激元之方法比起采用传统的金属薄膜具有低耗可调等诸多优点。

本课题采用半导体薄膜覆盖的被誉为光子学“硅”的铌酸锂极性晶片与钽酸锂晶片，结合静电改性所致的亚纳米层内实现高达金属电子密度界面二维电子气与位相光栅，巧妙地激发可见光波段界面等离激元，从而极大地提高电磁场强度而致使光学非线性效应显著增强。我们重点从实验出发研究二阶与三阶光学非线性效应相关的新频率光的产生与光学调制等现象，继而利用体块材料中非线性光学理论框架来建立适于亚波长尺度的理论模型。本课题所采用的材料系统与激励等离激元之方法比起采用传统的金属薄膜具有低耗可调等诸多优点。主要工作总结如下：.利用ITO覆盖掺铁铌酸锂晶片，观察到在ITO/LN界面处有点状与线状的折射率突变点，致使透过光的相位突变超过π。这样大的折射率突变是因为界面处的物理机制所导致，是近于原子内部的超高值静电场与非线性效应共同作用的综合结果。这充分证明有SPP激励所致的非线性的贡献。此实验证实了本课题立题时的推测分析是正确的。这项研究工作为无高损耗金属导体参与的可见光波段SPP激励与等离非线性光学的研究开辟了一条新的途径。 .提出对液晶分子垂直取向的新型物理机制，简化了把液晶层集成进半导体器件的难度，提高了系统的载流子密度和迁移率；将液晶取向层的材料从传统的高聚物拓展为半导体，探索了利用半导体薄膜对液晶取向的可能性。.利用ITO/LN材料系统，我们还较系统地研究了可见光波段SPP激励所致一系列新奇效应。如（1）基于SPP表面传输的杨氏双光束干涉。这证实了SPP在ITO/LN界面经过厘米量级传输后再转换到可传播光时，并不丧失去相干性；（2）将此SPP激励推广至过渡金属元素掺杂的钽酸锂与钾钠铌酸锶钡晶体系列..报道了一种铁电铌酸锂和覆盖单层石墨烯之间的电荷转移以及随后的效应。研究发现，石墨烯包覆LN更有利于实现可见SPP的激发和合成能量耦合，即反射增加和更大的动态范围。这项工作揭示了二维双层二维电子系统之间的相互作用，为实现可调谐非金属等离子体器件铺平了道路。

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