Nonlinear pulse dynamics in plasmonic waveguides

我们目前对光的本质及其与物质的相互作用的理解，使科学家和工程师能够引入各种技术解决方案，这些解决方案已成为日常生活中不可或缺的一部分。这些技术的范围从激光笔，在很远的距离上聚焦一个微小的光点，到光纤电缆，它取代了电报线，并通过以惊人的速度传输数据，彻底改变了电信行业。波导——电线的光学类似物——在任何此类设备的操作中起着基本作用。波导实现两个主要功能。它们引导光线沿着预期的路径，抑制其自然扩散的趋势，并占据所有可用的空间。此外，通过在波导核心中保持高局部光强度，它们触发介质的非线性响应。后者对于执行超快速信号处理至关重要：从频率转换和脉冲重整形到光携带信号的逻辑运算。当今的光导技术依靠的原理与光学透镜中光束聚焦的原理相同：光“更喜欢”在密集介质中传播，而不是在稀薄介质中传播。一根由致密材料（如硅玻璃）制成的细线，例如被空气包围，就构成了光的导向结构。然而，这种技术有一个主要的限制：波导核心的尺寸不能太小。这里的最小尺度是由光的波长决定的——微米量级。虽然比人的头发丝细得多，但仍不足以满足现代技术的要求。例如，如果要用光学类似物取代现代微处理器的所有电子元件（目标是将性能提高1000倍甚至更多），每个元件的尺寸应该在几十纳米左右。把光线挤得这么紧是一项具有挑战性的任务。有望取代传统波导的候选者是所谓的等离子波导，目前正在研究实验室中开发。它们代表了透明电介质背景中几纳米厚的微小金属夹杂物，或者，反之亦然，金属表面上的狭窄凹槽/楔形或金属膜上的孔。在这种复合金属/电介质结构中，光的引导是通过在金属和电介质之间的表面上激发特定波来实现的。这种波将光子与金属内部的等离子体振荡耦合在一起，被称为“等离子体激元”。最重要的是，等离子体波导的最小尺寸没有限制，事实上，它们在纳米尺度上定位和引导光。虽然等离子体波导的主要指导原理已被很好地理解，但迄今为止对等离子体的不同非线性过程知之甚少。原因是最先进的理论是建立在模型的基础上的，这些模型是在假设材料特性的变化规模与光的波长相当或远远大于光的波长的情况下推导出来的。显然，这对等离子波导设置不再适用。本研究的目的是建立适当的理论，并从根本上探索与光在由金属和电介质组成的复合纳米结构中的传播有关的新的非线性过程。旨在理解和探索新的物理效应，将为未来高性能、便携式、可调谐、自适应和可重构光器件的发展奠定坚实的基础。

Our current understanding of light nature and its interaction with matter has allowed scientists and engineers to introduce a variety of technological solutions, which have become integral parts of everyday life. These range from laser pointers, focusing a tiny spot of light over large distances, to optical cables which replaced telegraph wires and revolutionized telecommunication industry by allowing data transmissions at incredible rates. The fundamental role in operation of any such device is played by waveguides - optical analogues of electric wires. The waveguides implement two major functions. They guide light along desired paths suppressing its natural tendency to spread and occupy all the available space. Also, by sustaining high local light intensity in the waveguide core, they trigger nonlinear response of the medium. The latter is vital to perform ultra-fast signal processing: from frequency conversion and pulse re-shaping to logical operations with signal carried by light. The present-day techniques of light guiding rely on the same principles that ensure beam focusing in an optical lens: the fact that light "prefers" to propagate in a dense medium rather than in a rarefied one. A thin wire made of a dense material, such as silica glass, surrounded, for example, by air makes a guiding structure for light. This technique, however, has the major limitation: the size of a waveguide core cannot be too small. The minimal scale here is dictated by the wavelength of light - of the order of micrometer. Being much thinner than a human hair, this is still not small enough to comply with demands of modern technology. For instance, if one is to replace all electronic components of a modern microprocessor by their optical analogues (aiming to boost the performance by a thousand times and beyond), the size of each element should be of the order of few tens of nanometers. Squeezing light this tight is a challenging task.Promising candidates to replace conventional waveguides - are the so-called plasmonic waveguides, currently being developed in research labs. They represent few nanometers thick tiny metal inclusions in a transparent dielectric background, or, vice versa, narrow grooves/wedges on a metallic surface or holes in metal films. Light guiding in such composite metal/dielectric structures is done by virtue of exciting specific waves on a surface between a metal and a dielectric. Such waves couple photons with plasma oscillations inside the metal and are called 'plasmons'. Crucially, there is no limitation as to the minimal size of a plasmonic waveguide, indeed they localize and guide light at nanometer scale.While the major guiding principles of plasmonic waveguides are well understood, little is known about different nonlinear processes with plasmons so far. The reason is that the state-of-the art theory is based on models, derived under the assumption that the material properties change on a scale comparable to or much larger than the light wavelength. Apparently, this is no longer true for plasmonic waveguide setups. It is the purpose of this research to develop appropriate theories and explore fundamentally new nonlinear processes associated with light propagation in composite nano-structures consisting of metals and dielectrics. Aiming to understand and explore novel physical effects, it will form the solid basis for future development of high performance, portable, tuneable, adaptive and reconfigurable optical devices.