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，这些解决方案已经成为日常生活中不可或缺的一部分。它们的范围从激光指示器，在很长的距离内聚焦一个微小的光点，到光缆，它取代了电报，并通过允许以令人难以置信的速度传输数据来革命电信行业。在任何这类设备的运行中，基本的作用都是通过波导--类似于电线的光学材料--来实现的。该波导实现了两个主要功能。它们引导光沿着所需的路径传播，抑制其自然传播并占据所有可用空间的趋势。此外，通过在波导芯中保持较高的局部光强度，它们会触发介质的非线性响应。后者对于执行超高速信号处理至关重要：从频率转换和脉冲重塑到对光携带的信号进行逻辑运算。当今的导光技术依赖于确保光束在光学透镜中聚焦的相同原理：即光更喜欢在密集介质中传播，而不是在稀薄介质中传播。例如，一根由致密材料(如二氧化硅玻璃)制成的细丝被空气包围，构成了一种光的引导结构。然而，这项技术有一个主要的限制：波导芯的尺寸不能太小。这里的最小刻度是由光的波长决定的--大约是微米级。尽管它比人的头发要细得多，但它仍然不够小，不能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(目标是将性能提高一千倍甚至更多)，每个元件的尺寸应该在几十纳米左右。将光挤得这么紧是一项具有挑战性的任务。有可能取代传统波导的候选材料是目前正在研究实验室中开发的所谓等离子体波导管。它们代表了透明电介质背景中几个纳米厚的微小金属夹杂物，反之亦然，金属表面上的狭窄凹槽/楔形或金属薄膜上的孔洞。这种复合金属/电介质结构中的光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“等离子激元”。最重要的是，等离子体波导管的最小尺寸没有限制，事实上，它们在纳米尺度上定域和引导光。虽然等离子体波导管的主要指导原理已经被很好地理解，但到目前为止，关于等离子体波不同的非线性过程知之甚少。原因是最先进的理论是基于模型的，这些模型是在假设材料属性的变化幅度与光波长相当或远大于光波长的情况下推导出来的。显然，这不再适用于等离子体波导管装置。这项研究的目的是发展合适的理论，从根本上探索与光在金属和介质组成的复合纳米结构中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、自适应和可重构光学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.