Plasmonics and Near-Field Optics: Towards the limits of electromagnetic energy confinement

在过去的几十年里，电子设备的尺寸以惊人的速度减小，这使得高度集成的微电子芯片的制造成为可能，微电子芯片彻底改变了我们的日常生活，从快速的个人电脑到现代汽车中的复杂电子设备，无处不在。虽然晶体管的尺寸正在迅速接近10纳米（一毫米的百万分之一）量级的典型尺寸，但是诸如光导和用于光存储的腔的光学器件的集成已经落后，使得在最先进的电子器件和光学器件之间存在大约10-100倍的尺寸差距。为了创造出能够通过光在芯片之间和芯片上传输信息的超快计算机，或者为了对单个分子进行光学研究，必须缩小这种尺寸差距。为了实现这一愿景，必须打破限制超小光束产生的基本障碍-所谓的衍射极限，即光不能被限制在小于感兴趣材料波长一半的尺寸内。然而，有一种方法可以绕过这个障碍，通过在金属和电介质之间的界面上产生低维光，即所谓的表面等离子体激元。这些激发是通过光与传导电子在金属界面处的运动的耦合而产生的，最近已经表明，光可以以这种方式被限制在光的衍射极限以下的尺寸。这使得能够创建许多等离子体激元器件，例如波导和纳米颗粒谐振器。在这里，我们提出了一些目前在这一领域遇到的主要挑战，即这种微小的光学器件与传统的光学光导，如光纤的有效接口，以及在等离子体结构中的高定位和（加热）损耗之间的固有权衡的调查。作为主要的研究工具，我们将使用扫描近场光学显微镜（SNOM），它通过一个非常小的孔（直径约100纳米）的尖端局部照明或收集来自纳米结构的光。这种显微镜的分辨率不受衍射极限的限制，但由孔的大小单独。然而，从这样的孔出射的光的形状没有被很好地限定，并且吞吐量通常很低，使得等离子体激元波导或谐振器的受控研究很困难。我们将克服这个障碍，通过使用薄的光纤命名为锥，它可以泄漏光在一个控制的方式，作为近场探头。这将使我们能够开发方法来进行金属纳米结构的白光光谱学，例如使用该原理的复杂形状的单个颗粒，波导和谐振器，致力于传统光学和纳米级金属光导之间的高效接口。这些努力的结果将影响近场光学的进一步建立一个常规的实验室技术，和等离子体光学基础设施的设计。在这方面，新的几何形状的光的引导和限制，如金属表面或同心环之间的小间隙将研究与这种技术，并通过使用强大的计算机模拟。这些研究将致力于低损耗波导与高限制光存储腔的集成，这对于创建全光学生物传感器和开关非常重要。

The size of electronic devices has decreased at an amazing speed during the last decades, and this has made possible the fabrication of highly integrated microelectronic chips, which have revolutionized our daily life and are found everywhere from fast personal computers to complex electronics in modern automobiles. While dimensions of transistors are fastly approaching typical sizes on the order of 10 nanometres (the millionth part of a millimetre), the integration of optical devices such as light guides and cavities for light storage has lagged behind, so that there exists a size gap of about a factor 10-100 between state-of-the-art electronic and optical devices. In order to create ultrafast computers that transmit information between and on chips via light, or to optically investigate single molecules, this size gap must be closed. For this vision to come true, a fundamental barrier that limits the creation of ultrasmall light beams has to be broken - the so-called diffraction limit , which states that light cannot be confined to dimensions smaller than about half the wavelength in the material of interest. There is a way around this obstacle, however, via the creation of lower dimensional light confined to the interface between a metal and a dielectric, so called surface plasmon-polaritons. These excitations are created via the coupling of light to the motion of the conduction electrons at a metallic interface, and it has been shown recently that light can be confined this way to dimensions below the diffraction limit of light. This has enabled the creation of a number of plasmonic devices such as waveguides and nanoparticle resonators. Here we propose to work on some of the major challenges currently encountered in this field, namely the efficient interfacing of such tiny optical devices with conventional optical light guides such as fibres, and the investigation of the inherent trade-off between high localization and (heating) loss in plasmonic structures. As a main research tool, we will use a scanning near-field optical microscope (SNOM), which locally illuminates or collects light from nanoscale structures via a tip with a very small hole (about 100 nanometre diameter) in it. The resolution of such a microscope is not limited by the diffraction limit, but by the hole size alone. However, the shape of the light emerging from such an aperture is not defined very well, and the throughput is generally low, making the controlled investigation of plasmon waveguides or resonators difficult. We will overcome this obstacle by using thinned optical fibres named tapers , which can leak out light in a controlled way, as near-field probes. This will allow us to develop ways to carry out white-light spectroscopy of metallic nanostructures such as single particles of complex shapes, waveguides and resonators using this principle, working towards highly efficient interfaces between conventional optical and nanoscale metallic light guides. The outcome of these efforts will affect both the further establishment of near-field optics a routine laboratory technique, and the design of a plasmon optics infrastructure. In this regard, novel geometries for the guiding and confinement of light such as small gaps between metallic surfaces or concentric rings will be investigated with this technique and by using powerful computer simulations. These studies will work towards the integration of low-loss waveguides with high confinement light storage cavities, which is important for the creation of all-optical biological sensors and switches.