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

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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.