A central topic in science and technology is the exploration and exploitation of smaller and smaller systems by optical techniques. Often, the wavelength is dictated by the system of interest: Biological systems emit and absorb visible light, or photovoltaic devices convert from the solar spectrum. For a long time, the achievable optical resolution has seemed principally bound by the diffraction limit.

However, the advances in nanoscience and nanotechnology have led to the fabrication of structures with ever smaller feature sizes, such that the length scale of fabricable features has reached dimensions far below the wavelength of visible light.

Visible light interacts resonantly with metallic structures that have characteristic dimensions of around 100nm. A strong resonant interaction of light with appropriately designed structures presents a manifold of new tools for the study of new optical phenomena in science and technology, for which the tight control of optical fields is a prerequisite. Plasmonic nanostructures strongly confine, enhance and thus control light on the nanometre scale. This thesis centers around the challenge of the precise control of optical fields on the nanoscale. An overview of near-field optics, its methods and challenges is presented in Chapter 1.

Nanotechnology relies largely on nanofabrication, which is a continuously developing topic. The feature size of under 100nm required for optically resonant nanostructures is within the range of the resolution of state-of-the-art nanofabrication tools. The fabrication of such nanostructures using Focused Ion Beam technology is discussed in Chapter 2.

Optical antennas have proven to efficiently link free radiation to objects through localized fields. The object can be a single molecule, a non-linear medium or a semiconductor, depending on the purpose of the device. With increasing complexity of optical antennas the need arises to precisely investigate and control their modal local field distribution. In Chapter 3, I present the investigation of local antenna fields by deterministic control of a nanometric fluorescent bead as the local field probe. The bead accurately maps the optical modes of an antenna, for the first time optically resolving modal features of 35nm FWHM. Moreover, the antenna resonance is revealed.

A critical point in the interaction of light with matter is the matching of the impedance of all components involved in the interaction. Chapter 4 demonstrates how intrinsically very different structures, a tapered waveguide and a sub-wavelength aperture, are impedance-matched at the wavelength of operation to improve the transmission of the aperture.

Near-field Scanning Optical Microscopy is a standard tool to image a variety of samples with nanometric resolution. The low transmissivity of conventional probes with sub-wavelength apertures imposes a strong limitation to its popularity. As reported in Chapter 5, a redesign of the probe removes the lossy sub-wavelength components and improves the feed to the aperture. The throughput increases by 100x and the damage threshold by 40x. As this increase in brightness allows to employ smaller apertures, single molecules are imaged with a true optical resolution of as good as 60nm FWHM. No fitting algorithms are required.

As the results presented in this thesis show, localized fields and therefore the functioning of nanostructures such as optical antennas can be precisely assessed by a mapping with fluorescent nanosources. The mapping provides a flexible tool to tune the nanostructures and increase the level of control exerted on optical fields. In reverse, an optimized nanostructure will efficiently control single emitters in its vicinity. Benefiting applications include high resolution imaging, high sensitivity sensing and photo detection, photovoltaics and non-linear optics.


Wednesday April 18, 11:00. ICFO Auditorium

Thesis Advisor: Prof. Niek Van Hulst