Research activities Photonic Materials

Surface plasmon nano-optics

We study the propagation, confinement and dispersion of surface plasmon polaritons on metal nanostructures. The taper shown in the figure is used to concentrate light with a wavelength of 1500 nm to a "hot spot" with a diameter smaller than 100 nm. We study the process of adiabatically guiding towards the taper tip and use these tapers to couple light into metallic nanowire waveguides.

Surface plasmon nanocavities

We study the confinement of light in the smallest possible optical cavities. The ring shown in the figure is made by focused ion beam milling in single-crystalline gold, and has a diameter of only 600 nm. Light is confined in a whispering-gallery mode that propagates at the bottom of the groove. Such plasmonic nanocavities have a very high ratio of Q/V (quality factor / mode volume), which is of interest in the control of spontaneous emission and low-threshold lasing.

Plasmonic photovoltaics

We study the integration of metal nanostructures with thin film photovoltaic solar cells. Light is scattered from the metal nanoparticles and subsequently coupled into the thin-film semiconductor layer over a wide range of angles, thereby enhancing the effective path length (and thus absorption) in the layer. We study fundamental aspects of this effect and apply it in experiments on thin-film silicon solar cells.

Optical metamaterials

Optical metamaterials are materials with a nanoscale structure that is engineered to lead to novel/unusual optical properties. For example, we have studied metal-insulator-metal waveguides that, for a particular geometry shows a negative refractive index for light propagating in the dielectric. Negative-index materials find applications in optical imaging below the diffraction limit, or invisibility cloaking.

Cathodoluminescence spectroscopy

Cathodoluminescence imaging spectroscopy is a new technique that we have developed to study optical phenomena at the nanoscale. An electron beam, incident on the surface of a metal nanostructure will generate transition radiation into the far field, and surface plasmons that propagate over the surface. By detecting the emitted light as a function of position of the electron beam, we are able to determine two-dimensional images of the optical density of states. The resolution of the technique is only determined by the electron-beam spot size, typically < 10 nm.

Nanofabrication

In order to perform leading experiments in the field of nanophotonics, it is essential to develop develop novel methods to fabricate optical materials at the nanoscale. The figure shows a plasmonic nanolense that is made by assembling Au nanoparticles (diameters 5, 8 and 15 nm) by using a DNA templating technique. Other nanofabrication tools that we continuously develop and improve are focussed ion beam milling, electron beam lithography and colloidal self-assembly.